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Dziri S, Hassen I, Fatnassi S, Mrabet Y, Casabianca H, Hanchi B, Hosni K. Phenolic constituents, antioxidant and antimicrobial activities of rosy garlic (Allium roseum var. odoratissimum). J Funct Foods 2012. [DOI: 10.1016/j.jff.2012.01.010] [Citation(s) in RCA: 65] [Impact Index Per Article: 5.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/25/2022] Open
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A DFT study on the structural, electronic properties and radical scavenging mechanisms of calycosin, glycitein, pratensein and prunetin. COMPUT THEOR CHEM 2012. [DOI: 10.1016/j.comptc.2012.01.028] [Citation(s) in RCA: 40] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/19/2022]
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103
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Park J, Morimoto Y, Lee YM, Nam W, Fukuzumi S. Proton-Promoted Oxygen Atom Transfer vs Proton-Coupled Electron Transfer of a Non-Heme Iron(IV)-Oxo Complex. J Am Chem Soc 2012; 134:3903-11. [DOI: 10.1021/ja211641s] [Citation(s) in RCA: 76] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Jiyun Park
- Department of Bioinspired Science, Ewha Womans University, Seoul 120-750, Korea
| | - Yuma Morimoto
- Department of Material and Life
Science, Graduate School of Engineering, Osaka University, ALCA, Japan Science and Technology Agency (JST), Suita, Osaka 565-0871,
Japan
| | - Yong-Min Lee
- Department of Bioinspired Science, Ewha Womans University, Seoul 120-750, Korea
| | - Wonwoo Nam
- Department of Bioinspired Science, Ewha Womans University, Seoul 120-750, Korea
| | - Shunichi Fukuzumi
- Department of Bioinspired Science, Ewha Womans University, Seoul 120-750, Korea
- Department of Material and Life
Science, Graduate School of Engineering, Osaka University, ALCA, Japan Science and Technology Agency (JST), Suita, Osaka 565-0871,
Japan
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104
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Fukuzumi S, Ohkubo K, Morimoto Y. Mechanisms of metal ion-coupled electron transfer. Phys Chem Chem Phys 2012; 14:8472-84. [DOI: 10.1039/c2cp40459a] [Citation(s) in RCA: 60] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/29/2023]
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105
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Woldu AS, Mai J. Computation of the bond dissociation enthalpies and free energies of hydroxylic antioxidants using the ab initio Hartree-Fock method. Redox Rep 2012; 17:252-74. [PMID: 23339861 PMCID: PMC6837695 DOI: 10.1179/1351000212y.0000000030] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/31/2022] Open
Abstract
INTRODUCTION A new method for calculating theoretical bond dissociation enthalpy (BDE) and bond dissociation free energy (BDFE) of hydroxylic antioxidants is forwarded. BDE and BDFE may be understood as activation energies accompanying the formation of transition states, which may undergo downhill homolytic dissociation. The new method does not involve the complete fission of O-H bonds. METHOD Theoretical gas phase BDE values were calculated with the ab initio unrestricted Hartree-Fock (UHF) method, as changes in enthalpy between ground singlet states (GS) and triplet dissociative states (DS). Similarly, gas phase BDFEs were estimated from the corresponding changes in Gibbs free energy. The results were then compared with reliable experimental reports. RESULTS The proposed theoretical approach of BDE and BDFE determination was tested using 10 simple phenols, 5 flavonoids, and l-ascorbic acid derivatives. The agreement between our calculated gas phase results and the adopted experimental values were generally within 0.5 kcal mol(-1), with a very few exceptions. DISCUSSION Generally, steric interactions as well as intramolecular hydrogen bonding involving the dissociating OH group should be minimized in the GS. The DS are both electronically and vibrationally exited transition states. They have one unpaired electron on the carbon atom, which bears the homolytically dissociating OH group and are second order saddle points with a fixed CONCLUSION It was concluded that ab initio UHF was well suited for the estimation of gas phase BDE and BDFE. The method presented has a good potential for application across a range of hydroxylic antioxidants. Currently, work is underway to extend its application in other class of antioxidants.
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Affiliation(s)
- Ameha Seyoum Woldu
- Department of Pharmaceutical Chemistry and Pharmacognosy, School of Pharmacy, College of Health Sciences, Addis Ababa University, Addis Ababa, Ethiopia.
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Bonin J, Robert M. Photoinduced Proton-Coupled Electron Transfers in Biorelevant Phenolic Systems. Photochem Photobiol 2011; 87:1190-203. [DOI: 10.1111/j.1751-1097.2011.00996.x] [Citation(s) in RCA: 32] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
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108
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Gerken JB, McAlpin JG, Chen JYC, Rigsby ML, Casey WH, Britt RD, Stahl SS. Electrochemical Water Oxidation with Cobalt-Based Electrocatalysts from pH 0–14: The Thermodynamic Basis for Catalyst Structure, Stability, and Activity. J Am Chem Soc 2011; 133:14431-42. [DOI: 10.1021/ja205647m] [Citation(s) in RCA: 611] [Impact Index Per Article: 47.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022]
Affiliation(s)
- James B. Gerken
- Department of Chemistry, University of Wisconsin−Madison, 1101 University Avenue, Madison, Wisconsin 53706-1322, United States
| | - J. Gregory McAlpin
- Department of Chemistry, University of California, 1 Shields Avenue, Davis, California 95616-0935, United States
| | - Jamie Y. C. Chen
- Department of Chemistry, University of Wisconsin−Madison, 1101 University Avenue, Madison, Wisconsin 53706-1322, United States
| | - Matthew L. Rigsby
- Department of Chemistry, University of Wisconsin−Madison, 1101 University Avenue, Madison, Wisconsin 53706-1322, United States
| | - William H. Casey
- Department of Chemistry, University of California, 1 Shields Avenue, Davis, California 95616-0935, United States
| | - R. David Britt
- Department of Chemistry, University of California, 1 Shields Avenue, Davis, California 95616-0935, United States
| | - Shannon S. Stahl
- Department of Chemistry, University of Wisconsin−Madison, 1101 University Avenue, Madison, Wisconsin 53706-1322, United States
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109
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Keough JM, Jenson DL, Zuniga AN, Barry BA. Proton coupled electron transfer and redox-active tyrosine Z in the photosynthetic oxygen-evolving complex. J Am Chem Soc 2011; 133:11084-7. [PMID: 21714528 PMCID: PMC3246746 DOI: 10.1021/ja2041139] [Citation(s) in RCA: 40] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Proton coupled electron transfer (PCET) reactions play an essential role in many enzymatic processes. In PCET, redox-active tyrosines may be involved as intermediates when the oxidized phenolic side chain deprotonates. Photosystem II (PSII) is an excellent framework for studying PCET reactions, because it contains two redox-active tyrosines, YD and YZ, with different roles in catalysis. One of the redox-active tyrosines, YZ, is essential for oxygen evolution and is rapidly reduced by the manganese-catalytic site. In this report, we investigate the mechanism of YZ PCET in oxygen-evolving PSII. To isolate YZ(•) reactions, but retain the manganese-calcium cluster, low temperatures were used to block the oxidation of the metal cluster, high microwave powers were used to saturate the YD(•) EPR signal, and YZ(•) decay kinetics were measured with EPR spectroscopy. Analysis of the pH and solvent isotope dependence was performed. The rate of YZ(•) decay exhibited a significant solvent isotope effect, and the rate of recombination and the solvent isotope effect were pH independent from pH 5.0 to 7.5. These results are consistent with a rate-limiting, coupled proton electron transfer (CPET) reaction and are contrasted to results obtained for YD(•) decay kinetics at low pH. This effect may be mediated by an extensive hydrogen-bond network around YZ. These experiments imply that PCET reactions distinguish the two PSII redox-active tyrosines.
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Affiliation(s)
- James M. Keough
- School of Chemistry and Biochemistry and the Petit Institute for Bioengineering and Bioscience, Georgia Institute of Technology, Atlanta, Georgia 30332
| | - David L. Jenson
- School of Chemistry and Biochemistry and the Petit Institute for Bioengineering and Bioscience, Georgia Institute of Technology, Atlanta, Georgia 30332
| | - Ashley N. Zuniga
- School of Chemistry and Biochemistry and the Petit Institute for Bioengineering and Bioscience, Georgia Institute of Technology, Atlanta, Georgia 30332
| | - Bridgette A. Barry
- School of Chemistry and Biochemistry and the Petit Institute for Bioengineering and Bioscience, Georgia Institute of Technology, Atlanta, Georgia 30332
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Viuda-Martos M, Ruiz-Navajas Y, Fernández-López J, Sendra E, Sayas-Barberá E, Pérez-Álvarez JA. Antioxidant properties of pomegranate (Punica granatum L.) bagasses obtained as co-product in the juice extraction. Food Res Int 2011. [DOI: 10.1016/j.foodres.2010.10.057] [Citation(s) in RCA: 66] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
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111
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Marković ZS, Dimitrić Marković JM, Milenković D, Filipović N. Mechanistic study of the structure-activity relationship for the free radical scavenging activity of baicalein. J Mol Model 2011; 17:2575-84. [PMID: 21229369 DOI: 10.1007/s00894-010-0942-y] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/18/2010] [Accepted: 12/20/2010] [Indexed: 10/18/2022]
Abstract
Density functional theory calculations were performed to evaluate the antioxidant activity of baicalein. The conformational behaviors of both the isolated and the aqueous-solvated species (simulated with the conductor-like polarizable continuum solvation model) were analyzed at the M052X/6-311 + G(d,p) level. The most stable tautomers of various forms of baicalein displayed three IHBs between O4 and OH5, O5 and OH6, and O6 and OH7. The most stable tautomer of the baicalein radical was obtained by dehydrogenating the hydroxyl at C6, while the most stable anion tautomer was obtained by deprotonating the C7 hydroxyl in gaseous and aqueous phases. The expected antioxidant activity of baicalein was explained by its ionization potentials (IPs) and homolytic O-H bond dissociation enthalpies (BDEs), which were obtained via the UM052X optimization level of the corresponding radical species. Heterolytic O-H bond cleavages (proton dissociation enthalpies, PDEs) were also computed. The calculated IP, BDE, and PDE values suggested that one-step H-atom transfer, rather than sequential proton loss-electron transfer or electron transfer-proton transfer, would be the most favorable mechanism for explaining the antioxidant activity of baicalein in the gas phase and in nonpolar solvents. In aqueous solution, the SPLET mechanism was more important.
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Affiliation(s)
- Zoran S Marković
- Department of Biochemical and Medical Sciences, State University of Novi Pazar, Vuka Karadžića bb, Novi Pazar 36300, Republic of Serbia
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112
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Zhang W, Burgess IJ. Step-wise proton-coupled electron transfer extended to aminobenzoquinone modified monolayers. Phys Chem Chem Phys 2011; 13:2151-9. [DOI: 10.1039/c0cp01251c] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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113
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Kawashima T, Ohkubo K, Fukuzumi S. Stepwise vs. concerted pathways in scandium ion-coupled electron transfer from superoxide ion to p-benzoquinone derivatives. Phys Chem Chem Phys 2011; 13:3344-52. [DOI: 10.1039/c0cp00916d] [Citation(s) in RCA: 27] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
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114
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Kumar A, Sevilla MD. Proton-coupled electron transfer in DNA on formation of radiation-produced ion radicals. Chem Rev 2010; 110:7002-23. [PMID: 20443634 PMCID: PMC2947616 DOI: 10.1021/cr100023g] [Citation(s) in RCA: 154] [Impact Index Per Article: 11.0] [Reference Citation Analysis] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2023]
Affiliation(s)
- Anil Kumar
- Department of Chemistry, Oakland University, Rochester, MI 48309
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Affiliation(s)
- Jillian L. Dempsey
- Beckman Institute, California Institute of Technology, Pasadena, CA 91125
| | - Jay R. Winkler
- Beckman Institute, California Institute of Technology, Pasadena, CA 91125
| | - Harry B. Gray
- Beckman Institute, California Institute of Technology, Pasadena, CA 91125
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Gerken JB, Landis EC, Hamers RJ, Stahl SS. Fluoride-modulated cobalt catalysts for electrochemical oxidation of water under non-alkaline conditions. CHEMSUSCHEM 2010; 3:1176-1179. [PMID: 20725926 DOI: 10.1002/cssc.201000161] [Citation(s) in RCA: 34] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/29/2023]
Affiliation(s)
- James B Gerken
- Department of Chemistry, University of Wisconsin-Madison, 1101 University Ave, 53706-1322, USA
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Sichula V, Hu Y, Mirzakulova E, Manzer SF, Vyas S, Hadad CM, Glusac KD. Mechanism of N(5)-Ethyl-flavinium Cation Formation Upon Electrochemical Oxidation of N(5)-Ethyl-4a-hydroxyflavin Pseudobase. J Phys Chem B 2010; 114:9452-61. [PMID: 20597524 DOI: 10.1021/jp104443y] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Vincent Sichula
- Department of Chemistry, Bowling Green State University, Bowling Green, Ohio 43403, and Department of Chemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210
| | - Ying Hu
- Department of Chemistry, Bowling Green State University, Bowling Green, Ohio 43403, and Department of Chemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210
| | - Ekaterina Mirzakulova
- Department of Chemistry, Bowling Green State University, Bowling Green, Ohio 43403, and Department of Chemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210
| | - Samuel F. Manzer
- Department of Chemistry, Bowling Green State University, Bowling Green, Ohio 43403, and Department of Chemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210
| | - Shubham Vyas
- Department of Chemistry, Bowling Green State University, Bowling Green, Ohio 43403, and Department of Chemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210
| | - Christopher M. Hadad
- Department of Chemistry, Bowling Green State University, Bowling Green, Ohio 43403, and Department of Chemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210
| | - Ksenija D. Glusac
- Department of Chemistry, Bowling Green State University, Bowling Green, Ohio 43403, and Department of Chemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210
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118
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Yan S, Kang S, Hayashi T, Mukamel S, Lee JY. Computational studies on electron and proton transfer in phenol-imidazole-base triads. J Comput Chem 2010; 31:393-402. [PMID: 19479733 DOI: 10.1002/jcc.21339] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
Abstract
The electron and proton transfer in phenol-imidazole-base systems (base = NH(2)(-) or OH(-)) were investigated by density-functional theory calculations. In particular, the role of bridge imidazole on the electron and proton transfer was discussed in comparison with the phenol-base systems (base = imidazole, H(2)O, NH(3), OH(-), and NH(2)(-)). In the gas phase phenol-imidazole-base system, the hydrogen bonding between the phenol and the imidazole is classified as short strong hydrogen bonding, whereas that between the imidazole and the base is a conventional hydrogen bonding. The n value in sp(n) hybridization of the oxygen and carbon atoms of the phenolic CO sigma bond was found to be closely related to the CO bond length. From the potential energy surfaces without and with zero point energy correction, it can be concluded that the separated electron and proton transfer mechanism is suitable for the gas-phase phenol-imidazole-base triads, in which the low-barrier hydrogen bond is found and the delocalized phenolic proton can move freely in the single-well potential. For the gas-phase oxidized systems and all of the triads in water solvent, the homogeneous proton-coupled electron transfer mechanism prevails.
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Affiliation(s)
- Shihai Yan
- Department of Chemistry, SungKyunKwan University, Suwon 440-746, Korea
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120
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Fukuzumi S, Kotani H, Suenobu T, Hong S, Lee YM, Nam W. Contrasting Effects of Axial Ligands on Electron-Transfer Versus Proton-Coupled Electron-Transfer Reactions of Nonheme Oxoiron(IV) Complexes. Chemistry 2010; 16:354-61. [DOI: 10.1002/chem.200901163] [Citation(s) in RCA: 40] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
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121
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Miyazaki S, Kojima T, Mayer JM, Fukuzumi S. Proton-coupled electron transfer of ruthenium(III)-pterin complexes: a mechanistic insight. J Am Chem Soc 2009; 131:11615-24. [PMID: 19722655 DOI: 10.1021/ja904386r] [Citation(s) in RCA: 56] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Ruthenium(II) complexes having pterins of redox-active heteroaromatic coenzymes as ligands were demonstrated to perform multistep proton transfer (PT), electron transfer (ET), and proton-coupled electron transfer (PCET) processes. Thermodynamic parameters including pK(a) and bond dissociation energy (BDE) of multistep PCET processes in acetonitrile (MeCN) were determined for ruthenium-pterin complexes, [Ru(II)(Hdmp)(TPA)](ClO(4))(2) (1), [Ru(II)(Hdmdmp)(TPA)](ClO(4))(2) (2), [Ru(II)(dmp(-))(TPA)]ClO(4) (3), and [Ru(II)(dmdmp(-))(TPA)]ClO(4) (4) (Hdmp = 6,7-dimethylpterin, Hdmdmp = N,N-dimethyl-6,7-dimethylpterin, TPA = tris(2-pyridylmethyl)amine), all of which had been isolated and characterized before. The BDE difference between 1 and one-electron oxidized species, [Ru(III)(dmp(-))(TPA)](2+), was determined to be 89 kcal mol(-1), which was large enough to achieve hydrogen atom transfer (HAT) from phenol derivatives. In the HAT reactions from phenol derivatives to [Ru(III)(dmp(-))(TPA)](2+), the second-order rate constants (k) were determined to exhibit a linear relationship with BDE values of phenol derivatives with a slope (-0.4), suggesting that this HAT is simultaneous proton and electron transfer. As for HAT reaction from 2,4,6-tri-tert-buthylphenol (TBP; BDE = 79.15 kcal mol(-1)) to [Ru(III)(dmp(-))(TPA)](2+), the activation parameters were determined to be DeltaH(double dagger) = 1.6 +/- 0.2 kcal mol(-1) and DeltaS(double dagger) = -36 +/- 2 cal K(-1) mol(-1). This small activation enthalpy suggests a hydrogen-bonded adduct formation prior to HAT. Actually, in the reaction of 4-nitrophenol with [Ru(III)(dmp(-))(TPA)](2+), the second-order rate constants exhibited saturation behavior at higher concentrations of the substrate, and low-temperature ESI-MS allowed us to detect the hydrogen-bonding adduct. This also lends credence to an associative mechanism of the HAT involving intermolecular hydrogen bonding between the deprotonated dmp ligand and the phenolic O-H to facilitate the reaction. In particular, a two-point hydrogen bonding between the complex and the substrate involving the 2-amino group of the deprotonated pterin ligand effectively facilitates the HAT reaction from the substrate to the Ru(III)-pterin complex.
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Affiliation(s)
- Soushi Miyazaki
- Department of Material and Life Science, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan
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Quantitative structure–activity relationships for estimating the aryl hydrocarbon receptor binding affinities of resveratrol derivatives and the antioxidant activities of hydroxystilbenes. Med Chem Res 2009. [DOI: 10.1007/s00044-009-9236-2] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/15/2023]
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123
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Mandal PC, Bhattacharyya J, Das S, Mukhopadhyay S, Kirschenbaum LJ. Mechanistic studies on the oxidation of pyruvic acid by an oxo-bridged diiron(III,III) complex in aqueous acidic media. Polyhedron 2009. [DOI: 10.1016/j.poly.2009.07.009] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/20/2022]
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124
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Wu A, Mader EA, Datta A, Hrovat DA, Borden WT, Mayer JM. Nitroxyl radical plus hydroxylamine pseudo self-exchange reactions: tunneling in hydrogen atom transfer. J Am Chem Soc 2009; 131:11985-97. [PMID: 19618933 PMCID: PMC2775461 DOI: 10.1021/ja904400d] [Citation(s) in RCA: 65] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Bimolecular rate constants have been measured for reactions that involve hydrogen atom transfer (HAT) from hydroxylamines to nitroxyl radicals, using the stable radicals TEMPO(*) (2,2,6,6-tetramethylpiperidine-1-oxyl radical), 4-oxo-TEMPO(*) (2,2,6,6-tetramethyl-4-oxo-piperidine-1-oxyl radical), di-tert-butylnitroxyl ((t)Bu(2)NO(*)), and the hydroxylamines TEMPO-H, 4-oxo-TEMPO-H, 4-MeO-TEMPO-H (2,2,6,6-tetramethyl-N-hydroxy-4-methoxy-piperidine), and (t)Bu(2)NOH. The reactions have been monitored by UV-vis stopped-flow methods, using the different optical spectra of the nitroxyl radicals. The HAT reactions all have |DeltaG (o)| < or = 1.4 kcal mol(-1) and therefore are close to self-exchange reactions. The reaction of 4-oxo-TEMPO(*) + TEMPO-H --> 4-oxo-TEMPO-H + TEMPO(*) occurs with k(2H,MeCN) = 10 +/- 1 M(-1) s(-1) in MeCN at 298 K (K(2H,MeCN) = 4.5 +/- 1.8). Surprisingly, the rate constant for the analogous deuterium atom transfer reaction is much slower: k(2D,MeCN) = 0.44 +/- 0.05 M(-1) s(-1) with k(2H,MeCN)/k(2D,MeCN) = 23 +/- 3 at 298 K. The same large kinetic isotope effect (KIE) is found in CH(2)Cl(2), 23 +/- 4, suggesting that the large KIE is not caused by solvent dynamics or hydrogen bonding to solvent. The related reaction of 4-oxo-TEMPO(*) with 4-MeO-TEMPO-H(D) also has a large KIE, k(3H)/k(3D) = 21 +/- 3 in MeCN. For these three reactions, the E(aD) - E(aH) values, between 0.3 +/- 0.6 and 1.3 +/- 0.6 kcal mol(-1), and the log(A(H)/A(D)) values, between 0.5 +/- 0.7 and 1.1 +/- 0.6, indicate that hydrogen tunneling plays an important role. The related reaction of (t)Bu(2)NO(*) + TEMPO-H(D) in MeCN has a large KIE, 16 +/- 3 in MeCN, and very unusual isotopic activation parameters, E(aD) - E(aH) = -2.6 +/- 0.4 and log(A(H)/A(D)) = 3.1 +/- 0.6. Computational studies, using POLYRATE, also indicate substantial tunneling in the (CH(3))(2)NO(*) + (CH(3))(2)NOH model reaction for the experimental self-exchange processes. Additional calculations on TEMPO((*)/H), (t)Bu(2)NO((*)/H), and Ph(2)NO((*)/H) self-exchange reactions reveal why the phenyl groups make the last of these reactions several orders of magnitude faster than the first two. By inference, the calculations also suggest why tunneling appears to be more important in the self-exchange reactions of dialkylhydroxylamines than of arylhydroxylamines.
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Affiliation(s)
- Adam Wu
- Department of Chemistry, Campus Box 351700, University of Washington, Seattle, Washington 98195-1700, USA
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Mishra R, Mukhopadhyay S, Banerjee R. Reaction of hydrogen peroxide with coordinated superoxide in [(NH3)5CoIII(micro-O2)CoIII(NH3)5]5+: a mechanistic study. Dalton Trans 2009:5469-73. [PMID: 19587989 DOI: 10.1039/b901280j] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
In aqueous acetate buffer media, hydrogen peroxide reduces the bridging superoxide in [(NH3)5CoIII(micro-O2)CoIII(NH3)5]5+ (1) to the corresponding peroxide in the complex, [(NH3)5CoIII(micro-O2H)CoIII(NH2)(NH3)4]4+ (2), itself being oxidized to HO2*. The complex 2 thus produced decomposes rapidly to the final products, CoII, NH3, etc. instead of reacting with a second molecule of hydrogen peroxide. In the presence of excess [H2O2] over (1), the reaction obeyed first-order kinetics and exhibited inverse proton dependence. [(NH3)5CoIII(micro-O2)CoIII((NH2)(NH3)4]4+ (3), a conjugate base of 1, seems to be the kinetically reactive species and the cause for the observed inverse proton dependence. Kinetics is little affected when one of the hydrogen atoms from hydrogen peroxide is replaced with an alkyl group, as in tert-butyl hydroperoxide. But replacement of both the H atoms with alkyl groups halts the reaction as seen with di-tert-butyl peroxides, and peroxodisulfate ion. The reaction rate with hydrogen peroxide significantly decreases with increasing proportion of D2O replacing water in the solvent and the rate-limiting step seems to be an H-atom transfer.
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Affiliation(s)
- Ritu Mishra
- Department of Chemistry, Jadavpur University, Kolkata, 700 032, India
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126
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Mohajeri A, Asemani SS. Theoretical investigation on antioxidant activity of vitamins and phenolic acids for designing a novel antioxidant. J Mol Struct 2009. [DOI: 10.1016/j.molstruc.2009.04.031] [Citation(s) in RCA: 60] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/24/2023]
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Viirlaid S, Mahlapuu R, Kilk K, Kuznetsov A, Soomets U, Järv J. Mechanism and stoichiometry of 2,2-diphenyl-1-picrylhydrazyl radical scavenging by glutathione and its novel alpha-glutamyl derivative. Bioorg Chem 2009; 37:126-32. [PMID: 19487009 DOI: 10.1016/j.bioorg.2009.05.001] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/17/2009] [Revised: 05/02/2009] [Accepted: 05/04/2009] [Indexed: 10/20/2022]
Abstract
Kinetic mechanism and stoichiometry of scavenging the 2,2-diphenyl-1-picrylhydrazyl radical by glutathione and its novel analog, containing alpha-glutamyl residue in place of the gamma-glutamyl moiety, were studied using different ratios of reagents. At low concentrations of the peptides, the process was described as a bimolecular reaction obeying the stoichiometric ratio 1:1. However, at excess of peptides the formation of a non-covalent complex between the reagents was discovered and characterized by dissociation constants K = 0.61 mM for glutathione and K = 0.27 mM for the glutathione alpha-glutamyl analog, respectively. The complex formation was followed by a reaction step that was characterized by the similar rate constant k = 0.02 s(-1) for both peptides. Thus, the apparently different antioxidant activity of these two peptides, observed under common assay conditions, was determined by differences in the formation of this non-covalent complex.
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Affiliation(s)
- Säde Viirlaid
- Institute of Chemistry, University of Tartu, 51014 Tartu, Estonia
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128
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Mader EA, Manner VW, Markle TF, Wu A, Franz JA, Mayer JM. Trends in ground-state entropies for transition metal based hydrogen atom transfer reactions. J Am Chem Soc 2009; 131:4335-45. [PMID: 19275235 PMCID: PMC2723939 DOI: 10.1021/ja8081846] [Citation(s) in RCA: 71] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Reported herein are thermochemical studies of hydrogen atom transfer (HAT) reactions involving transition metal H-atom donors M(II)LH and oxyl radicals. [Fe(II)(H(2)bip)(3)](2+), [Fe(II)(H(2)bim)(3)](2+), [Co(II)(H(2)bim)(3)](2+), and Ru(II)(acac)(2)(py-imH) [H(2)bip = 2,2'-bi-1,4,5,6-tetrahydropyrimidine, H(2)bim = 2,2'-bi-imidazoline, acac = 2,4-pentandionato, py-imH = 2-(2'-pyridyl)imidazole)] each react with TEMPO (2,2,6,6-tetramethyl-1-piperidinoxyl) or (t)Bu(3)PhO(*) (2,4,6-tri-tert-butylphenoxyl) to give the deprotonated, oxidized metal complex M(III)L and TEMPOH or (t)Bu(3)PhOH. Solution equilibrium measurements for the reaction of [Co(II)(H(2)bim)(3)](2+) with TEMPO show a large, negative ground-state entropy for hydrogen atom transfer, -41 +/- 2 cal mol(-1) K(-1). This is even more negative than the DeltaS(o)(HAT) = -30 +/- 2 cal mol(-1) K(-1) for the two iron complexes and the DeltaS(o)(HAT) for Ru(II)(acac)(2)(py-imH) + TEMPO, 4.9 +/- 1.1 cal mol(-1) K(-1), as reported earlier. Calorimetric measurements quantitatively confirm the enthalpy of reaction for [Fe(II)(H(2)bip)(3)](2+) + TEMPO, thus also confirming DeltaS(o)(HAT). Calorimetry on TEMPOH + (t)Bu(3)PhO(*) gives DeltaH(o)(HAT) = -11.2 +/- 0.5 kcal mol(-1) which matches the enthalpy predicted from the difference in literature solution BDEs. A brief evaluation of the literature thermochemistry of TEMPOH and (t)Bu(3)PhOH supports the common assumption that DeltaS(o)(HAT) approximately 0 for HAT reactions of organic and small gas-phase molecules. However, this assumption does not hold for transition metal based HAT reactions. The trend in magnitude of |DeltaS(o)(HAT)| for reactions with TEMPO, Ru(II)(acac)(2)(py-imH) << [Fe(II)(H(2)bip)(3)](2+) = [Fe(II)(H(2)bim)(3)](2+) < [Co(II)(H(2)bim)(3)](2+), is surprisingly well predicted by the trends for electron transfer half-reaction entropies, DeltaS(o)(ET), in aprotic solvents. This is because both DeltaS(o)(ET) and DeltaS(o)(HAT) have substantial contributions from vibrational entropy, which varies significantly with the metal center involved. The close connection between DeltaS(o)(HAT) and DeltaS(o)(ET) provides an important link between these two fields and provides a starting point from which to predict which HAT systems will have important ground-state entropy effects.
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Affiliation(s)
- Elizabeth A Mader
- Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195-1700, USA
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129
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Jena NR, Mishra PC, Suhai S. Protection Against Radiation-Induced DNA Damage by Amino Acids: A DFT Study. J Phys Chem B 2009; 113:5633-44. [DOI: 10.1021/jp810468m] [Citation(s) in RCA: 30] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Affiliation(s)
- N. R. Jena
- Division Molecular Biophysics (B020), Deutsches Krebsforschungszentrum (DKFZ), Im Neuenheimer Feld 580, D - 69120 Heidelberg, Germany, and Department of Physics, Banaras Hindu University, Varanasi-221005, India
| | - P. C. Mishra
- Division Molecular Biophysics (B020), Deutsches Krebsforschungszentrum (DKFZ), Im Neuenheimer Feld 580, D - 69120 Heidelberg, Germany, and Department of Physics, Banaras Hindu University, Varanasi-221005, India
| | - S. Suhai
- Division Molecular Biophysics (B020), Deutsches Krebsforschungszentrum (DKFZ), Im Neuenheimer Feld 580, D - 69120 Heidelberg, Germany, and Department of Physics, Banaras Hindu University, Varanasi-221005, India
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130
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Fukuzumi S. Roles of Metal Ions in Controlling Bioinspired Electron-Transfer Systems. Metal Ion-Coupled Electron Transfer. PROGRESS IN INORGANIC CHEMISTRY 2009. [DOI: 10.1002/9780470440124.ch2] [Citation(s) in RCA: 73] [Impact Index Per Article: 4.9] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/03/2022]
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131
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Wu A, Mayer JM. Hydrogen atom transfer reactions of a ruthenium imidazole complex: hydrogen tunneling and the applicability of the Marcus cross relation. J Am Chem Soc 2008; 130:14745-54. [PMID: 18841973 PMCID: PMC2633126 DOI: 10.1021/ja805067h] [Citation(s) in RCA: 54] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
The reaction of Ru(II)(acac)2(py-imH) (Ru(II)imH) with TEMPO(*) (2,2,6,6-tetramethylpiperidine-1-oxyl radical) in MeCN quantitatively gives Ru(III)(acac)2(py-im) (Ru(III)im) and the hydroxylamine TEMPO-H by transfer of H(*) (H(+) + e(-)) (acac = 2,4-pentanedionato, py-imH = 2-(2'-pyridyl)imidazole). Kinetic measurements of this reaction by UV-vis stopped-flow techniques indicate a bimolecular rate constant k(3H) = 1400 +/- 100 M(-1) s(-1) at 298 K. The reaction proceeds via a concerted hydrogen atom transfer (HAT) mechanism, as shown by ruling out the stepwise pathways of initial proton or electron transfer due to their very unfavorable thermochemistry (Delta G(o)). Deuterium transfer from Ru(II)(acac)2(py-imD) (Ru(II)imD) to TEMPO(*) is surprisingly much slower at k(3D) = 60 +/- 7 M(-1) s(-1), with k(3H)/k(3D) = 23 +/- 3 at 298 K. Temperature-dependent measurements of this deuterium kinetic isotope effect (KIE) show a large difference between the apparent activation energies, E(a3D) - E(a3H) = 1.9 +/- 0.8 kcal mol(-1). The large k(3H)/k(3D) and DeltaE(a) values appear to be greater than the semiclassical limits and thus suggest a tunneling mechanism. The self-exchange HAT reaction between Ru(II)imH and Ru(III)im, measured by (1)H NMR line broadening, occurs with k(4H) = (3.2 +/- 0.3) x 10(5) M(-1) s(-1) at 298 K and k(4H)/k(4D) = 1.5 +/- 0.2. Despite the small KIE, tunneling is suggested by the ratio of Arrhenius pre-exponential factors, log(A(4H)/A(4D)) = -0.5 +/- 0.3. These data provide a test of the applicability of the Marcus cross relation for H and D transfers, over a range of temperatures, for a reaction that involves substantial tunneling. The cross relation calculates rate constants for Ru(II)imH(D) + TEMPO(*) that are greater than those observed: k(3H,calc)/k(3H) = 31 +/- 4 and k(3D,calc)/k(3D) = 140 +/- 20 at 298 K. In these rate constants and in the activation parameters, there is a better agreement with the Marcus cross relation for H than for D transfer, despite the greater prevalence of tunneling for H. The cross relation does not explicitly include tunneling, so close agreement should not be expected. In light of these results, the strengths and weaknesses of applying the cross relation to HAT reactions are discussed.
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Affiliation(s)
- Adam Wu
- Department of Chemistry, University of Washington, Campus Box 351700, Seattle, WA, 98195-1700, USA
| | - James M. Mayer
- Department of Chemistry, University of Washington, Campus Box 351700, Seattle, WA, 98195-1700, USA
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132
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Yuasa J, Fukuzumi S. A mechanistic dichotomy in concertedversusstepwise pathways in hydride and hydrogen transfer reactions of NADH analogues. J PHYS ORG CHEM 2008. [DOI: 10.1002/poc.1367] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/05/2022]
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133
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Electrochemical kinetic analysis of a 1,4-hydroxynaphthoquinone self-assembled monolayer. J Electroanal Chem (Lausanne) 2008. [DOI: 10.1016/j.jelechem.2008.04.030] [Citation(s) in RCA: 36] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022]
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134
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Estévez L, Mosquera RA. Molecular structure and antioxidant properties of delphinidin. J Phys Chem A 2008; 112:10614-23. [PMID: 18821739 DOI: 10.1021/jp8043237] [Citation(s) in RCA: 52] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Density functional theory calculations were performed to evaluate the antioxidant activity of delphinidin, taking into account its acid/base equilibrium. The conformational behavior of both the isolated and the aqueous solvation species (simulated with the polarizable continuum model) were analyzed at the B3LYP/6-31++G(d,p) level, considering the cationic, neutral, and anionic forms, the latter two forms consisting of diverse tautomers. The analysis of their electron density distributions, using the quantum theory of atoms in molecules, reveals several facts that are not in line with their usual Lewis structures. The prototropic preferences observed in the gas phase and in solution are similar. Thus, in both phases, most stable tautomer of neutral delphinidin is obtained by deprotonating the hydroxyl at C4', and the most stable tautomer of the anion is obtained by deprotonating the hydroxyls at C4' and C5. All the planar conformers obtained display an intramolecular hydrogen bond (IHB) between O3 and H6'. Furthermore, the most stable tautomers of the neutral and anionic forms display two IHBs between O4' and H3' and H5'. To obtain ionization potentials (IPs) and homolytic O-H bond dissociation enthalpies (BDEs), the corresponding radical species were optimized at the UB3LYP level. Heterolytic O-H bond dissociation enthalpies (proton dissociation enthalpies, PDEs) were also computed. The expected important antioxidant activity can be justified from these results. IP, O-H BDE, and O-H PDE values suggest that one-step H atom transfer rather than sequential proton loss-electron transfer or electron transfer-proton transfer would be the most favored mechanisms for explaining the antioxidant activity of delphinidin in nonpolar solvents as well as in aqueous solution.
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Affiliation(s)
- Laura Estévez
- Departamento de Química Física, Facultade de Química, Universidade de Vigo, Lagoas-Marcosende s/n 36310-Vigo, Galicia, Spain
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135
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Temprado M, McDonough JE, Mendiratta A, Tsai YC, Fortman GC, Cummins CC, Rybak-Akimova EV, Hoff CD. Thermodynamic and kinetic studies of H atom transfer from HMo(CO)3(eta(5)-C5H5) to Mo(N[t-Bu]Ar)3 and (PhCN)Mo(N[t-Bu]Ar)3: direct insertion of benzonitrile into the Mo-H bond of HMo(N[t-Bu]Ar)3 forming (Ph(H)C=N)Mo(N[t-Bu]Ar)3. Inorg Chem 2008; 47:9380-9. [PMID: 18788794 DOI: 10.1021/ic800945m] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Synthetic studies are reported that show that the reaction of either H2SnR2 (R = Ph, n-Bu) or HMo(CO)3(Cp) (1-H, Cp = eta(5)-C5H5) with Mo(N[t-Bu]Ar)3 (2, Ar = 3,5-C6H3Me2) produce HMo(N[t-Bu]Ar)3 (2-H). The benzonitrile adduct (PhCN)Mo(N[t-Bu]Ar)3 (2-NCPh) reacts rapidly with H2SnR2 or 1-H to produce the ketimide complex (Ph(H)C=N)Mo(N[t-Bu]Ar)3 (2-NC(H)Ph). The X-ray crystal structures of both 2-H and 2-NC(H)Ph are reported. The enthalpy of reaction of 1-H and 2 in toluene solution has been measured by solution calorimetry (DeltaH = -13.1 +/- 0.7 kcal mol(-1)) and used to estimate the Mo-H bond dissociation enthalpy (BDE) in 2-H as 62 kcal mol(-1). The enthalpy of reaction of 1-H and 2-NCPh in toluene solution was determined calorimetrically as DeltaH = -35.1 +/- 2.1 kcal mol(-1). This value combined with the enthalpy of hydrogenation of [Mo(CO)3(Cp)]2 (1(2)) gives an estimated value of 90 kcal mol(-1) for the BDE of the ketimide C-H of 2-NC(H)Ph. These data led to the prediction that formation of 2-NC(H)Ph via nitrile insertion into 2-H would be exothermic by approximately 36 kcal mol(-1), and this reaction was observed experimentally. Stopped flow kinetic studies of the rapid reaction of 1-H with 2-NCPh yielded DeltaH(double dagger) = 11.9 +/- 0.4 kcal mol(-1), DeltaS(double dagger) = -2.7 +/- 1.2 cal K(-1) mol(-1). Corresponding studies with DMo(CO)3(Cp) (1-D) showed a normal kinetic isotope effect with kH/kD approximately 1.6, DeltaH(double dagger) = 13.1 +/- 0.4 kcal mol(-1) and DeltaS(double dagger) = 1.1 +/- 1.6 cal K(-1) mol(-1). Spectroscopic studies of the much slower reaction of 1-H and 2 yielding 2-H and 1/2 1(2) showed generation of variable amounts of a complex proposed to be (Ar[t-Bu]N)3Mo-Mo(CO)3(Cp) (1-2). Complex 1-2 can also be formed in small equilibrium amounts by direct reaction of excess 2 and 1(2). The presence of 1-2 complicates the kinetic picture; however, in the presence of excess 2, the second-order rate constant for H atom transfer from 1-H has been measured: 0.09 +/- 0.01 M(-1) s(-1) at 1.3 degrees C and 0.26 +/- 0.04 M(-1) s(-1) at 17 degrees C. Study of the rate of reaction of 1-D yielded kH/kD = 1.00 +/- 0.05 consistent with an early transition state in which formation of the adduct (Ar[t-Bu]N)3Mo...HMo(CO)3(Cp) is rate limiting.
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Affiliation(s)
- Manuel Temprado
- Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA
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136
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Abstract
The current understanding of the antioxidant properties of phenols (in homogeneous solutions) is reviewed, with particular emphasis on the role of the solvent. Phenols (ArOH) are known to reduce the rates of oxidation of organic matter by transferring a H atom (from their OH groups) to the chain-carrying ROO* radicals, a mechanism that most likely involves a concerted transfer of the hydrogen as a proton and of one electron between the two oxygen atoms, O-H---O* (proton-coupled electron transfer mechanism). The antioxidant capabilities of phenols are strongly reduced by hydrogen-bond accepting solvents since the hydrogen-bonded molecules ArOH---S are virtually unreactive toward ROO* radicals. The magnitude of these kinetic solvent effects is determined by the solute acidity alpha(2)(H) of ArOH (range 0 to 1) and solvent basicity beta(2)(H) (range 0 to 1). Hydroxyl solvents (alcohols) have a double effect on ArOH. On the one hand, they act as hydrogen-bond accepting solvents and reduce the conventional rates of the ArOH + ROO* reaction. On the other hand, these solvents favour the ionization of ArOH into their phenoxide anions ArO(-), which may react with ROO* very rapidly by electron transfer (sequential proton loss electron transfer mechanism). The overall effect is therefore determined by the ionization degree of ArOH. Other aspects of the kinetics and thermodynamics of ArOH + ROO* are also discussed.
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Affiliation(s)
- Mario C Foti
- Instituto di Chimica Biomolecolare del CNR, Via del Santuario, 110 Valverde (CT) 95028, Italy.
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137
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Hammarström L, Styring S. Coupled electron transfers in artificial photosynthesis. Philos Trans R Soc Lond B Biol Sci 2008; 363:1283-91; discussion 1291. [PMID: 17954432 DOI: 10.1098/rstb.2007.2225] [Citation(s) in RCA: 53] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
Light-induced charge separation in molecular assemblies has been widely investigated in the context of artificial photosynthesis. Important progress has been made in the fundamental understanding of electron and energy transfer and in stabilizing charge separation by multi-step electron transfer. In the Swedish Consortium for Artificial Photosynthesis, we build on principles from the natural enzyme photosystem II and Fe-hydrogenases. An important theme in this biomimetic effort is that of coupled electron-transfer reactions, which have so far received only little attention. (i) Each absorbed photon leads to charge separation on a single-electron level only, while catalytic water splitting and hydrogen production are multi-electron processes; thus there is the need for controlling accumulative electron transfer on molecular components. (ii) Water splitting and proton reduction at the potential catalysts necessarily require the management of proton release and/or uptake. Far from being just a stoichiometric requirement, this controls the electron transfer processes by proton-coupled electron transfer (PCET). (iii) Redox-active links between the photosensitizers and the catalysts are required to rectify the accumulative electron-transfer reactions, and will often be the starting points of PCET.
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Affiliation(s)
- Leif Hammarström
- Department of Photochemistry and Molecular Science, Uppsala University, PO Box 523, 751 20 Uppsala, Sweden.
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138
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Yuasa J, Yamada S, Fukuzumi S. One-step versus stepwise mechanism in protonated amino acid-promoted electron-transfer reduction of a quinone by electron donors and two-electron reduction by a dihydronicotinamide adenine dinucleotide analogue. Interplay between electron transfer and hydrogen bonding. J Am Chem Soc 2008; 130:5808-20. [PMID: 18386924 DOI: 10.1021/ja8001452] [Citation(s) in RCA: 55] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Semiquinone radical anion of 1-(p-tolylsulfinyl)-2,5-benzoquinone (TolSQ(*-)) forms a strong hydrogen bond with protonated histidine (TolSQ(*-)/His x 2 H(+)), which was successfully detected by electron spin resonance. Strong hydrogen bonding between TolSQ(*-) and His x 2 H(+) results in acceleration of electron transfer (ET) from ferrocenes [R2Fc, R = C5H5, C5H4(n-Bu), C5H4Me] to TolSQ, when the one-electron reduction potential of TolSQ is largely shifted to the positive direction in the presence of His x 2 H(+). The rates of His x 2 H(+)-promoted ET from R2Fc to TolSQ exhibit deuterium kinetic isotope effects due to partial dissociation of the N-H bond in His x 2 H(+) at the transition state, when His x 2 H(+) is replaced by the deuterated compound (His x 2 D(+)-d6). The observed deuterium kinetic isotope effect (kH/kD) decreases continuously with increasing the driving force of ET to approach kH/kD = 1.0. On the other hand, His x 2 H(+) also promotes a hydride reduction of TolSQ by an NADH analogue, 9,10-dihydro-10-methylacridine (AcrH2). The hydride reduction proceeds via the one-step hydride-transfer pathway. In such a case, a large deuterium kinetic isotope effect is observed in the rate of the hydride transfer, when AcrH2 is replaced by the dideuterated compound (AcrD2). In sharp contrast to this, no deuterium kinetic isotope effect is observed, when His x 2 H(+) is replaced by His x 2 D(+)-d6. On the other hand, direct protonation of TolSQ and 9,10-phenanthrenequinone (PQ) also results in efficient reductions of TolSQH(+) and PQH(+) by AcrH2, respectively. In this case, however, the hydride-transfer reactions occur via the ET pathway, that is, ET from AcrH2 to TolSQH(+) and PQH(+) occurs in preference to direct hydride transfer from AcrH2 to TolSQH(+) and PQH(+), respectively. The AcrH2(*+) produced by the ET oxidation of AcrH2 by TolSQH(+) and PQH(+) was directly detected by using a stopped-flow technique.
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Affiliation(s)
- Junpei Yuasa
- Department of Material and Life Science, Division of Advanced Science and Biotechnology, Graduate School of Engineering, Osaka University, SORST, Japan Science and Technology Agency, Suita, Osaka 565-0871, Japan
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139
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Lassalle-Kaiser B, Guillot R, Sainton J, Charlot MF, Aukauloo A. Proton-mediated redox control in a nickel(II)-bisimidazolate complex: spectroscopic characterisation and density functional analysis. Chemistry 2008; 14:4307-17. [PMID: 18351697 DOI: 10.1002/chem.200701662] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
The synthesis and characterisation of the pro-ligand LH4, in which L is the o-phenylenebisamide-2-imidazole and its nickel(II) complexes are reported. The X-ray structures of the fully protonated [NiLH2] and deprotonated [NiL] complexes are presented. The effects of the deprotonation of the imidazole functions on the electronic structure of the complexes are analysed by (1)H NMR, UV/Vis and IR spectroscopy and cyclic voltammetry. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations support the analysis based on experimental data. The singly oxidised form of the deprotonated complex [NiL] was generated by preparative electrolysis and its electronic structure was investigated. Spectroelectrochemistry shows the appearance of intense transitions in the region lambda = 600-900 nm with several isosbestic points. X-band EPR spectroscopy presents an isotropic signal at g = 2.03, whereas the Q-band EPR reveals a weak anisotropic signal characteristic of a metalloradical species. DFT and TDDFT data support the description of the species as a nickel(II)-radical form, with a major contribution of the spin density on the phenylene ring and the amidate functions with a negligible participation of the imidazolate groups. This finding is in sharp contrast with those obtained from the one-electron-oxidised form of nickel(II) complexes containing phenolate groups.
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Affiliation(s)
- Benedikt Lassalle-Kaiser
- Institut de Chimie Moléculaire et des Matériaux d'Orsay, UMR-CNRS 8182, Université de Paris-Sud, F-91405 Orsay, France
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140
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Liu F, Concepcion JJ, Jurss JW, Cardolaccia T, Templeton JL, Meyer TJ. Mechanisms of Water Oxidation from the Blue Dimer to Photosystem II. Inorg Chem 2008; 47:1727-52. [DOI: 10.1021/ic701249s] [Citation(s) in RCA: 352] [Impact Index Per Article: 22.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Affiliation(s)
- Feng Liu
- Department of Chemistry, University of North Carolina at Chapel Hill, CB 3290, Chapel Hill, North Carolina 27599-3290
| | - Javier J. Concepcion
- Department of Chemistry, University of North Carolina at Chapel Hill, CB 3290, Chapel Hill, North Carolina 27599-3290
| | - Jonah W. Jurss
- Department of Chemistry, University of North Carolina at Chapel Hill, CB 3290, Chapel Hill, North Carolina 27599-3290
| | - Thomas Cardolaccia
- Department of Chemistry, University of North Carolina at Chapel Hill, CB 3290, Chapel Hill, North Carolina 27599-3290
| | - Joseph L. Templeton
- Department of Chemistry, University of North Carolina at Chapel Hill, CB 3290, Chapel Hill, North Carolina 27599-3290
| | - Thomas J. Meyer
- Department of Chemistry, University of North Carolina at Chapel Hill, CB 3290, Chapel Hill, North Carolina 27599-3290
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141
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Wu A, Masland J, Swartz RD, Kaminsky W, Mayer JM. Synthesis and characterization of ruthenium bis(beta-diketonato) pyridine-imidazole complexes for hydrogen atom transfer. Inorg Chem 2007; 46:11190-201. [PMID: 18052056 PMCID: PMC2596074 DOI: 10.1021/ic7015726] [Citation(s) in RCA: 68] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Ruthenium bis(beta-diketonato) complexes have been prepared at both the RuII and RuIII oxidation levels and with protonated and deprotonated pyridine-imidazole ligands. RuII(acac)2(py-imH) (1), [RuIII(acac)2(py-imH)]OTf (2), RuIII(acac)2(py-im) (3), RuII(hfac)2(py-imH) (4), and [DBU-H][RuII(hfac)2(py-im)] (5) have been fully characterized, including X-ray crystal structures (acac = 2,4-pentanedionato, hfac = 1,1,1,5,5,5-hexafluoro-2,4-pentanedionato, py-imH = 2-(2'-pyridyl)imidazole, DBU = 1,8-diazabicyclo[5.4.0]undec-7-ene). For the acac-imidazole complexes 1 and 2, cyclic voltammetry in MeCN shows the RuIII/II reduction potential (E1/2) to be -0.64 V versus Cp2Fe+/0. E1/2 for the deprotonated imidazolate complex 3 (-1.00 V) is 0.36 V more negative. The RuII bis-hfac analogues 4 and 5 show the same DeltaE1/2 = 0.36 V but are 0.93 V harder to oxidize than the acac derivatives (0.29 and -0.07 V). The difference in acidity between the acac and hfac derivatives is much smaller, with pKa values of 22.1 and 19.3 in MeCN for 1 and 4, respectively. From the E1/2 and pKa values, the bond dissociation free energies (BDFEs) of the N-H bonds in 1 and 4 are calculated to be 62.0 and 79.6 kcal mol(-1) in MeCN - a remarkable difference of 17.6 kcal mol(-1) for such structurally similar compounds. Consistent with these values, there is a facile net hydrogen atom transfer from 1 to TEMPO* (2,2,6,6-tetramethylpiperidine-1-oxyl radical) to give 3 and TEMPO-H. The DeltaG degrees for this reaction is -4.5 kcal mol(-1). 4 is not oxidized by TEMPO* (DeltaG degrees = +13.1 kcal mol(-1)), but in the reverse direction TEMPO-H readily reduces in situ generated RuIII(hfac)2(py-im) (6). A RuII-imidazoline analogue of 1, RuII(acac)2(py-imnH) (7), reacts with 3 equiv of TEMPO* to give the imidazolate 3 and TEMPO-H, with dehydrogenation of the imidazoline ring.
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Affiliation(s)
- Adam Wu
- Department of Chemistry, University of Washington, Campus Box 351700, Seattle, WA, 98195-1700, USA
| | - Joshua Masland
- Department of Chemistry, University of Washington, Campus Box 351700, Seattle, WA, 98195-1700, USA
| | | | | | - James M. Mayer
- Department of Chemistry, University of Washington, Campus Box 351700, Seattle, WA, 98195-1700, USA
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142
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Seyedsayamdost MR, Xie J, Chan CTY, Schultz PG, Stubbe J. Site-Specific Insertion of 3-Aminotyrosine into Subunit α2 of E. coli Ribonucleotide Reductase: Direct Evidence for Involvement of Y730 and Y731 in Radical Propagation. J Am Chem Soc 2007; 129:15060-71. [DOI: 10.1021/ja076043y] [Citation(s) in RCA: 117] [Impact Index Per Article: 6.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Mohammad R. Seyedsayamdost
- Contribution from the Department of Chemistry and Biology, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139-4307, and Department of Chemistry and the Skaggs Institute for Chemical Biology, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037
| | - Jianming Xie
- Contribution from the Department of Chemistry and Biology, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139-4307, and Department of Chemistry and the Skaggs Institute for Chemical Biology, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037
| | - Clement T. Y. Chan
- Contribution from the Department of Chemistry and Biology, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139-4307, and Department of Chemistry and the Skaggs Institute for Chemical Biology, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037
| | - Peter G. Schultz
- Contribution from the Department of Chemistry and Biology, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139-4307, and Department of Chemistry and the Skaggs Institute for Chemical Biology, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037
| | - JoAnne Stubbe
- Contribution from the Department of Chemistry and Biology, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139-4307, and Department of Chemistry and the Skaggs Institute for Chemical Biology, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037
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143
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Tan L, Zhang XF, Yan BZ, Shi HM, Du LB, Zhang YZ, Wang LF, Tang YL, Liu Y. A novel flavonoid from Lespedeza virgata (Thunb.) DC.: Structural elucidation and antioxidative activity. Bioorg Med Chem Lett 2007; 17:6311-5. [PMID: 17890083 DOI: 10.1016/j.bmcl.2007.09.003] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/10/2007] [Revised: 08/13/2007] [Accepted: 09/03/2007] [Indexed: 10/22/2022]
Abstract
Bioactivity guided fractionation of the ethanolic extract of the whole plants of Lespedeza virgata (Thunb.) DC. resulted in the isolation of a novel flavonoid (1) along with five known compounds 2-6. The molecular and structural formula as well as the stereochemistry of compound 1 were determined using data obtained from (1)H and (13)C NMR spectra, DEPT135 and by 2D HSQC, HMBC, (1)H-(1)H correlated spectroscopy ((1)H-(1)H COSY), and nuclear overhauser effect spectroscopy (NOESY) experiments. The superoxide anion scavenging activities of all isolated compounds were evaluated by the hypoxanthine nitro blue tetrazolium and ESR methods and the new compound 1 showed the strongest antioxidative activity 95.79% (IC(50)=0.14 mg/ml).
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Affiliation(s)
- Li Tan
- State Key Laboratory for SCUSS, Institute of Chemistry, The Chinese Academy of Sciences, Beijing, PR China
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144
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Affiliation(s)
- My Hang V Huynh
- DE-1: High Explosive Science and Technology Group, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
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145
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Klein E, Lukes V. DFT/B3LYP study of the substituent effect on the reaction enthalpies of the individual steps of single electron transfer-proton transfer and sequential proton loss electron transfer mechanisms of phenols antioxidant action. J Phys Chem A 2007; 110:12312-20. [PMID: 17078630 DOI: 10.1021/jp063468i] [Citation(s) in RCA: 93] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
The reaction enthalpies related to the individual steps of two phenolic antioxidants action mechanisms, single electron transfer-proton transfer (SET-PT) and sequential proton loss electron transfer (SPLET), for 30 meta and para-substituted phenols (ArOH) were calculated using DFT/B3LYP method. These mechanisms represent the alternative ways to the extensively studied hydrogen atom transfer (HAT) mechanism. Except the comparison of calculated reaction enthalpies with available experimental and/or theoretical values, obtained enthalpies were correlated with Hammett constants. We have found that electron-donating substituents induce the rise in the enthalpy of proton dissociation (PDE) from ArOH+* radical cation (second step in SET-PT) and in the proton affinities of phenoxide ions ArO- (reaction enthalpy of the first step in SPLET). Electron-withdrawing groups cause the increase in the reaction enthalpies of the processes where electron is abstracted, i.e., in the ionization potentials of ArOH (first step in SET-PT) and in the enthalpy of electron transfer from ArO- (second step in SPLET). Found results indicate that all dependences of reaction enthalpies on Hammett constants of the substituents are linear. The calculations of liquid-phase reaction enthalpies for several para-substituted phenols indicate that found trends hold also in water, although substituent effects are weaker. From the thermodynamic point of view, entering SPLET mechanism represents the most probable process in water.
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Affiliation(s)
- Erik Klein
- Institute of Physical Chemistry and Chemical Physics, Slovak University of Technology, Radlinského 9, SK-812 37 Bratislava, Slovak Republic.
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146
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Vuina D, Pilepić V, Ljubas D, Sanković K, Sajenko I, Uršić S. PCET in the oxidation of ascorbate. Dramatic change of the kinetic isotope effect on the change in solvent polarity. Tetrahedron Lett 2007. [DOI: 10.1016/j.tetlet.2007.03.140] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/23/2022]
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147
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Mader EA, Davidson ER, Mayer JM. Large ground-state entropy changes for hydrogen atom transfer reactions of iron complexes. J Am Chem Soc 2007; 129:5153-66. [PMID: 17402735 PMCID: PMC2628630 DOI: 10.1021/ja0686918] [Citation(s) in RCA: 116] [Impact Index Per Article: 6.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Reported herein are the hydrogen atom transfer (HAT) reactions of two closely related dicationic iron tris(alpha-diimine) complexes. FeII(H2bip) (iron(II) tris[2,2'-bi-1,4,5,6-tetrahydropyrimidine]diperchlorate) and FeII(H2bim) (iron(II) tris[2,2'-bi-2-imidazoline]diperchlorate) both transfer H* to TEMPO (2,2,6,6-tetramethyl-1-piperidinoxyl) to yield the hydroxylamine, TEMPO-H, and the respective deprotonated iron(III) species, FeIII(Hbip) or FeIII(Hbim). The ground-state thermodynamic parameters in MeCN were determined for both systems using both static and kinetic measurements. For FeII(H2bip) + TEMPO, DeltaG degrees = -0.3 +/- 0.2 kcal mol-1, DeltaH degrees = -9.4 +/- 0.6 kcal mol-1, and DeltaS degrees = -30 +/- 2 cal mol-1 K-1. For FeII(H2bim) + TEMPO, DeltaG degrees = 5.0 +/- 0.2 kcal mol-1, DeltaH degrees = -4.1 +/- 0.9 kcal mol-1, and DeltaS degrees = -30 +/- 3 cal mol-1 K-1. The large entropy changes for these reactions, |TDeltaS degrees | = 9 kcal mol-1 at 298 K, are exceptions to the traditional assumption that DeltaS degrees approximately 0 for simple HAT reactions. Various studies indicate that hydrogen bonding, solvent effects, ion pairing, and iron spin equilibria do not make major contributions to the observed DeltaS degrees HAT. Instead, this effect arises primarily from changes in vibrational entropy upon oxidation of the iron center. Measurement of the electron-transfer half-reaction entropy, |DeltaS degrees Fe(H2bim)/ET| = 29 +/- 3 cal mol-1 K-1, is consistent with a vibrational origin. This conclusion is supported by UHF/6-31G* calculations on the simplified reaction [FeII(H2N=CHCH=NH2)2(H2bim)]2+...ONH2 left arrow over right arrow [FeII(H2N=CHCH=NH2)2(Hbim)]2+...HONH2. The discovery that DeltaS degrees HAT can deviate significantly from zero has important implications on the study of HAT and proton-coupled electron-transfer (PCET) reactions. For instance, these results indicate that free energies, rather than enthalpies, should be used to estimate the driving force for HAT when transition-metal centers are involved.
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Affiliation(s)
- Elizabeth A. Mader
- University of Washington, Campus Box 351700, Seattle, WA, 98195-1700, USA, E-mail:
| | - Ernest R. Davidson
- University of Washington, Campus Box 351700, Seattle, WA, 98195-1700, USA, E-mail:
| | - James M. Mayer
- University of Washington, Campus Box 351700, Seattle, WA, 98195-1700, USA, E-mail:
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148
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Costentin C, Robert M, Savéant JM. Concerted Proton−Electron Transfer Reactions in Water. Are the Driving Force and Rate Constant Depending on pH When Water Acts as Proton Donor or Acceptor? J Am Chem Soc 2007; 129:5870-9. [PMID: 17428051 DOI: 10.1021/ja067950q] [Citation(s) in RCA: 91] [Impact Index Per Article: 5.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The competition between stepwise and concerted (CPET) pathways in proton-coupled electron-transfer reactions in water is discussed on thermodynamic and kinetic bases. In the case where water is the proton acceptor, the CPET pathway may compete favorably with the stepwise pathway. The main parameter of the competition is pK of the oxidized form of the substrate being smaller or larger than 0. The driving force of the forward reaction is however independent of pH, despite the equilibrium redox potential of the proton-electron system being a function of pH. At high pH values, CPET reactions involving OH- as proton acceptor may likewise compete favorably with stepwise pathways. The overall reaction rate constant is an increasing function of pH, not because the driving force depends on pH but because OH- is a reactant. In buffered media, association of the substrate with the basic components of the buffer offers an alternative CPET route; the driving force comes closer to that offered by the pH-dependent equilibrium redox potential.
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Affiliation(s)
- Cyrille Costentin
- Laboratoire d'Electrochimie Moléculaire, Unité Mixte de Recherche Université, CNRS No 7591, Université de Paris 7-Denis Diderot, 2 place Jussieu, 75251 Paris Cedex 05, France
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149
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DFT/B3LYP study of the substituent effect on the reaction enthalpies of the individual steps of sequential proton loss electron transfer mechanism of phenols antioxidant action: Correlation with phenolic CO bond length. ACTA ACUST UNITED AC 2007. [DOI: 10.1016/j.theochem.2006.11.002] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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150
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Nehru K, Seo MS, Kim J, Nam W. Oxidative N-Dealkylation Reactions by Oxoiron(IV) Complexes of Nonheme and Heme Ligands. Inorg Chem 2006; 46:293-8. [PMID: 17198439 DOI: 10.1021/ic0614014] [Citation(s) in RCA: 91] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Nonheme and heme iron monooxygenases participate in oxidative N-dealkylation reactions in nature, and high-valent oxoiron(IV) species have been invoked as active oxidants that effect the oxygenation of organic substrates. The present study describes the first example of the oxidative N-dealkylation of N,N-dialkylamines by synthetic nonheme oxoiron(IV) complexes and the reactivity comparisons of nonheme and heme oxoiron(IV) complexes. Detailed mechanistic studies were performed with various N,N-dialkylaniline substrates such as para-substituted N,N-dimethylanilines, para-chloro-N-ethyl-N-methylaniline, para-chloro-N-cyclopropyl-N-isopropylaniline, and deuteriated N,N-dimethylanilines. The results of a linear free-energy correlation, inter- and intramolecular kinetic isotope effects, and product analysis studied with the mechanistic probes demonstrate that the oxidative N-dealkylation reactions by nonheme and heme oxoiron(IV) complexes occur via an electron transfer-proton transfer (ET-PT) mechanism.
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Affiliation(s)
- Kasi Nehru
- Department of Chemistry, Division of Nano Sciences, and Center for Biomimetic Systems, Ewha Womans University, Seoul 120-750, Korea
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