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Hussein AA, Jafar NNA, Ma Y. Elucidating the Mechanism of Tetrahydrofuran-Diol Formation through Os(VI)-Catalyzed Oxidative Cyclization of 5,6-Dihydroxyalkenes Ligated by Citric Acid. J Org Chem 2024; 89:6892-6902. [PMID: 38701335 DOI: 10.1021/acs.joc.4c00268] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 05/05/2024]
Abstract
A computational study is reported here on the mechanism of tetrahydrofuran (THF)-diol formation from the Os(VI)-catalyzed oxidative cyclization of 5,6-dihydroxyalkene ligated with citric acid and in the presence of Bro̷nsted acid. Initiated by Os(VI) dioxo citrate formation, coordination of co-oxidant pyridine-N-oxide (PNO) and protonation of its oxo group generate the active catalyst. The catalytic cycle commences through successive steps, including dihydroxyalkene addition to the active catalyst in a concerted mechanism to form hexacoordinated alkoxy-protonated PNO-complexed Os(VI) bisglycolate as a turnover-limiting step (TLS), cyclization to Os(IV) THF-diolate, reoxidation to Os(VI) THF-diolate, and hydrolysis via a dissociative mechanism to furnish the THF-diol and regenerate the active species, sustaining the catalytic cycle through an Os(VI)/Os(IV) cycle. Despite the overall exergonic nature of catalytic cycle (ΔGrcycle = -45.0 kcal/mol), the TLS is accelerated by the formation of an open-valence 16-electron Os(VI) intermediate but decelerated by the undesired formation of a saturated/hexacoordinate 18-electron Os(VI) intermediate. Bro̷nsted acid plays crucial roles in the formation of Os(VI) citrate and the active catalyst, impediment of the second cycle, and the cyclization step. Additionally, besides its role as a co-oxidant, and in the presence of acid, PNO is found to assist the insertion of dihydroxyalkene and, importantly, in releasing the THF-diol to regenerate the active intermediate.
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Affiliation(s)
- Aqeel A Hussein
- Department of Biology, College of Science, Al-Qasim Green University, Al-Qasim, Babylon 51013, Iraq
- Al-Zahraa Center for Medical and Pharmaceutical Research Sciences (ZCMRS), Al-Zahraa University for Women, Karbala 56001, Iraq
| | - Nadhir N A Jafar
- Al-Zahraa Center for Medical and Pharmaceutical Research Sciences (ZCMRS), Al-Zahraa University for Women, Karbala 56001, Iraq
| | - Yumiao Ma
- BSJ Institute, Beijing 100084, People's Republic of China
- Beijing Orienda Instrument Co. Ltd., Beijing 102200, People's Republic of China
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2
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Kirk ML, Lepluart J, Yang J. Resonance Raman spectroscopy of pyranopterin molybdenum enzymes. J Inorg Biochem 2022; 235:111907. [PMID: 35932756 PMCID: PMC10575615 DOI: 10.1016/j.jinorgbio.2022.111907] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/30/2022] [Revised: 05/16/2022] [Accepted: 06/16/2022] [Indexed: 10/17/2022]
Abstract
Resonance Raman spectroscopy (rR) is a powerful spectroscopic probe that is widely used for studying the geometric and electronic structure of metalloproteins. In this focused review, we detail how resonance Raman spectroscopy has contributed to a greater understanding of electronic structure, geometric structure, and the reaction mechanisms of pyranopterin molybdenum enzymes. The review focuses on the enzymes sulfite oxidase (SO), dimethyl sulfoxide reductase (DMSOR), xanthine oxidase (XO), and carbon monoxide dehydrogenase. Specifically, we highlight how Mo-Ooxo, Mo-Ssulfido, Mo-Sdithiolene, and dithiolene CC vibrational modes, isotope and heavy atom perturbations, resonance enhancement, and associated Raman studies of small molecule analogs have provided detailed insight into the nature of these metalloenzyme active sites.
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Affiliation(s)
- Martin L Kirk
- Department of Chemistry and Chemical Biology, The University of New Mexico, MSC03 2060, 1 University of New Mexico, Albuquerque, NM 87131-0001, United States.
| | - Jesse Lepluart
- Department of Chemistry and Chemical Biology, The University of New Mexico, MSC03 2060, 1 University of New Mexico, Albuquerque, NM 87131-0001, United States
| | - Jing Yang
- Department of Chemistry and Chemical Biology, The University of New Mexico, MSC03 2060, 1 University of New Mexico, Albuquerque, NM 87131-0001, United States
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3
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Ha Y, Dille SA, Braun A, Colston K, Hedman B, Hodgson KO, Basu P, Solomon EI. S K-edge XAS of Cu II, Cu I, and Zn II oxidized Dithiolene complexes: Covalent contributions to structure and the Jahn-Teller effect. J Inorg Biochem 2022; 230:111752. [PMID: 35202982 PMCID: PMC9680909 DOI: 10.1016/j.jinorgbio.2022.111752] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/02/2021] [Revised: 02/01/2022] [Accepted: 02/03/2022] [Indexed: 11/26/2022]
Abstract
Reduced dithiolene ligands are bound to high valent Mo centers in the active site of the oxotransferase family of enzymes. Related model complexes have been studied with great insight by Prof. Holm and his colleagues. This study focuses on the other limit of dithiolene chemistry: an investigation of the 2-electron oxidized dithiolene bound to low-valent late transition metal (TM) ions (ZnII, CuI, and CuII). The bonding descriptions of the oxidized dithiolene [N,N-dimethyl piperazine 2,3-dithione (Me2Dt0)] complexes are probed using S K-edge X-ray absorption spectroscopy (XAS) and the results are correlated to density functional theory (DFT) calculations. These experimentally supported calculations are then extended to explain the different geometric structures of the three complexes. The ZnII(Me2Dt0)2 complex has only ligand-ligand repulsion so it is stabilized at the D2d symmetry limit. The CuI(Me2Dt0)2 complex has additional weak backbonding thus distorts somewhat from D2d toward D2h symmetry. The CuII(Me2Dt0)2 complex has a strong σ donor bond that leads to both a large Jahn-Teller stabilization to D2h and an additional covalent contribution to the geometry. The combined strong stabilization results in the square planar, D2h structure. This study quantifies the competition between the ligand-ligand repulsion and the change in electronic structures in determining the final geometric structures of the oxidized dithiolene complexes, and provides quantitative insights into the Jahn-Teller stabilization energy and its origin.
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Affiliation(s)
- Yang Ha
- Department of Chemistry, Stanford University, Stanford, CA 94035, United States; Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Stanford University, Menlo Park, CA 94025, United States; Advanced Light Source, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, United States
| | - Sara A Dille
- Department of Chemistry and Chemical Biology, Indiana University-Purdue University Indianapolis, 402 N Blackford St, Indianapolis, IN 46202, United States
| | - Augustin Braun
- Department of Chemistry, Stanford University, Stanford, CA 94035, United States; Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Stanford University, Menlo Park, CA 94025, United States
| | - Kyle Colston
- Department of Chemistry and Chemical Biology, Indiana University-Purdue University Indianapolis, 402 N Blackford St, Indianapolis, IN 46202, United States
| | - Britt Hedman
- Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Stanford University, Menlo Park, CA 94025, United States
| | - Keith O Hodgson
- Department of Chemistry, Stanford University, Stanford, CA 94035, United States; Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Stanford University, Menlo Park, CA 94025, United States
| | - Partha Basu
- Department of Chemistry and Chemical Biology, Indiana University-Purdue University Indianapolis, 402 N Blackford St, Indianapolis, IN 46202, United States
| | - Edward I Solomon
- Department of Chemistry, Stanford University, Stanford, CA 94035, United States; Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Stanford University, Menlo Park, CA 94025, United States.
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Le CC, Bae M, Kiamehr S, Balskus EP. Emerging Chemical Diversity and Potential Applications of Enzymes in the DMSO Reductase Superfamily. Annu Rev Biochem 2022; 91:475-504. [PMID: 35320685 DOI: 10.1146/annurev-biochem-032620-110804] [Citation(s) in RCA: 4] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
Molybdenum- and tungsten-dependent proteins catalyze essential processes in living organisms and biogeochemical cycles. Among these enzymes, members of the dimethyl sulfoxide (DMSO) reductase superfamily are considered the most diverse, facilitating a wide range of chemical transformations that can be categorized as oxygen atom installation, removal, and transfer. Importantly, DMSO reductase enzymes provide high efficiency and excellent selectivity while operating under mild conditions without conventional oxidants such as oxygen or peroxides. Despite the potential utility of these enzymes as biocatalysts, such applications have not been fully explored. In addition, the vast majority of DMSO reductase enzymes still remain uncharacterized. In this review, we describe the reactivities, proposed mechanisms, and potential synthetic applications of selected enzymes in the DMSO reductase superfamily. We also highlight emerging opportunities to discover new chemical activity and current challenges in studying and engineering proteins in the DMSO reductase superfamily. Expected final online publication date for the Annual Review of Biochemistry, Volume 91 is June 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
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Affiliation(s)
- Chi Chip Le
- Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts, USA;
| | - Minwoo Bae
- Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts, USA;
| | - Sina Kiamehr
- Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts, USA;
| | - Emily P Balskus
- Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts, USA;
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Hussein AA, Ma Y, Moustafa GAI. Predominance of the second cycle in homogeneous Os-catalyzed dihydroxylation: the nature of Os( vi) → Os( viii) reoxidation and unprecedented roles of amine- N-oxides. Catal Sci Technol 2022. [DOI: 10.1039/d1cy02107a] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/12/2022]
Abstract
Our detailed DFT study of Os-catalyzed alkene dihydroxylation revealed that the reaction predominantly proceeds via a second cycle initiated by the formation of a putative Os(viii)trioxoglycolate as a highly reactive intermediate.
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Affiliation(s)
- Aqeel A. Hussein
- Department of Pharmacy, College of Medicine, Komar University of Science and Technology, Sulaymaniyah, Kurdistan Region, Iraq
| | - Yumiao Ma
- BSJ Institue, Haidian, Beijing, 100084, People's Republic of China
- Hangzhou Yanqu Information Technology Co., Ltd., Xixi Legu Creative Pioneering Park, No. 712 Wen'er West Road, Xihu District, Hangzhou City, Zhejiang Province, 310003, People's Republic of China
| | - Gamal A. I. Moustafa
- Department of Medicinal Chemistry, Faculty of Pharmacy, Minia University, Minia 61519, Egypt
- Department of Chemistry, University of Southampton, Southampton, Hampshire, SO17 1BJ, UK
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6
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Hussein AA. Ru-catalysed oxidative cyclisation of 1,5-dienes: an unprecedented role for the co-oxidant. RSC Adv 2020; 10:15228-15238. [PMID: 35495449 PMCID: PMC9052297 DOI: 10.1039/d0ra02303e] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/11/2020] [Accepted: 03/30/2020] [Indexed: 11/21/2022] Open
Abstract
The Ru-mediated oxidative cyclisation of 1,5-dienes to furnish 2,5-dihydroxyalkyl-substituted tetrahydrofuran-diols (THF-diols) represents a practical approach for the synthesis of many bioactive natural products. In the current study, we reported profound findings obtained by density functional theory (DFT) simulations, and they were consistent with the experimental conditions. The results set out a catalytic cycle within intermediacy of NaIO4-complexed Ru(vi) species. Importantly, the co-oxidant played a critical role in the cyclisation step and subsequently the release of THF-diols. Following the formation of Ru(vi) glycolate, cyclisation and THF-diol release proceeded through NaIO4-coordinated Ru(vi) intermediates, outpacing the Ru(viii) glycolate or THF-diolate intermediates and subsequently entering “second cycle” type pathways. The results indicated a cycle involving Ru(viii)/Ru(vi)/Ru(iv)/Ru(vi) rather than Ru(viii)/Ru(vi)/Ru(viii)/Ru(vi)/Ru(viii). Additionally, the existence of an electron-withdrawing group (EWG) on one of the double bonds of 1,5-dienes revealed that the regioselectivity of the Ru-catalysed oxidative cyclisation was predominantly initiated at the electron-rich alkene. Overall, this study offers new insights, which were ignored by earlier experimentalists and theoreticians, into the Ru-catalysed functionalizations of alkenes and 1,5-dienes. The Ru-mediated oxidative cyclisation of 1,5-dienes to THF-diols proceeds with the intermediacy of NaIO4-complexed Ru(vi) species and offers new insights into the Ru-catalysed functionalizations of alkenes and 1,5-dienes.![]()
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Affiliation(s)
- Aqeel A Hussein
- School of Chemistry, University of Southampton Southampton Hampshire SO17 1BJ UK .,College of Dentistry, University of Al-Ameed PO Box 198 Karbala Iraq
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7
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Kirk ML, Kc K. Molybdenum and Tungsten Cofactors and the Reactions They Catalyze. Met Ions Life Sci 2020; 20:/books/9783110589757/9783110589757-015/9783110589757-015.xml. [PMID: 32851830 PMCID: PMC8176780 DOI: 10.1515/9783110589757-015] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2023]
Abstract
The last 20 years have seen a dramatic increase in our mechanistic understanding of the reactions catalyzed by pyranopterin Mo and W enzymes. These enzymes possess a unique cofactor (Moco) that contains a novel ligand in bioinorganic chemistry, the pyranopterin ene-1,2-dithiolate. A synopsis of Moco biosynthesis and structure is presented, along with our current understanding of the role Moco plays in enzymatic catalysis. Oxygen atom transfer (OAT) reactivity is discussed in terms of breaking strong metal-oxo bonds and the mechanism of OAT catalyzed by enzymes of the sulfite oxidase (SO) family that possess dioxo Mo(VI) active sites. OAT reactivity is also discussed in members of the dimethyl sulfoxide (DMSO) reductase family, which possess des-oxo Mo(IV) sites. Finally, we reveal what is known about hydride transfer reactivity in xanthine oxidase (XO) family enzymes and the formate dehydrogenases. The formal hydride transfer reactivity catalyzed by xanthine oxidase family enzymes is complex and cleaves substrate C-H bonds using a mechanism that is distinct from monooxygenases. The chapter primarily highlights developments in the field that have occurred since ~2000, which have contributed to our collective structural and mechanistic understanding of the three canonical pyranopterin Mo enzymes families: XO, SO, and DMSO reductase.
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Duffus BR, Schrapers P, Schuth N, Mebs S, Dau H, Leimkühler S, Haumann M. Anion Binding and Oxidative Modification at the Molybdenum Cofactor of Formate Dehydrogenase from Rhodobacter capsulatus Studied by X-ray Absorption Spectroscopy. Inorg Chem 2019; 59:214-225. [PMID: 31814403 DOI: 10.1021/acs.inorgchem.9b01613] [Citation(s) in RCA: 15] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Formate dehydrogenase (FDH) enzymes are versatile catalysts for CO2 conversion. The FDH from Rhodobacter capsulatus contains a molybdenum cofactor with the dithiolene functions of two pyranopterin guanine dinucleotide molecules, a conserved cysteine, and a sulfido group bound at Mo(VI). In this study, we focused on metal oxidation state and coordination changes in response to exposure to O2, inhibitory anions, and redox agents using X-ray absorption spectroscopy (XAS) at the Mo K-edge. Differences in the oxidative modification of the bis-molybdopterin guanine dinucleotide (bis-MGD) cofactor relative to samples prepared aerobically without inhibitor, such as variations in the relative numbers of sulfido (Mo═S) and oxo (Mo═O) bonds, were observed in the presence of azide (N3-) or cyanate (OCN-). Azide provided best protection against O2, resulting in a quantitatively sulfurated cofactor with a displaced cysteine ligand and optimized formate oxidation activity. Replacement of the cysteine ligand by a formate (HCO2-) ligand at the molybdenum in active enzyme is compatible with our XAS data. Cyanide (CN-) inactivated the enzyme by replacing the sulfido ligand at Mo(VI) with an oxo ligand. Evidence that the sulfido group may become protonated upon molybdenum reduction was obtained. Our results emphasize the role of coordination flexibility at the molybdenum center during inhibitory and catalytic processes of FDH enzymes.
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Affiliation(s)
- Benjamin R Duffus
- Institut für Biochemie und Biologie, Molekulare Enzymologie , Universität Potsdam , Karl-Liebknecht Strasse 24-25 , 14476 Potsdam , Germany
| | - Peer Schrapers
- Institut für Experimentalphysik , Freie Universität Berlin , Arnimallee 14 , 14195 Berlin , Germany
| | - Nils Schuth
- Institut für Experimentalphysik , Freie Universität Berlin , Arnimallee 14 , 14195 Berlin , Germany
| | - Stefan Mebs
- Institut für Experimentalphysik , Freie Universität Berlin , Arnimallee 14 , 14195 Berlin , Germany
| | - Holger Dau
- Institut für Experimentalphysik , Freie Universität Berlin , Arnimallee 14 , 14195 Berlin , Germany
| | - Silke Leimkühler
- Institut für Biochemie und Biologie, Molekulare Enzymologie , Universität Potsdam , Karl-Liebknecht Strasse 24-25 , 14476 Potsdam , Germany
| | - Michael Haumann
- Institut für Experimentalphysik , Freie Universität Berlin , Arnimallee 14 , 14195 Berlin , Germany
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9
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Hussein AA, Phipps MJS, Skylaris CK, Brown RCD. Mechanism of Os-Catalyzed Oxidative Cyclization of 1,5-Dienes. J Org Chem 2019; 84:15173-15183. [PMID: 31664829 DOI: 10.1021/acs.joc.9b02174] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/12/2023]
Abstract
The oxidative cyclization of 1,5-dienes by metal-oxo species is a powerful method for stereocontrolled synthesis of tetrahydrofuran diols (THF-diols), structural motifs present in many bioactive natural products. Oxidative cyclization of (2E,6E)-octa-2,6-diene catalyzed by OsO4/NMO has been studied using density functional theory (DFT) calculations (M06-2X/aug-cc-pVDZ/Hay-Wadt VDZ (n+1) ECP), highlighting the remarkable effect of acid on the fate of the first intermediate, an Os(VI) dioxoglycolate. A strong acid promotes cyclization of the Os(VI) dioxoglycolate, or its NMO complex, through protonation of an oxo ligand to give more electrophilic species. By contrast, in the absence of acid, reoxidation may occur to afford the Os(VIII) trioxoglycolate, which is shown to favor conventional "second cycle" dihydroxylation reactivity rather than cyclization. The results of the calculations are consistent with experimental results for reactions of OsO4/NMO with 1,5-dienes with acid (oxidative cyclization) and without acid (second cycle osmylation/dihydroxylation). Detailed evaluation of potential catalytic cycles supports oxidation of the cyclized Os(IV) THF-diolate intermediate to the corresponding Os(VI) species followed by slow hydrolysis and, finally, regeneration of OsO4.
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Affiliation(s)
- Aqeel A Hussein
- Department of Chemistry , University of Southampton , Southampton , Hampshire SO17 1BJ , U.K.,Faculty of Dentistry , University of Al-Ameed , Karbala , P.O. Box No. 198, Iraq
| | - Maximillian J S Phipps
- Department of Chemistry , University of Southampton , Southampton , Hampshire SO17 1BJ , U.K
| | - Chris-Kriton Skylaris
- Department of Chemistry , University of Southampton , Southampton , Hampshire SO17 1BJ , U.K
| | - Richard C D Brown
- Department of Chemistry , University of Southampton , Southampton , Hampshire SO17 1BJ , U.K
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10
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Li Y, Chen X, Gong Y. Synthesis of a dinuclear europium( iii) complex through deprotonation and oxygen-atom transfer of trimethylamine N-oxide. Dalton Trans 2019; 48:17158-17162. [DOI: 10.1039/c9dt04234b] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/24/2023]
Abstract
A dinuclear europium complex was synthesized via unprecedented deprotonation and oxygen-atom transfer of Me3NO.
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Affiliation(s)
- Yangjuan Li
- Department of Radiochemistry
- Shanghai Institute of Applied Physics
- Chinese Academy of Sciences
- Shanghai
- China
| | - Xiuting Chen
- Department of Radiochemistry
- Shanghai Institute of Applied Physics
- Chinese Academy of Sciences
- Shanghai
- China
| | - Yu Gong
- Department of Radiochemistry
- Shanghai Institute of Applied Physics
- Chinese Academy of Sciences
- Shanghai
- China
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11
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Majumder S, Pasayat S, Roy S, Dash SP, Dhaka S, Maurya MR, Reichelt M, Reuter H, Brzezinski K, Dinda R. Dioxidomolybdenum(VI) complexes bearing sterically constrained aroylazine ligands: Synthesis, structural investigation and catalytic evaluation. Inorganica Chim Acta 2018. [DOI: 10.1016/j.ica.2017.09.043] [Citation(s) in RCA: 20] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
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12
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Synthesis and solution structure of desoxotungsten(IV) and monooxotungsten(VI) benzenedithiolate complexes containing two intramolecular NH⋯S hydrogen bonds. Inorganica Chim Acta 2017. [DOI: 10.1016/j.ica.2017.08.036] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/23/2022]
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13
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Dong G, Ryde U. Effect of the protein ligand in DMSO reductase studied by computational methods. J Inorg Biochem 2017; 171:45-51. [DOI: 10.1016/j.jinorgbio.2017.03.004] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/25/2016] [Revised: 02/07/2017] [Accepted: 03/19/2017] [Indexed: 11/27/2022]
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14
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Fortino M, Marino T, Russo N, Sicilia E. Mechanistic investigation of trimethylamine-N-oxide reduction catalysed by biomimetic molybdenum enzyme models. Phys Chem Chem Phys 2017; 18:8428-36. [PMID: 26932500 DOI: 10.1039/c5cp07278f] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
In this paper, we report a theoretical investigation of the reduction reaction mechanism of Me3NO using molybdenum containing systems that are functional and structural analogues of trimethylamine N-oxide reductase mononuclear molybdenum enzyme. The reactivity of the monooxomolybdenum(IV) benzenedithiolato complex and its derivatives to carbamoyl (t-BuNHCO) and acylamino (t-BuCONH) substituents on the benzene rings in both cis and trans arrangements was explored. The calculated energy profiles describing the steps of two mechanisms of attack considered viable (named cis- and trans-attack) by the Me3NO substrate at cis and trans positions with respect to the oxo ligand show that the attack on cis is energetically more favourable than the attack on trans. Along the pathway for the cis-attack the first step of the reaction, that is rate-determining for all the studied compounds, is the approach of the substrate to the Mo centre in cis to the oxo ligand that causes a distortion of the initial square-pyramidal geometry of the complex. The reaction steps involved in the trans position attack were also explored. Calculations confirm that, as previously suggested, the introduction of ligands able to form intramolecular NH···S hydrogen bonds accelerates the reduction of the Me3NO substrate and contributes to the tuning of the reactivity of molybdoenzyme models.
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Affiliation(s)
- M Fortino
- Department of Chemistry and Chemical Technologies, Università della Calabria, 87036, Arcavacata di Rende, Italy.
| | - T Marino
- Department of Chemistry and Chemical Technologies, Università della Calabria, 87036, Arcavacata di Rende, Italy.
| | - N Russo
- Department of Chemistry and Chemical Technologies, Università della Calabria, 87036, Arcavacata di Rende, Italy.
| | - E Sicilia
- Department of Chemistry and Chemical Technologies, Università della Calabria, 87036, Arcavacata di Rende, Italy.
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15
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Maia LB, Moura I, Moura JJ. Molybdenum and tungsten-containing formate dehydrogenases: Aiming to inspire a catalyst for carbon dioxide utilization. Inorganica Chim Acta 2017. [DOI: 10.1016/j.ica.2016.07.010] [Citation(s) in RCA: 71] [Impact Index Per Article: 10.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
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16
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Fernández L, Pérez-Pla FF, Tuñón I, Llopis E. DFT Study on the Interaction of Tris(benzene-1,2-dithiolato)molybdenum Complex with Water. A Hydrolysis Mechanism Involving a Feasible Seven-Coordinate Aquomolybdenum Intermediate. J Phys Chem A 2016; 120:9636-9646. [PMID: 27933913 DOI: 10.1021/acs.jpca.6b10233] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
In the present work, the reactivity of the tris(benzene-1,2-dithiolato)molybdenum complex ([Mo(bdt)3]) toward water is studied by means of the density functional theory (DFT). DFT calculations were performed using the M06, B3P86, and B3PW91 hybrid functionals for comparison purposes. The M06 method was employed to elucidate the reaction pathway, relative stability of the intermediate products, nature of the Mo-S bond cleavage, and electronic structure of the involved molybdenum species. This functional was also used to study the transference of electrons from the molybdenum center toward the ligands. The reaction pathway confirms that [Mo(bdt)3] undergoes hydrolysis, yielding dihydroxo-bis(benzene-1,2-dithiolato)molybdenum complex ([Mo(OH)2(bdt)2]) and benzenedithiol. The reaction takes place through seven transition structures, one of them involving an aquo seven-coordinate molybdenum intermediate stabilized by a lone pair (LP) LPO→LPMo hyperconjugative interaction. This heptacoordinate species allows understanding of the observed oxygen atom exchange between water and tertiary phosphines mediated by these complexes. Calculations also show that [Mo(C2H4S2)3] and [Mo(OH)2(C2H4S2)2] have d2 and d0 electronic configuration, and hence an electron pair must be transferred during the course of the hydrolysis. The frontier molecular orbital (FMO) analysis concludes that the electron pair is transferred in the rupture of the second Mo-S bond, from the occupied donating Mo dx2-y2 orbital to the unoccupied C2H4(SH)2 S-C σ* ligand orbital. This result is supported by the bond dissociation energy calculations, which demonstrate that the neutral dissociation of the second Mo-S bond is energetically the more favorable.
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Affiliation(s)
- Lorenzo Fernández
- Institut de Cíencia dels Materials (ICMUV) , c/Catedrático Beltrán 2, 46980, Valencia, Spain
| | - Francisco F Pérez-Pla
- Institut de Cíencia dels Materials (ICMUV) , c/Catedrático Beltrán 2, 46980, Valencia, Spain
| | - Iñaki Tuñón
- Departamento de Química Física, Universitat de València , Dr. Moliner 50, 46100, Burjassot, Valencia, Spain
| | - Elisa Llopis
- Institut de Cíencia dels Materials (ICMUV) , c/Catedrático Beltrán 2, 46980, Valencia, Spain
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Hu L, Chen H. Assessment of DFT Methods for Computing Activation Energies of Mo/W-Mediated Reactions. J Chem Theory Comput 2015; 11:4601-14. [DOI: 10.1021/acs.jctc.5b00373] [Citation(s) in RCA: 41] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/07/2023]
Affiliation(s)
- Lianrui Hu
- Beijing National Laboratory
for Molecular Sciences (BNLMS), CAS Key Laboratory of Photochemistry,
Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
| | - Hui Chen
- Beijing National Laboratory
for Molecular Sciences (BNLMS), CAS Key Laboratory of Photochemistry,
Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
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18
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Mogesa B, Perera E, Rhoda HM, Gibson JK, Oomens J, Berden G, van Stipdonk MJ, Nemykin VN, Basu P. Solution, Solid, and Gas Phase Studies on a Nickel Dithiolene System: Spectator Metal and Reactor Ligand. Inorg Chem 2015; 54:7703-16. [PMID: 26244772 DOI: 10.1021/acs.inorgchem.5b00531] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
The syntheses of cationic nickel complexes using N,N'-dimethyl piperazine 2,3-dithione (Me2Dt(0)) and N,N'-diisopropyl piperazine 2,3-dithione ((i)Pr2Dt(0)) ligands are reported. These ligands were used in synthesizing bis and tris(dithione)Ni(II) complexes as tetrafluoroborate or hexafluorophosphate salts, i.e., [Ni((i)Pr2Dt(0))2][BF4]2 ([1a][BF4]2), [Ni((i)Pr2Dt(0))2][PF6]2 ([1a][PF6]2), [Ni(Me2Dt(0))2][BF4]2 ([1b][BF4]2), [Ni((i)Pr2Dt(0))3][BF4]2 ([2a][BF4]2), and [Ni((i)Pr2Dt(0))3][PF6]2 ([2a][PF6]2), respectively. Complex [2a][PF6]2 was isolated from a methanolic solution of [1a][PF6]2. Compound [1a][BF4]2 crystallizes in a trigonal crystal system (space group, P31/c) and exhibits unique packing features, whereas [2a][BF4]2 crystallizes in a monoclinic (P21/n) space group. Cyclic voltammograms of [1a][BF4]2 and [1b][BF4]2 are indicative of four reduction processes associated with stepwise single-electron reduction of the ligands. Spectroelectrochemical experiments on [1a][BF4]2 exhibit an intervalence charge transfer (IVCT) transition as a spectroscopic signature of the mixed-valence [Ni((i)Pr2Dt(0))((i)Pr2Dt(1-))](-) species. Analysis of this IVCT band suggests that this ligand based mixed valence complex, [Ni((i)Pr2Dt(0))((i)Pr2Dt(1-))](-), behaves more like a traditional class II/III metal based mixed-valence complex. The density functional theory (DFT) and time dependent DFT calculations provide a theoretical framework for understanding the electronic structures and the nature of excited states of the target compounds that are consistent with their spectroscopic and redox properties. Vibrational spectra of [1a](2+) and [2a](2+) were investigated as discrete species in the gas phase using infrared multiple photon dissociation (IRMPD) spectroscopy.
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Affiliation(s)
- Benjamin Mogesa
- †Department of Chemistry and Biochemistry, Duquesne University, Pittsburgh, Pennsylvania 15282, United States
| | - Eranda Perera
- †Department of Chemistry and Biochemistry, Duquesne University, Pittsburgh, Pennsylvania 15282, United States
| | - Hannah M Rhoda
- ‡Department of Chemistry and Biochemistry, University of Minnesota-Duluth, Duluth, Minnesota 55812, United States
| | - John K Gibson
- §Chemical Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, United States
| | - Jos Oomens
- ∥Radboud University, Institute for Molecules and Materials, FELIX Laboratory, Toernooiveld 7c, 6525ED Nijmegen, The Netherlands
| | - Giel Berden
- ∥Radboud University, Institute for Molecules and Materials, FELIX Laboratory, Toernooiveld 7c, 6525ED Nijmegen, The Netherlands
| | - Michael J van Stipdonk
- †Department of Chemistry and Biochemistry, Duquesne University, Pittsburgh, Pennsylvania 15282, United States
| | - Victor N Nemykin
- ‡Department of Chemistry and Biochemistry, University of Minnesota-Duluth, Duluth, Minnesota 55812, United States
| | - Partha Basu
- †Department of Chemistry and Biochemistry, Duquesne University, Pittsburgh, Pennsylvania 15282, United States
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19
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Li J, Andrejić M, Mata RA, Ryde U. A Computational Comparison of Oxygen Atom Transfer Catalyzed by Dimethyl Sulfoxide Reductase with Mo and W. Eur J Inorg Chem 2015. [DOI: 10.1002/ejic.201500209] [Citation(s) in RCA: 21] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/15/2022]
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20
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Schrapers P, Hartmann T, Kositzki R, Dau H, Reschke S, Schulzke C, Leimkühler S, Haumann M. Sulfido and cysteine ligation changes at the molybdenum cofactor during substrate conversion by formate dehydrogenase (FDH) from Rhodobacter capsulatus. Inorg Chem 2015; 54:3260-71. [PMID: 25803130 DOI: 10.1021/ic502880y] [Citation(s) in RCA: 48] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/09/2023]
Abstract
Formate dehydrogenase (FDH) enzymes are attractive catalysts for potential carbon dioxide conversion applications. The FDH from Rhodobacter capsulatus (RcFDH) binds a bis-molybdopterin-guanine-dinucleotide (bis-MGD) cofactor, facilitating reversible formate (HCOO(-)) to CO2 oxidation. We characterized the molecular structure of the active site of wildtype RcFDH and protein variants using X-ray absorption spectroscopy (XAS) at the Mo K-edge. This approach has revealed concomitant binding of a sulfido ligand (Mo=S) and a conserved cysteine residue (S(Cys386)) to Mo(VI) in the active oxidized molybdenum cofactor (Moco), retention of such a coordination motif at Mo(V) in a chemically reduced enzyme, and replacement of only the S(Cys386) ligand by an oxygen of formate upon Mo(IV) formation. The lack of a Mo=S bond in RcFDH expressed in the absence of FdsC implies specific metal sulfuration by this bis-MGD binding chaperone. This process still functioned in the Cys386Ser variant, showing no Mo-S(Cys386) ligand, but retaining a Mo=S bond. The C386S variant and the protein expressed without FdsC were inactive in formate oxidation, supporting that both Mo-ligands are essential for catalysis. Low-pH inhibition of RcFDH was attributed to protonation at the conserved His387, supported by the enhanced activity of the His387Met variant at low pH, whereas inactive cofactor species showed sulfido-to-oxo group exchange at the Mo ion. Our results support that the sulfido and S(Cys386) ligands at Mo and a hydrogen-bonded network including His387 are crucial for positioning, deprotonation, and oxidation of formate during the reaction cycle of RcFDH.
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Affiliation(s)
- Peer Schrapers
- †Institut für Experimentalphysik, Freie Universität Berlin, 14195 Berlin, Germany
| | - Tobias Hartmann
- ‡Institut für Biochemie und Biologie, Molekulare Enzymologie, Universität Potsdam, 14476 Potsdam, Germany
| | - Ramona Kositzki
- †Institut für Experimentalphysik, Freie Universität Berlin, 14195 Berlin, Germany
| | - Holger Dau
- †Institut für Experimentalphysik, Freie Universität Berlin, 14195 Berlin, Germany
| | - Stefan Reschke
- ‡Institut für Biochemie und Biologie, Molekulare Enzymologie, Universität Potsdam, 14476 Potsdam, Germany
| | - Carola Schulzke
- §Institut für Biochemie, Bioanorganische Chemie, Ernst-Moritz-Arndt-Universität Greifswald, 17487 Greifswald, Germany
| | - Silke Leimkühler
- ‡Institut für Biochemie und Biologie, Molekulare Enzymologie, Universität Potsdam, 14476 Potsdam, Germany
| | - Michael Haumann
- †Institut für Experimentalphysik, Freie Universität Berlin, 14195 Berlin, Germany
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21
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Andrejić M, Mata RA. Local Hybrid QM/QM Calculations of Reaction Pathways in Metallobiosites. J Chem Theory Comput 2014; 10:5397-404. [DOI: 10.1021/ct5008313] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Milica Andrejić
- Institut
für Physikalische
Chemie, Georg-August-Universität Göttingen, Tammannstrasse
6, D-37077 Göttingen, Germany
| | - Ricardo A. Mata
- Institut
für Physikalische
Chemie, Georg-August-Universität Göttingen, Tammannstrasse
6, D-37077 Göttingen, Germany
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22
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Li J, Ryde U. Comparison of the Active-Site Design of Molybdenum Oxo-Transfer Enzymes by Quantum Mechanical Calculations. Inorg Chem 2014; 53:11913-24. [DOI: 10.1021/ic5010837] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Jilai Li
- Department of Theoretical Chemistry, Chemical
Centre, Lund University, P.O. Box 124, SE-221 00 Lund, Sweden
- State Key Laboratory of Theoretical and
Computational Chemistry, Institute of Theoretical Chemistry, Jilin University, Changchun 130023, People’s Republic of China
| | - Ulf Ryde
- Department of Theoretical Chemistry, Chemical
Centre, Lund University, P.O. Box 124, SE-221 00 Lund, Sweden
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23
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DFT study on the oxygen transfer mechanism in nitroethenediamine based H2-receptor antagonists using the bis-dithiolene complex as the model catalyst for N-oxide reductase enzyme. J Inorg Biochem 2014; 142:84-91. [PMID: 25450022 DOI: 10.1016/j.jinorgbio.2014.09.011] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/04/2014] [Revised: 09/17/2014] [Accepted: 09/18/2014] [Indexed: 12/24/2022]
Abstract
Nitroethenediamine is an important functional unit, which is present in H2-receptor antagonists. These drugs show low bioavailability due to the bacterial degradation caused by the N-oxide reductase type of enzymes present in the human colon. Quantum chemical studies have been carried out to elucidate the mechanism of metabolic degradation of nitroethenediamine in the active site of N-oxide reductase. Three different pathways have been explored for the N-oxide bond cleavage by the model system, Mo(IV) bis-dithiolene complex [Mo(OMe)(mdt)2](-), (where mdt=1,2-dimethyl-ethene-1,2-dithiolate) using B3LYP/6-311+G(d,p) and M06/6-311+G(d,p) Density Functional Theory methods. The oxygen atom transfer from the nitrogen atom of nitroethenediamine to the Mo(IV) complex, involves simultaneous weakening of the N-oxide bond and the formation of Mo-O bond through a least motion path. During this transfer, Mo center is converted from a square pyramidal geometry to a distorted octahedral geometry, to facilitate the process of oxygen atom transfer. The energy barrier for the oxygen atom transfer from the imine tautomer has been estimated to be 25.9kcal/mol however, the overall reaction has been found to be endothermic. On the other hand, oxygen transfer reaction from the nitronic acid tautomer requires 30.5kcal/mol energy leading to a highly exothermic metabolite (M-1) directly hence, this path can be considered thermodynamically favorable for this metabolite. The alternative path involving the oxygen atom transfer from the enamine tautomer requires comparatively a higher energy barrier (32.6kcal/mol) and leads to a slightly endothermic metabolite. This study established the structural and energetic details associated with the Mo(IV) bis-dithiolene complex that catalyzes the degradation of nitroethenediamine based drug molecules.
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24
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Ha Y, Tenderholt AL, Holm RH, Hedman B, Hodgson KO, Solomon EI. Sulfur K-edge X-ray absorption spectroscopy and density functional theory calculations on monooxo Mo(IV) and bisoxo Mo(VI) bis-dithiolenes: insights into the mechanism of oxo transfer in sulfite oxidase and its relation to the mechanism of DMSO reductase. J Am Chem Soc 2014; 136:9094-105. [PMID: 24884723 PMCID: PMC4073832 DOI: 10.1021/ja503316p] [Citation(s) in RCA: 20] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/10/2014] [Indexed: 12/25/2022]
Abstract
Sulfur K-edge X-ray absorption spectroscopy (XAS) and density functional theory (DFT) calculations have been used to determine the electronic structures of two complexes [Mo(IV)O(bdt)2](2-) and [Mo(VI)O2(bdt)2](2-) (bdt = benzene-1,2-dithiolate(2-)) that relate to the reduced and oxidized forms of sulfite oxidase (SO). These are compared with those of previously studied dimethyl sulfoxide reductase (DMSOr) models. DFT calculations supported by the data are extended to evaluate the reaction coordinate for oxo transfer to a phosphite ester substrate. Three possible transition states are found with the one at lowest energy, stabilized by a P-S interaction, in good agreement with experimental kinetics data. Comparison of both oxo transfer reactions shows that in DMSOr, where the oxo is transferred from the substrate to the metal ion, the oxo transfer induces electron transfer, while in SO, where the oxo transfer is from the metal site to the substrate, the electron transfer initiates oxo transfer. This difference in reactivity is related to the difference in frontier molecular orbitals (FMO) of the metal-oxo and substrate-oxo bonds. Finally, these experimentally related calculations are extended to oxo transfer by sulfite oxidase. The presence of only one dithiolene at the enzyme active site selectively activates the equatorial oxo for transfer, and allows facile structural reorganization during turnover.
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Affiliation(s)
- Yang Ha
- Department
of Chemistry, Stanford University, Stanford, California 94305, United States
| | - Adam L. Tenderholt
- Department
of Chemistry, Stanford University, Stanford, California 94305, United States
| | - Richard H. Holm
- Department
of Chemistry and Chemical Biology, Harvard
University, Cambridge, Massachusetts 02138, United States
| | - Britt Hedman
- Stanford
Synchrotron Radiation Lightsource, SLAC, Stanford University, Menlo Park, California 94025, United States
| | - Keith O. Hodgson
- Department
of Chemistry, Stanford University, Stanford, California 94305, United States
- Stanford
Synchrotron Radiation Lightsource, SLAC, Stanford University, Menlo Park, California 94025, United States
| | - Edward I. Solomon
- Department
of Chemistry, Stanford University, Stanford, California 94305, United States
- Stanford
Synchrotron Radiation Lightsource, SLAC, Stanford University, Menlo Park, California 94025, United States
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25
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Blomberg MRA, Borowski T, Himo F, Liao RZ, Siegbahn PEM. Quantum chemical studies of mechanisms for metalloenzymes. Chem Rev 2014; 114:3601-58. [PMID: 24410477 DOI: 10.1021/cr400388t] [Citation(s) in RCA: 436] [Impact Index Per Article: 43.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/08/2023]
Affiliation(s)
- Margareta R A Blomberg
- Department of Organic Chemistry, Arrhenius Laboratory, Stockholm University , SE-106 91 Stockholm, Sweden
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26
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Li JL, Mata RA, Ryde U. Large Density-Functional and Basis-Set Effects for the DMSO Reductase Catalyzed Oxo-Transfer Reaction. J Chem Theory Comput 2013; 9:1799-807. [DOI: 10.1021/ct301094r] [Citation(s) in RCA: 39] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022]
Affiliation(s)
- Ji-Lai Li
- Department of Theoretical Chemistry,
Lund University, Chemical Centre, P.O. Box 124, SE-221 00 Lund, Sweden
- State Key Laboratory of Theoretical
and Computational Chemistry, Institute of Theoretical Chemistry, Jilin
University, Changchun 130023, People’s Republic of China
| | - Ricardo A. Mata
- Institut für Physikalische
Chemie, Georg-August-Universität Göttingen, Tammannstrasse
6, D-37077, Göttingen, Germany
| | - Ulf Ryde
- Department of Theoretical Chemistry,
Lund University, Chemical Centre, P.O. Box 124, SE-221 00 Lund, Sweden
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27
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Why is the molybdenum-substituted tungsten-dependent formaldehyde ferredoxin oxidoreductase not active? A quantum chemical study. J Biol Inorg Chem 2013; 18:175-181. [PMID: 23183892 DOI: 10.1007/s00775-012-0961-5] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/26/2012] [Accepted: 11/13/2012] [Indexed: 01/12/2023]
Abstract
Formaldehyde ferredoxin oxidoreductase is a tungsten-dependent enzyme that catalyzes the oxidative degradation of formaldehyde to formic acid. The molybdenum ion can be incorporated into the active site to displace the tungsten ion, but is without activity. Density functional calculations have been employed to understand the incapacitation of the enzyme caused by molybdenum substitution. The calculations show that the enzyme with molybdenum (Mo-FOR) has higher redox potential than that with tungsten, which makes the formation of the Mo(VI)=O complex endothermic by 14 kcal/mol. Following our previously suggested mechanism for this enzyme, the formaldehyde substrate oxidation was also investigated for Mo-FOR using the same quantum-mechanics-only model, except for the displacement of tungsten by molybdenum. The calculations demonstrate that formaldehyde oxidation occurs via a sequential two-step mechanism. Similarly to the tungsten-catalyzed reaction, the Mo(VI)=O species performs the nucleophilic attack on the formaldehyde carbon, followed by proton transfer in concert with two-electron reduction of the metal center. The first step is rate-limiting, with a total barrier of 28.2 kcal/mol. The higher barrier is mainly due to the large energy penalty for the formation of the Mo(VI)=O species.
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