1
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Vu K, Pandian J, Zhang B, Annas C, Parker AJ, Mancini JS, Wang EB, Saldana-Greco D, Nelson ES, Springsted G, Lischka H, Plasser F, Parish CA. Multireference Averaged Quadratic Coupled Cluster (MR-AQCC) Study of the Geometries and Energies for ortho-, meta- and para-Benzyne. J Phys Chem A 2024. [PMID: 39240216 DOI: 10.1021/acs.jpca.4c04099] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 09/07/2024]
Abstract
The diradical benzyne isomers are excellent prototypes for evaluating the ability of an electronic structure method to describe static and dynamic correlation. The benzyne isomers are also interesting molecules with which to study the fundamentals of through-space and through-bond diradical coupling that is important in so many electronic device applications. In the current study, we utilize the multireference methods MC-SCF, MR-CISD, MR-CISD+Q, and MR-AQCC with an (8,8) complete active space that includes the σ, σ*, π and π* orbitals, to characterize the electronic structure of ortho-, meta- and para-benzyne. We also determine the adiabatic and vertical singlet-triplet splittings for these isomers. MR-AQCC and MR-CISD+Q produced energy gaps in good agreement with previously obtained experimental values. Geometries, orbital energies and unpaired electron densities show significant through-space coupling in the o- and m-benzynes, while p-benzyne shows through-bond coupling, explaining the dramatically different singlet-triplet gaps between the three isomers.
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Affiliation(s)
- Khanh Vu
- Department of Chemistry, Gottwald Center for the Sciences, University of Richmond, Richmond, Virginia 23173, United States
| | - Joshua Pandian
- Department of Chemistry, Gottwald Center for the Sciences, University of Richmond, Richmond, Virginia 23173, United States
| | - Boyi Zhang
- Department of Chemistry, Gottwald Center for the Sciences, University of Richmond, Richmond, Virginia 23173, United States
| | - Christina Annas
- Department of Chemistry, Gottwald Center for the Sciences, University of Richmond, Richmond, Virginia 23173, United States
| | - Anna J Parker
- Department of Chemistry, Gottwald Center for the Sciences, University of Richmond, Richmond, Virginia 23173, United States
| | - John S Mancini
- Department of Chemistry, Gottwald Center for the Sciences, University of Richmond, Richmond, Virginia 23173, United States
| | - Evan B Wang
- Department of Chemistry, Gottwald Center for the Sciences, University of Richmond, Richmond, Virginia 23173, United States
| | - Diomedes Saldana-Greco
- Department of Chemistry, Gottwald Center for the Sciences, University of Richmond, Richmond, Virginia 23173, United States
| | - Emily S Nelson
- Department of Chemistry, Gottwald Center for the Sciences, University of Richmond, Richmond, Virginia 23173, United States
| | - Greg Springsted
- Department of Chemistry, Gottwald Center for the Sciences, University of Richmond, Richmond, Virginia 23173, United States
| | - Hans Lischka
- Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409, United States
| | - Felix Plasser
- Department of Chemistry, Loughborough University, Ashby Road, Loughborough LE11 3TU, Leicestershire, U.K
| | - Carol A Parish
- Department of Chemistry, Gottwald Center for the Sciences, University of Richmond, Richmond, Virginia 23173, United States
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2
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Murtada R, Finn S, Gao J. Development of mass spectrometric glycan characterization tags using acid-base chemistry and/or free radical chemistry. MASS SPECTROMETRY REVIEWS 2024; 43:269-288. [PMID: 36161326 DOI: 10.1002/mas.21810] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/07/2022] [Revised: 08/26/2022] [Accepted: 09/05/2022] [Indexed: 06/16/2023]
Abstract
Despite recent advances in glycomics, glycan characterization still remains an analytical challenge. Accordingly, numerous glycan-tagging reagents with different chemistries were developed, including those involving acid-base chemistry and/or free radical chemistry. Acid-base chemistry excels at dissociating glycans into their constituent components in a systematic and predictable manner to generate cleavages at glycosidic bonds. Glycans are also highly susceptible to depolymerization by free radical processes, which is supported by results observed from electron-activated dissociation techniques. Therefore, the free radical activated glycan sequencing (FRAGS) reagent was developed so as to possess the characteristics of both acid-base and free radical chemistry, thus generating information-rich glycosidic bond and cross-ring cleavages. Alternatively, the free radical processes can be induced via photodissociation of the specific carbon-iodine bond which gives birth to similar fragmentation patterns as the FRAGS reagent. Furthermore, the methylated-FRAGS (Me-FRAGS) reagent was developed to eliminate glycan rearrangements by way of a fixed charged as opposed to a labile proton, which would otherwise yield additional, yet unpredictable, fragmentations including internal residue losses or multiple external residue losses. Lastly, to further enhance glycan enrichment and characterization, solid-support FRAGS was developed.
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Affiliation(s)
- Rayan Murtada
- Department of Chemistry and Biochemistry, Montclair State University, Montclair, New Jersey, USA
| | - Shane Finn
- Department of Chemistry and Biochemistry, Montclair State University, Montclair, New Jersey, USA
| | - Jinshan Gao
- Department of Chemistry and Biochemistry, Montclair State University, Montclair, New Jersey, USA
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3
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Báez-Grez R, Pino-Rios R. On the aromaticity and stability of benzynes in the ground and lowest-lying triplet excited states. J Comput Chem 2024; 45:6-12. [PMID: 37671655 DOI: 10.1002/jcc.27214] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/05/2023] [Revised: 08/02/2023] [Accepted: 08/09/2023] [Indexed: 09/07/2023]
Abstract
In this work, we have revisited the aromaticity of benzyne isomers at the unrestricted density functional theory level (UDFT) using the energetic, magnetic, and delocalization criteria. In addition, this last criterion has also been analyzed employing complete active space (CASSCF) calculations. The results show conservation of aromaticity in these monocycles. Additionally it is observed that this trend is maintained in polycyclic aromatic hydrocarbon derivatives such as biradical didehydrophenanthrenes. Do these results imply a violation of Baird's rule? The answer is No, because this conservation in aromaticity is due to the loss of hydrogen atoms affects only the electronic σ skeleton and exerts a minor influence on the π cloud. Additionally, we have analyzed the relative stability of benzyne isomers and their relationship with experimental ΔES-T values. According to the literature, the stability of the benzynes in the singlet state is due to an effective interaction between the electrons of the biradical centers; however, this effect is completely reversed in the triplet state, which explains why the para isomer has the lowest ΔES-T gap.
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Affiliation(s)
- Rodrigo Báez-Grez
- Departamento de Química, Facultad de Ciencias Exactas, Universidad Andres Bello, Santiago, Chile
| | - Ricardo Pino-Rios
- Instituto de Estudios de la Salud, Universidad Arturo, Chile
- Química y Farmacia, Facultad de Ciencias de la Salud, Universidad Arturo Prat, Chile
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4
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Ding D, Feng E, Kotha RR, Chapman NC, Jiang H, Nash JJ, Kenttämaa HI. Spin-Spin Coupling Controls the Gas-Phase Reactivity of Aromatic σ-Type Triradicals. Chemistry 2021; 28:e202102968. [PMID: 34786768 DOI: 10.1002/chem.202102968] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/14/2021] [Indexed: 11/11/2022]
Abstract
Examination of the reactions of σ-type quinolinium-based triradicals with cyclohexane in the gas phase demonstrated that the radical site that is the least strongly coupled to the other two radical sites reacts first, independent of the intrinsic reactivity of this radical site, in contrast to related biradicals that first react at the most electron-deficient radical site. Abstraction of one or two H atoms and formation of an ion that formally corresponds to a combination of the ion and cyclohexane accompanied by elimination of a H atom ("addition-H") were observed. In all cases except one, the most reactive radical site of the triradicals is intrinsically less reactive than the other two radical sites. The product complex of the first H atom abstraction either dissociates to give the H-atom-abstraction product and the cyclohexyl radical or the more reactive radical site in the produced biradical abstracts a H atom from the cyclohexyl radical. The monoradical product sometimes adds to cyclohexene followed by elimination of a H atom, generating the "addition-H" products. Similar reaction efficiencies were measured for three of the triradicals as for relevant monoradicals. Surprisingly, the remaining three triradicals (all containing a meta-pyridyne moiety) reacted substantially faster than the relevant monoradicals. This is likely due to the exothermic generation of a meta-pyridyne analog that has enough energy to attain the dehydrocarbon atom separation common for H-atom-abstraction transition states of protonated meta-pyridynes.
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Affiliation(s)
- Duanchen Ding
- Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, IN, 47907, USA
| | - Erlu Feng
- Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, IN, 47907, USA
| | - Raghavendhar R Kotha
- Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, IN, 47907, USA
| | - Nathan C Chapman
- Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, IN, 47907, USA
| | - Hanning Jiang
- Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, IN, 47907, USA
| | - John J Nash
- Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, IN, 47907, USA
| | - Hilkka I Kenttämaa
- Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, IN, 47907, USA
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5
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Milton JR, Jankiewicz BJ, Max JP, Vinueza NR, Kirkpatrick LM, Campbell K, Gallardo VA, Reece JN, Kenttämaa HI. Study on the Gas-Phase Reactivity of Charged Pyridynes. J Org Chem 2021; 86:9979-9993. [PMID: 34242505 DOI: 10.1021/acs.joc.1c00618] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The reactivities of three isomeric, charged ortho-pyridynes, the 1,2-, 2,3-, and 3,4-didehydropyridinium cations, were examined in the gas phase using Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometry. The structures of selected product ions were probed using collision-activated dissociation (CAD) experiments in a linear quadrupole ion trap (LQIT) mass spectrometer. Mechanisms based on quantum chemical calculations are proposed for the formation of all major products. The products of the reactions of the charged ortho-pyridynes in the gas phase were found to closely resemble those formed upon reactions of neutral ortho-arynes in solution, but the mechanisms of these reactions exhibit striking differences. Additionally, no radical reactions were observed for any of the charged ortho-pyridynes examined, in contrast to previous proposals that ortho-benzyne can occasionally react via radical mechanisms. Finally, the relative reactivities of those charged gaseous ortho-pyridynes that yielded similar product distributions were found to be affected mainly by the (calculated) vertical electron affinities of the dehydrocarbon sites, which suggests that the reactivity of these species is controlled by polar effects.
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Affiliation(s)
- Jacob R Milton
- Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907-2084, United States
| | - Bartłomiej J Jankiewicz
- Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907-2084, United States.,Institute of Optoelectronics, Military University of Technology, gen. S. Kaliskiego 2, 00-908 Warsaw, Poland
| | - Joann P Max
- Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907-2084, United States
| | - Nelson R Vinueza
- Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907-2084, United States
| | - Lindsey M Kirkpatrick
- Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907-2084, United States.,Ryan White Center for Pediatric Infectious Disease, Indiana University School of Medicine, 705 Riley Hospital Drive, Room 3032, Indianapolis, Indiana 46202, United States
| | - Karinna Campbell
- Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907-2084, United States
| | - Vanessa A Gallardo
- Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907-2084, United States
| | - Jennifer N Reece
- Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907-2084, United States
| | - Hilkka I Kenttämaa
- Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907-2084, United States
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6
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Ma X, Feng E, Jiang H, Boulos VM, Gao J, Nash JJ, Kenttämaa HI. Protonated Ground-State Singlet meta-Pyridynes React from an Excited Triplet State. J Org Chem 2021; 86:3249-3260. [PMID: 33555870 DOI: 10.1021/acs.joc.0c02594] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The gaseous 2,6-didehydropyridinium cation and its derivatives transfer a proton to reagents for which the reaction for their singlet ground states is too endothermic to be observed. These reactions occur from the lowest-energy excited triplet states, which has not been observed (or reported) for other meta-benzyne analogues. Quantum chemical calculations indicate that the (excited) triplet states are stronger Brønsted acids than their (ground) singlet states, likely due to unfavorable three-center, four-electron interactions in the singlet-state conjugate bases. The cations have substantially smaller (calculated) singlet-triplet (S-T) splittings (ranging from ca. -11 to -17 kcal mol-1) than other related meta-benzyne analogues (e.g., -23.4 kcal mol-1 for the 3,5-isomer). This is rationalized by the destabilization of the singlet states (relative to the triplet states) by reduced (spatial) overlap of the nonbonding molecular orbitals due to the presence of the nitrogen atom between the radical sites (making the ring more rigid). Both the singlet and triplet states are believed to be generated upon formation of these biradicals via energetic collisions due to their small S-T splittings. It appears that once the triplet states are formed, the rate of proton transfer is faster than the rate of intersystem crossing unless the biradicals contain heavy atoms.
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Affiliation(s)
- Xin Ma
- Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907, United States
| | - Erlu Feng
- Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907, United States
| | - Hanning Jiang
- Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907, United States
| | - Victoria M Boulos
- Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907, United States
| | - Jinshan Gao
- Department of Chemistry and Biochemistry, Center for Quantitative Obesity Research, Montclair State University, 1 Normal Avenue, Montclair, New Jersey 07043, United States
| | - John J Nash
- Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907, United States
| | - Hilkka I Kenttämaa
- Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907, United States
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7
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Ding D, Jiang H, Ma X, Nash JJ, Kenttämaa HI. Effects of the Distance between Radical Sites on the Reactivities of Aromatic Biradicals. J Org Chem 2020; 85:8415-8428. [PMID: 32482062 DOI: 10.1021/acs.joc.0c00658] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/25/2023]
Abstract
Coupling of the radical sites in isomeric benzynes is known to hinder their radical reactivity. In order to determine how far apart the radical sites must be for them not to interact, the gas-phase reactivity of several isomeric protonated (iso)quinoline- and acridine-based biradicals was examined. All the (iso)quinolinium-based biradicals were found to react slower than the related monoradicals with similar vertical electron affinities (i.e., similar polar effects). In sharp contrast, the acridinium-based biradicals, most with the radical sites farther apart than in the (iso)quinolinium-based systems, showed greater reactivities than the relevant monoradicals with similar vertical electron affinities. The greater distances between the two radical sites in these biradicals lead to very little or no spin-spin coupling, and no suppression of radical reactivity was observed. Therefore, the radical sites can still interact if they are located on adjacent benzene rings and only after being separated further than that does no coupling occur. The most reactive radical site of each biradical was experimentally determined to be the one predicted to be more reactive based on the monoradical reactivity data. Therefore, the calculated vertical electron affinities of relevant monoradicals can be used to predict which radical site is most reactive in the biradicals.
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Affiliation(s)
- Duanchen Ding
- Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907, United States
| | - Hanning Jiang
- Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907, United States
| | - Xin Ma
- Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907, United States
| | - John J Nash
- Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907, United States
| | - Hilkka I Kenttämaa
- Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907, United States
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8
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Fabijanczuk K, Gaspar K, Desai N, Lee J, Thomas DA, Beauchamp JL, Gao J. Resin and Magnetic Nanoparticle-Based Free Radical Probes for Glycan Capture, Isolation, and Structural Characterization. Anal Chem 2019; 91:15387-15396. [PMID: 31718152 DOI: 10.1021/acs.analchem.9b01303] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/07/2023]
Abstract
By combining the merits of solid supports and free radical activated glycan sequencing (FRAGS) reagents, we develop a multifunctional solid-supported free radical probe (SS-FRAGS) that enables glycan enrichment and characterization. SS-FRAGS comprises a solid support, free radical precursor, disulfide bond, pyridyl, and hydrazine moieties. Thio-activated resin and magnetic nanoparticles (MNPs) are chosen as the solid support to selectively capture free glycans via the hydrazine moiety, allowing for their enrichment and isolation. The disulfide bond acts as a temporary covalent linkage between the solid support and the captured glycan, allowing the release of glycans via the cleavage of the disulfide bond by dithiothreitol. The basic pyridyl functional group provides a site for the formation of a fixed charge, enabling detection by mass spectrometry and avoiding glycan rearrangement during collisional activation. The free radical precursor generates a nascent free radical upon collisional activation and thus simultaneously induces systematic and predictable fragmentation for glycan structure elucidation. A radical-driven glycan deconstruction diagram (R-DECON) is developed to visually summarize the MS2 results and thus allow for the assembly of the glycan skeleton, making the differentiation of isobaric glycan isomers unambiguous. For application to a real-world sample, we demonstrate the efficacy of the SS-FRAGS by analyzing glycan structures enzymatically cleaved from RNase-B.
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Affiliation(s)
- Kimberly Fabijanczuk
- Department of Chemistry and Biochemistry and Center for Quantitative Obesity Research , Montclair State University , Montclair , New Jersey 07043 , United States
| | - Kaylee Gaspar
- Department of Chemistry and Biochemistry and Center for Quantitative Obesity Research , Montclair State University , Montclair , New Jersey 07043 , United States
| | - Nikunj Desai
- Department of Chemistry and Biochemistry and Center for Quantitative Obesity Research , Montclair State University , Montclair , New Jersey 07043 , United States
| | - Jungeun Lee
- Department of Chemistry and Biochemistry and Center for Quantitative Obesity Research , Montclair State University , Montclair , New Jersey 07043 , United States
| | - Daniel A Thomas
- Arthur Amos Noyes Laboratory of Chemical Physics , California Institute of Technology , Pasadena , California 91125 , United States
| | - J L Beauchamp
- Arthur Amos Noyes Laboratory of Chemical Physics , California Institute of Technology , Pasadena , California 91125 , United States
| | - Jinshan Gao
- Department of Chemistry and Biochemistry and Center for Quantitative Obesity Research , Montclair State University , Montclair , New Jersey 07043 , United States
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9
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Stuyver T, Chen B, Zeng T, Geerlings P, De Proft F, Hoffmann R. Do Diradicals Behave Like Radicals? Chem Rev 2019; 119:11291-11351. [DOI: 10.1021/acs.chemrev.9b00260] [Citation(s) in RCA: 139] [Impact Index Per Article: 27.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/28/2022]
Affiliation(s)
- Thijs Stuyver
- Algemene Chemie, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium
| | - Bo Chen
- Department of Chemistry and Chemical Biology, Cornell University, Ithaca New York 14853, United States
| | - Tao Zeng
- Department of Chemistry, York University, Toronto, Ontario M3J1P3, Canada
- Department of Chemistry, Carleton University, Ottawa, Ontario K1S5B6, Canada
| | - Paul Geerlings
- Algemene Chemie, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium
| | - Frank De Proft
- Algemene Chemie, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium
| | - Roald Hoffmann
- Department of Chemistry and Chemical Biology, Cornell University, Ithaca New York 14853, United States
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10
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Kotha RR, Yerabolu R, Aqueel MS, Riedeman JS, Szalwinski L, Ding D, Nash JJ, Kenttämaa HI. Quinoline Triradicals: A Reactivity Study. J Am Chem Soc 2019; 141:6672-6679. [PMID: 30945540 DOI: 10.1021/jacs.9b01740] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/21/2023]
Abstract
The gas-phase reactivities of several protonated quinoline-based σ-type (carbon-centered) mono-, bi-, and triradicals toward dimethyl disulfide (DMDS) were studied by using a linear quadrupole ion trap mass spectrometer. The mono- and biradicals produce abundant thiomethyl abstraction products and small amounts of DMDS radical cation, as expected. Surprisingly, all triradicals produce very abundant DMDS radical cations. A single-step mechanism involving electron transfer from DMDS to the triradicals is highly unlikely because the (experimental) adiabatic ionization energy of DMDS is almost 3 eV greater than the (calculated) adiabatic electron affinities of the triradicals. The unexpected reactivity can be explained based on an unprecedented two-step mechanism wherein the protonated triradical first transfers a proton to DMDS, which is then followed by hydrogen atom abstraction from the protonated sulfur atom in DMDS by the radical site in the benzene ring of the deprotonated triradical to generate the conventional DMDS radical cation and a neutral biradical. Quantum chemical calculations as well as examination of deuterated and methylated triradicals provide support for this mechanism. The proton affinities of the neutral triradicals (and DMDS) influence the first step of the reaction while the vertical electron affinities and spin-spin coupling of the neutral triradicals influence the second step. The calculated total reaction exothermicities for the triradicals studied range from 27.6 up to 29.9 kcal mol-1.
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Affiliation(s)
- Raghavendhar R Kotha
- Department of Chemistry , Purdue University , 560 Oval Drive , West Lafayette , Indiana 47907 , United States
| | - Ravikiran Yerabolu
- Department of Chemistry , Purdue University , 560 Oval Drive , West Lafayette , Indiana 47907 , United States
| | - Mohammad Sabir Aqueel
- Department of Chemistry , Purdue University , 560 Oval Drive , West Lafayette , Indiana 47907 , United States
| | - James S Riedeman
- Department of Chemistry , Purdue University , 560 Oval Drive , West Lafayette , Indiana 47907 , United States
| | - Lucas Szalwinski
- Department of Chemistry , Purdue University , 560 Oval Drive , West Lafayette , Indiana 47907 , United States
| | - Duanchen Ding
- Department of Chemistry , Purdue University , 560 Oval Drive , West Lafayette , Indiana 47907 , United States
| | - John J Nash
- Department of Chemistry , Purdue University , 560 Oval Drive , West Lafayette , Indiana 47907 , United States
| | - Hilkka I Kenttämaa
- Department of Chemistry , Purdue University , 560 Oval Drive , West Lafayette , Indiana 47907 , United States
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11
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Ma X, Jin C, Wang D, Nash JJ, Kenttämaa HI. Relative Reactivities of Three Isomeric Aromatic Biradicals with a 1,4‐Biradical Topology Are Controlled by Polar Effects. Chemistry 2019; 25:6355-6361. [PMID: 30811712 DOI: 10.1002/chem.201806106] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/08/2018] [Revised: 02/24/2019] [Indexed: 01/22/2023]
Affiliation(s)
- Xin Ma
- Department of ChemistryPurdue University 560 Oval Drive West Lafayette Indiana 47907 USA
| | - Chunfen Jin
- Department of ChemistryPurdue University 560 Oval Drive West Lafayette Indiana 47907 USA
| | - Duanda Wang
- Department of ChemistryPurdue University 560 Oval Drive West Lafayette Indiana 47907 USA
| | - John J. Nash
- Department of ChemistryPurdue University 560 Oval Drive West Lafayette Indiana 47907 USA
| | - Hilkka I. Kenttämaa
- Department of ChemistryPurdue University 560 Oval Drive West Lafayette Indiana 47907 USA
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12
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Kotha RR, Yerabolu R, Ding D, Szalwinski L, Ma X, Wittrig A, Kong J, Nash JJ, Kenttämaa HI. Spin–Spin Coupling Between Two
meta
‐Benzyne Moieties In a Quinolinium Tetraradical Cation Increases Their Reactivities. Chemistry 2019; 25:4472-4477. [PMID: 30648302 DOI: 10.1002/chem.201806096] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/07/2018] [Revised: 01/12/2019] [Indexed: 11/11/2022]
Affiliation(s)
| | | | - Duanchen Ding
- Department of Chemistry Purdue University West Lafayette IN 47907 USA
| | - Lucas Szalwinski
- Department of Chemistry Purdue University West Lafayette IN 47907 USA
| | - Xin Ma
- Department of Chemistry Purdue University West Lafayette IN 47907 USA
| | - Ashley Wittrig
- ExxonMobil Research & Engineering Company Annandale NJ 08801 USA
| | - John Kong
- Department of Chemistry Purdue University West Lafayette IN 47907 USA
| | - John J. Nash
- Department of Chemistry Purdue University West Lafayette IN 47907 USA
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13
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Gao J, Jankiewicz BJ, Sheng H, Kirkpatrick L, Ma X, Nash JJ, Kenttämaa HI. Substituent Effects on the Reactivity of the 2,4,6-Tridehydropyridinium Cation, an Aromatic σ,σ,σ-Triradical. European J Org Chem 2018; 2018:6582-6589. [PMID: 31692928 PMCID: PMC6830858 DOI: 10.1002/ejoc.201801249] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/12/2018] [Indexed: 11/06/2022]
Abstract
2,4,6-Tridehydropyridinium cation (7) undergoes three consecutive atom or atom group abstractions from reagent molecules in the gas phase. By placing a π-electron-donating hydroxyl group between two radical sites, their reactivity can be quenched by enhancing their through-space coupling via a favorable resonance structure. Indeed, 3-hydroxy-2,4,6-tridehydropyridinium cation (8) abstracts only one atom or group of atoms from reagents. On the other hand, an electron-withdrawing cyano group between two of the radical sites (9) destabilizes the analogous resonance structure and diminishes through-space coupling between the radical sites, resulting in abstraction of three atoms, just like 7. However, the cyano-substituent also increases acidity to the point that 9 reacts pre-dominantly via proton transfer instead of undergoing radical reactions. Therefore, acidic triradicals may undergo nonradical, barrierless proton transfer reactions faster than radical reactions, which are usually accompanied by barriers. Examination of the analogous cyano-substituted mono-and biradicals revealed behavior similar to that of the corresponding unsubstituted species, with the exception of substantially greater reactivities due to their greater (calculated) vertical electron affinities. Finally, the 3-cyano-2,6-didehydropyridinium cation with a singlet ground state (S-T splitting: -11.9 kcal mol-1) was found to react exclusively from the lowest-energy triplet state by fast proton transfer reactions.
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Affiliation(s)
- Jinshan Gao
- Department of Chemistry and Biochemistry, Center for Quantitative Obesity Research, Montclair State University, 1 Normal Avenue, Montclair, NJ 07043, USA
| | | | | | | | - Xin Ma
- Department of Chemistry, Purdue University,West Lafayette, IN 47907, USA
| | - John J Nash
- Department of Chemistry, Purdue University,West Lafayette, IN 47907, USA
| | - Hilkka I Kenttämaa
- Department of Chemistry, Purdue University,West Lafayette, IN 47907, USA
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14
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Sheng H, Ma X, Lei HR, Milton J, Tang W, Jin C, Gao J, Wittrig AM, Archibold EF, Nash JJ, Kenttämaa HI. Polar Effects Control the Gas-Phase Reactivity of para-Benzyne Analogs. Chemphyschem 2018; 19:2839-2842. [PMID: 30203923 DOI: 10.1002/cphc.201800646] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/05/2018] [Indexed: 11/05/2022]
Abstract
We report herein a gas-phase reactivity study on a para-benzyne cation and its three cyano-substituted, isomeric derivatives performed using a dual-linear quadrupole ion trap mass spectrometer. All four biradicals were found to undergo primary and secondary radical reactions analogous to those observed for the related monoradicals, indicating the presence of two reactive radical sites. The reactivity of all biradicals is substantially lower than that of the related monoradicals, as expected based on the singlet ground states of the biradicals. The cyano-substituted biradicals show substantially greater reactivity than the analogous unsubstituted biradical. The greater reactivity is rationalized by the substantially greater (calculated) electron affinity of the radical sites of the cyano-substituted biradicals, which results in stabilization of their transition states through polar effects. This finding is in contrast to the long-standing thinking that the magnitude of the singlet-triplet splitting controls the reactivity of para-benzynes.
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Affiliation(s)
- Huaming Sheng
- Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, IN 47907, USA
| | - Xin Ma
- Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, IN 47907, USA
| | - Hao-Ran Lei
- Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, IN 47907, USA
| | - Jacob Milton
- Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, IN 47907, USA
| | - Weijuan Tang
- Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, IN 47907, USA
| | - Chunfen Jin
- Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, IN 47907, USA
| | - Jinshan Gao
- Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, IN 47907, USA
| | - Ashley M Wittrig
- Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, IN 47907, USA
| | - Enada F Archibold
- Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, IN 47907, USA
| | - John J Nash
- Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, IN 47907, USA
| | - Hilkka I Kenttämaa
- Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, IN 47907, USA
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15
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Wenthold PG, Winter AH. Nucleophilic Addition to Singlet Diradicals: Homosymmetric Diradicals. J Org Chem 2018; 83:12390-12396. [DOI: 10.1021/acs.joc.8b01413] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Paul G. Wenthold
- The Department of Chemistry and Biochemistry, Purdue University, West Lafayette, Indiana 47906, United States
| | - Arthur H. Winter
- The Department of Chemistry and Biochemistry, Iowa State University, Ames, Iowa 52101, United States
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16
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Vinueza NR, Jankiewicz BJ, Gallardo VA, Nash JJ, Kenttämaa HI. Effects of hydrogen bonding on the gas-phase reactivity of didehydroisoquinolinium cation isomers. Phys Chem Chem Phys 2018; 20:21567-21572. [PMID: 30094447 DOI: 10.1039/c8cp03350a] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Two previously unreported isomeric biradicals with a 1,4-radical topology, the 1,5-didehydroisoquinolinium cation and the 4,8-didehydroisoquinolinium cation, and an additional, previously reported isomer, the 4,5-didehydroisoquinolinium cation, were studied to examine the importance of the exact location of the radical sites on their reactivities in the gas phase. The experimental results suggest that hydrogen bonding in the transition state enhances the reactivity of the 1,5-didehydroisoquinolinium cation towards tetrahydrofuran but not towards allyl iodide, dimethyl disulfide or tert-butyl isocyanide. The observation of no such enhancement of reactivity towards tetrahydrofuran for the 4,8-didehydroisoquinolinium and 4,5-didehydroisoquinolinium cations supports this hypothesis as these two biradicals are not able to engage in hydrogen bonding in their transition states for hydrogen atom abstraction from tetrahydrofuran. Quantum chemical transition state calculations indicate that abstraction of a hydrogen atom from tetrahydrofuran by the 1,5-didehydroisoquinolinium cation occurs at the C-1 radical site and that the transition state is stabilized by hydrogen bonding.
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Affiliation(s)
- Nelson R Vinueza
- Purdue University, Department of Chemistry, 560 Oval Drive, West Lafayette, IN 47907, USA.
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17
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Hu H, Zhang B, Luxon A, Scott T, Wang B, Parish CA. An Extended Multireference Study of the Singlet and Triplet States of the 9,10-didehydroanthracene Diradical. J Phys Chem A 2018; 122:3688-3696. [DOI: 10.1021/acs.jpca.8b01233] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Affiliation(s)
- Hui Hu
- Department of Chemistry, Gottwald Center for the Sciences, University of Richmond, Richmond, Virginia 23173, United States
| | - Boyi Zhang
- Department of Chemistry, Gottwald Center for the Sciences, University of Richmond, Richmond, Virginia 23173, United States
| | - Adam Luxon
- Department of Chemistry, Gottwald Center for the Sciences, University of Richmond, Richmond, Virginia 23173, United States
| | - Thais Scott
- Department of Chemistry, Gottwald Center for the Sciences, University of Richmond, Richmond, Virginia 23173, United States
| | - Baoshan Wang
- College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, Hubei Province 430072, P.R. China
| | - Carol A. Parish
- Department of Chemistry, Gottwald Center for the Sciences, University of Richmond, Richmond, Virginia 23173, United States
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18
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Widjaja F, Max JP, Jin Z, Nash JJ, Kenttämaa HI. Gas-phase Reactivity of meta-Benzyne Analogs Toward Small Oligonucleotides of Differing Lengths. JOURNAL OF THE AMERICAN SOCIETY FOR MASS SPECTROMETRY 2017; 28:1392-1405. [PMID: 28456883 DOI: 10.1007/s13361-017-1655-6] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/21/2016] [Revised: 03/05/2017] [Accepted: 03/06/2017] [Indexed: 06/07/2023]
Abstract
The gas-phase reactivity of two aromatic carbon-centered σ,σ-biradicals (meta-benzyne analogs) and a related monoradical towards small oligonucleotides of differing lengths was investigated in a Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometer coupled with laser-induced acoustic desorption (LIAD). The mono- and biradicals were positively charged to allow for manipulation in the mass spectrometer. The oligonucleotides were evaporated into the gas phase as intact neutral molecules by using LIAD. One of the biradicals was found to be unreactive. The reactive biradical reacts with dinucleoside phosphates and trinucleoside diphosphates mainly by addition to a nucleobase moiety followed by cleavage of the glycosidic bond, leading to a nucleobase radical (e.g., base-H) abstraction. In some instances, after the initial cleavage, the unquenched radical site of the biradical abstracts a hydrogen atom from the neutral fragment, which results in a net nucleobase abstraction. In sharp contrast, the related monoradical mainly undergoes facile hydrogen atom abstraction from the sugar moiety. As the size of the oligonucleotides increases, the rate of hydrogen atom abstraction from the sugar moiety by the monoradical was found to increase due to the presence of more hydrogen atom donor sites, and it is the only reaction observed for tetranucleoside triphosphates. Hence, the monoradical only attacks sugar moieties in these substrates. The biradical also shows significant attack at the sugar moiety for tetranucleoside triphosphates. This drastic change in reactivity indicates that the size of the oligonucleotides plays a key role in the outcome of these reactions. This finding is attributed to more compact conformations in the gas phase for the tetranucleoside triphosphates than for the smaller oligonucleotides, which result from stronger stabilizing interactions between the nucleobases. Graphical Abstract ᅟ.
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Affiliation(s)
- Fanny Widjaja
- Department of Chemistry, Purdue University, West Lafayette, IN, 47906, USA
- , 1 Makeway Ave #15-03, Singapore, 228598, Singapore
| | - Joann P Max
- Department of Chemistry, Purdue University, West Lafayette, IN, 47906, USA
| | - Zhicheng Jin
- Department of Chemistry, Purdue University, West Lafayette, IN, 47906, USA
- Northeast Ohio Medical University, 4209 OH-44, Rootstown, OH, 44272, USA
| | - John J Nash
- Department of Chemistry, Purdue University, West Lafayette, IN, 47906, USA
| | - Hilkka I Kenttämaa
- Department of Chemistry, Purdue University, West Lafayette, IN, 47906, USA.
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19
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Kotha RR, Nash JJ, Kenttämaa HI. An Oxygen‐
peri
‐Bridged Quinolinium Cation and Its Monoradical Counterpart. European J Org Chem 2017. [DOI: 10.1002/ejoc.201601572] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Affiliation(s)
- Raghavendhar R. Kotha
- Department of Chemistry Purdue University 560 Oval Drive 47907‐2084 West Lafayette IN USA
| | - John J. Nash
- Department of Chemistry Purdue University 560 Oval Drive 47907‐2084 West Lafayette IN USA
| | - Hilkka I. Kenttämaa
- Department of Chemistry Purdue University 560 Oval Drive 47907‐2084 West Lafayette IN USA
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20
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de Oteyza DG, Pérez Paz A, Chen YC, Pedramrazi Z, Riss A, Wickenburg S, Tsai HZ, Fischer FR, Crommie MF, Rubio A. Noncovalent Dimerization after Enediyne Cyclization on Au(111). J Am Chem Soc 2016; 138:10963-7. [PMID: 27490459 DOI: 10.1021/jacs.6b05203] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/30/2023]
Abstract
We investigate the thermally induced cyclization of 1,2-bis(2-phenylethynyl)benzene on Au(111) using scanning tunneling microscopy and computer simulations. Cyclization of sterically hindered enediynes is known to proceed via two competing mechanisms in solution: a classic C(1)-C(6) (Bergman) or a C(1)-C(5) cyclization pathway. On Au(111), we find that the C(1)-C(5) cyclization is suppressed and that the C(1)-C(6) cyclization yields a highly strained bicyclic olefin whose surface chemistry was hitherto unknown. The C(1)-C(6) product self-assembles into discrete noncovalently bound dimers on the surface. The reaction mechanism and driving forces behind noncovalent association are discussed in light of density functional theory calculations.
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Affiliation(s)
- Dimas G de Oteyza
- Donostia International Physics Center , E-20018 San Sebastián, Spain.,Ikerbasque, Basque Foundation for Science , E-48011 Bilbao, Spain
| | - Alejandro Pérez Paz
- Nano-Bio Spectroscopy Group and ETSF, Universidad del País Vasco, CFM CSIC-UPV/EHU-MPC , 20018 San Sebastián, Spain
| | - Yen-Chia Chen
- Department of Physics, University of California , Berkeley, California 94720, United States
| | - Zahra Pedramrazi
- Department of Physics, University of California , Berkeley, California 94720, United States
| | - Alexander Riss
- Department of Physics, University of California , Berkeley, California 94720, United States
| | - Sebastian Wickenburg
- Department of Physics, University of California , Berkeley, California 94720, United States
| | - Hsin-Zon Tsai
- Department of Physics, University of California , Berkeley, California 94720, United States
| | - Felix R Fischer
- Department of Chemistry, University of California , Berkeley, California 94720, United States.,Materials Sciences Division, Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States.,Kavli Energy NanoSciences Institute at the University of California Berkeley and the Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States
| | - Michael F Crommie
- Department of Physics, University of California , Berkeley, California 94720, United States.,Materials Sciences Division, Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States.,Kavli Energy NanoSciences Institute at the University of California Berkeley and the Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States
| | - Angel Rubio
- Nano-Bio Spectroscopy Group and ETSF, Universidad del País Vasco, CFM CSIC-UPV/EHU-MPC , 20018 San Sebastián, Spain.,Max Planck Institute for the Structure and Dynamics of Matter , Luruper Chaussee 149, 22761 Hamburg, Germany.,Center for Free-electron Laser Science (CFEL) , Luruper Chaussee 149, 22761 Hamburg, Germany
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21
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Desai N, Thomas DA, Lee J, Gao J, Beauchamp JL. Eradicating mass spectrometric glycan rearrangement by utilizing free radicals. Chem Sci 2016; 7:5390-5397. [PMID: 30155192 PMCID: PMC6020757 DOI: 10.1039/c6sc01371f] [Citation(s) in RCA: 27] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/28/2016] [Accepted: 05/04/2016] [Indexed: 12/17/2022] Open
Abstract
We designed and synthesized a methylated free radical activated glycan sequencing reagent (Me-FRAGS) for eliminating mass spectrometric glycan rearrangement.
Mass spectrometric glycan rearrangement is problematic because it provides misleading structural information. Here we report on a new reagent, a methylated free radical activated glycan sequencing reagent (Me-FRAGS), which combines a free radical precursor with a methylated pyridine moiety that can be coupled to the reducing terminus of glycans. The collisional activation of Me-FRAGS-derivatized glycans generates a nascent free radical that concurrently induces abundant glycosidic bond and cross-ring cleavage without the need for subsequent activation. The product ions resulting from glycan rearrangement, including internal residue loss and multiple external residue losses, are precluded. Glycan structures can be easily assembled and visualized using a radical driven glycan deconstruction diagram (R-DECON diagram). The presence and location of N-acetylated saccharide units and branch sites can be identified from the characteristic dissociation patterns observed only at these locations. The mechanisms of dissociation are investigated and discussed. This Me-FRAGS based mass spectrometric approach creates a new blueprint for glycan structure analysis.
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Affiliation(s)
- Nikunj Desai
- Department of Chemistry and Biochemistry , Center for Quantitative Obesity Research , Montclair State University , 1 Normal Avenue , Montclair , NJ 07043 , USA .
| | - Daniel A Thomas
- Arthur Amos Noyes Laboratory of Chemical Physics , California Institute of Technology , 1200 East California Blvd , Pasadena , CA 91125 , USA .
| | - Jungeun Lee
- Department of Chemistry and Biochemistry , Center for Quantitative Obesity Research , Montclair State University , 1 Normal Avenue , Montclair , NJ 07043 , USA .
| | - Jinshan Gao
- Department of Chemistry and Biochemistry , Center for Quantitative Obesity Research , Montclair State University , 1 Normal Avenue , Montclair , NJ 07043 , USA .
| | - J L Beauchamp
- Arthur Amos Noyes Laboratory of Chemical Physics , California Institute of Technology , 1200 East California Blvd , Pasadena , CA 91125 , USA .
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22
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Vinueza NR, Jankiewicz BJ, Gallardo VA, LaFavers GZ, DeSutter D, Nash JJ, Kenttämaa HI. Reactivity Controlling Factors for an Aromatic Carbon-Centered σ,σ,σ-Triradical: The 4,5,8-Tridehydroisoquinolinium Ion. Chemistry 2015; 22:809-15. [DOI: 10.1002/chem.201502502] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/27/2015] [Revised: 09/27/2015] [Indexed: 11/07/2022]
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23
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Mu WH, Xia SY, Li JX, Fang DC, Wei G, Chass GA. Competing Mechanisms, Substituent Effects, and Regioselectivities of Nickel-Catalyzed [2 + 2 + 2] Cycloaddition between Carboryne and Alkynes: A DFT Study. J Org Chem 2015; 80:9108-17. [PMID: 26270257 DOI: 10.1021/acs.joc.5b01464] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Wei-Hua Mu
- Faculty
of Chemistry and Chemical Engineering, Yunnan Normal University, Kunming 650092, Yunnan, China
| | - Shu-Ya Xia
- Faculty
of Chemistry and Chemical Engineering, Yunnan Normal University, Kunming 650092, Yunnan, China
| | - Ji-Xiang Li
- Faculty
of Chemistry and Chemical Engineering, Yunnan Normal University, Kunming 650092, Yunnan, China
| | - De-Cai Fang
- Key
Laboratory of Theoretical and Computational Photochemistry, Ministry
of Education, College of Chemistry, Beijing Normal University, Beijing 100875, China
| | - Gang Wei
- CSIRO Manufacturing Flagship, Bradfield Road, West Lindfield, P.O. Box 218, Lindfield, NSW 2070, Australia
| | - Gregory A Chass
- School
of Biological and Chemical Sciences, Queen Mary University of London, London E1 4NS, U.K
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