title: α-Hydrogen Abstraction by •OH and •SH Radicals from Amino Acids and Their Peptide Derivatives.
abstract: We have used computational quantum chemistry to investigate the thermochemistry of α-hydrogen abstraction from the full set of amino acids normally found in proteins, as well as their peptide forms, by •OH and •SH radicals. These reactions, with their reasonable complexity in the electronic structure (at the α-carbon), are chosen as a consistent set of models for conducting a fairly robust assessment of theoretical procedures. Our benchmarking investigation shows that, in general, the performance for the various classes of theoretical methods improves in the order nonhybrid DFT → hybrid DFT → double-hybrid DFT → composite procedures. More specifically, we find that the DSD-PBE-P86 double-hybrid DFT procedure yields the best agreement with our high-level W1X-2 vibrationless barriers and reaction energies for this particular set of systems. A significant observation is that, when one considers relative instead of absolute values for the vibrationless barriers and reaction energies, even nonhybrid DFT procedures perform fairly well. To exploit this feature in a cost-effective manner, we have examined a number of multilayer schemes for the calculation of reaction energies and barriers for the abstraction reactions. We find that accurate values can be obtained when a "core" of glycine plus the abstracting radical is treated by DSD-PBE-P86, and the substituent effects are evaluated with M06-2X. Inspection of the set of calculated thermochemical data shows that the correlation between the free energy barriers and reaction free energies is strongest when the reactions are either endergonic or nearly thermoneutral.
authors.affiliation: School of Chemistry, University of Sydney , Sydney, NSW 2006, Australia.|ARC Centre of Excellence for Free Radical Chemistry and Biotechnology , Australia.
authors.forename: Bun
authors.initials: B
authors.lastname: Chan
authors.affiliation: School of Chemistry and Biochemistry, University of Western Australia , Perth, Washington 6009, Australia.
authors.forename: Amir
authors.initials: A
authors.lastname: Karton
authors.affiliation: ARC Centre of Excellence for Free Radical Chemistry and Biotechnology , Australia.|Research School of Chemistry, Australian National University , Canberra, ACT 2600, Australia.
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Easton
authors.affiliation: School of Chemistry, University of Sydney , Sydney, NSW 2006, Australia.|ARC Centre of Excellence for Free Radical Chemistry and Biotechnology , Australia.
authors.forename: Leo
authors.initials: L
authors.lastname: Radom
mesh_terms: D000596:Amino Acids / Q000737:chemistry*; D006859:Hydrogen / Q000737:chemistry*; D017665:Hydroxyl Radical / Q000737:chemistry*; D008956:Models, Chemical; D008958:Models, Molecular; D010455:Peptides / Q000737:chemistry*; D011789:Quantum Theory; D013438:Sulfhydryl Compounds / Q000737:chemistry; D013816:Thermodynamics
chemical_list: D000596:Amino Acids; D010455:Peptides; D013438:Sulfhydryl Compounds; D017665:Hydroxyl Radical; D006859:Hydrogen