title: Structure of radicals from X-irradiated guanine derivatives: an experimental and computational study of sodium guanosine dihydrate single crystals.
abstract: In sodium guanosine dihydrate single crystals, the guanine moiety is deprotonated at N1 due to growth from high-pH (>12) solutions. Electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) studies of crystals X-irradiated at 10 K detected evidence for three radical forms. Radical R1, characterized by two proton and two nitrogen hyperfine interactions, was identified as the product of net hydrogenation at N7 of the N1-deprotonated guanine unit. R1 exhibited an unusually distorted structure leading to net positive isotropic components of the hydrogen alpha-couplings. Radical R2, characterized by one proton and one nitrogen hyperfine coupling, was identified as the primary electron-loss product. This product is equivalent to that of deprotonation at N1 by the guanine cation and represents the first ENDOR characterization of that product. Radical R3, characterized by a single hydrogen hyperfine coupling, was identified as the product of net dehydrogenation at C1' of the ribose moiety. The identification of radicals R1-R3 was supported by density functional theory (DFT) calculations on several possible structures using the B3LYP/6-311G(2df,p)//6-31G(d,p) approach. Radical R4, detected after warming the crystals to room temperature, was identified as the well-known product of net hydrogenation of C8 of the (N1-deprotonated) guanine component. Radical R1, evidently formed by protonation of the primary electron addition product, was present as roughly 60% of the total radicals detected at 10 K. Radical R2 was present as roughly 27% of the total yield, and the concentration of R3 contributed the remaining 13%. R3 is evidently the product of one-electron oxidation followed by deprotonation; thus, the balance of oxidation and reduction products is approximately equal within experimental uncertainty.
authors.affiliation: Department of Physics and Astronomy, Georgia State University, P.O. Box 4106, Atlanta, Georgia 30302-4106, USA.
authors.forename: Nayana
authors.initials: N
authors.lastname: Jayatilaka
authors.forename: William H
authors.initials: WH
authors.lastname: Nelson
mesh_terms: D003460:Crystallization; D004247:DNA / Q000737:chemistry; D003848:Deoxyguanine Nucleotides / Q000737:chemistry*; D004578:Electron Spin Resonance Spectroscopy; D005609:Free Radicals; D006147:Guanine / Q000737:chemistry*; D006151:Guanosine / Q000031:analogs & derivatives* / Q000737:chemistry*; D006859:Hydrogen / Q000737:chemistry; D008956:Models, Chemical; D008968:Molecular Conformation; D009690:Nucleic Acid Conformation; D011522:Protons; D011687:Purines / Q000737:chemistry; D013696:Temperature; D014965:X-Rays
chemical_list: D003848:Deoxyguanine Nucleotides; D005609:Free Radicals; D011522:Protons; D011687:Purines; D006151:Guanosine; D006147:Guanine; C007257:2'-deoxyguanosine 5'-phosphate; D006859:Hydrogen; D004247:DNA