Structure of radicals from X-irradiated guanine derivatives: an experimental and computational study of sodium guanosine dihydrate single crystals

J Phys Chem B. 2007 Feb 1;111(4):800-10. doi: 10.1021/jp0659608.

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.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Crystallization
  • DNA / chemistry
  • Deoxyguanine Nucleotides / chemistry*
  • Electron Spin Resonance Spectroscopy
  • Free Radicals
  • Guanine / chemistry*
  • Guanosine / analogs & derivatives*
  • Guanosine / chemistry*
  • Hydrogen / chemistry
  • Models, Chemical
  • Molecular Conformation
  • Nucleic Acid Conformation
  • Protons
  • Purines / chemistry
  • Temperature
  • X-Rays

Substances

  • Deoxyguanine Nucleotides
  • Free Radicals
  • Protons
  • Purines
  • Guanosine
  • Guanine
  • 2'-deoxyguanosine 5'-phosphate
  • Hydrogen
  • DNA