The guanine cation radical: investigation of deprotonation states by ESR and DFT.
This work reports ESR studies that identify the favored site of deprotonation of the guanine cation radical (G*+) in an aqueous medium at 77 K. Using ESR and UV-visible spectroscopy, one-electron oxidized guanine is investigated in frozen aqueous D2O solutions of 2'-deoxyguanosine (dGuo) at low temperatures at various pHs at which the guanine cation radical, G*+ (pH 3-5), singly deprotonated species, G(-H)* (pH 7-9), and doubly deprotonated species, G(-2H)*- (pH > 11), are found. C-8-deuteration of dGuo to give 8-D-dGuo removes the major proton hyperfine coupling at C-8. This isolates the anisotropic nitrogen couplings for each of the three species and aids our analyses. These anisotropic nitrogen couplings were assigned to specific nitrogen sites by use of 15N-substituted derivatives at N1, N2, and N3 atoms in dGuo. Both ESR and UV-visible spectra are reported for each of the species: G*+, G(-H)*, and G(-2H)*-. The experimental anisotropic ESR hyperfine couplings are compared to those obtained from DFT calculations for the various tautomers of G(-H)*. Using the B3LYP/6-31G(d) method, the geometries and energies of G*+ and its singly deprotonated state in its two tautomeric forms, G(N1-H)* and G(N2-H)*, were investigated. In a nonhydrated state, G(N2-H)* is found to be more stable than G(N1-H)*, but on hydration with seven water molecules G(N1-H)* is found to be more stable than G(N2-H)*. The theoretically calculated hyperfine coupling constants (HFCCs) of G*+, G(N1-H)*, and G(-2H)*- match the experimentally observed HFCCs best on hydration with seven or more waters. For G(-2H)*-, the hyperfine coupling constant (HFCC) at the exocyclic nitrogen atom (N2) is especially sensitive to the number of hydrating water molecules; good agreement with experiment is not obtained until nine or 10 waters of hydration are included.
Adhikary
Amitava
A
Department of Chemistry, Oakland University, Rochester, Michigan 48309, USA.
Kumar
Anil
A
Becker
David
D
Sevilla
Michael D
MD
D000465:Algorithms; D016880:Anisotropy; D002412:Cations; D003849:Deoxyguanosine / Q000031:analogs & derivatives; D017666:Deuterium Oxide / Q000737:chemistry; D004578:Electron Spin Resonance Spectroscopy; D004583:Electrons; D005609:Free Radicals / Q000737:chemistry*; D006147:Guanine / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D007536:Isomerism; D008968:Molecular Conformation; D009584:Nitrogen / Q000737:chemistry; D010084:Oxidation-Reduction; D011522:Protons; D013056:Spectrophotometry, Ultraviolet; D013696:Temperature; D014867:Water / Q000737:chemistry
D002412:Cations; D005609:Free Radicals; D011522:Protons; D014867:Water; D006147:Guanine; D003849:Deoxyguanosine; D017666:Deuterium Oxide; D009584:Nitrogen