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 D 2

This work presents evidence that photo-excitation of guanine radical cations results in high yields of deoxyribose sugar radicals in DNA, guanine deoxyribonucleosides and deoxyribonucleotides. In dsDNA at low temperatures, formation of C1′• is observed from photo-excitation of G•+ in the 310–480 nm range with no C1′• formation observed ≥520 nm. Illumination of guanine radical cations in 2′dG, 3′-dGMP and 5′-dGMP in aqueous LiCl glasses at 143 K is found to result in remarkably high yields (∼85–95%) of sugar radicals, namely C1′•, C3′• and C5′•. The amount of each of the sugar radicals formed varies dramatically with compound structure and temperature of illumination. Radical assignments were confirmed using selective deuteration at C5′ or C3′ in 2′-dG and at C8 in all the guanine nucleosides/tides. Studies of the effect of temperature, pH, and wavelength of excitation provide important information about the mechanism of formation of these sugar radicals. Time-dependent density functional theory calculations verify that specific excited states in G•+ show considerable hole delocalization into the sugar structure, in accord with our proposed mechanism of action, namely deprotonation from the sugar moiety of the excited molecular radical cation.

In this work, it is shown that the incorporation of an 8-deuteroguanine (G*) moiety in DNA-oligomers allows for direct determination at 77 K of (i) the location of holes (i.e., the radical site) within dsDNA at specific base sites, even within stacks of G, as well as (ii) the protonation state of the hole at that site. These findings are based on our work and demonstrate that selective deuteration at C-8 on guanine moiety in dGuo results in an ESR signal from the guanine cation radical (G*• + ) which is easily distinguishable from that of the undeuterated guanine cation radical (G• + ). G*• + is also found to be easily distinguishable from its conjugate base, the N1-deprotonated radical, G*(−H)•. Our ESR results clearly establish that at 77 K (i) one-electron oxidized guanine in double stranded DNAoligomers exists as the deprotonated neutral radical G(−H)• as a result of facile proton transfer to the hydrogen bonded cytosine, and (ii) the hole is preferentially located at the 5′-end in several ds DNAoligomers with a GGG sequence.

In this study, the acid–base properties of the adenine cation radical are investigated by means of experiment and theory. Adenine cation radical (A•+) is produced by one-electron oxidation of dAdo and of the stacked DNA-oligomer (dA)6 by Cl2•− in aqueous glass (7.5 M LiCl in H2O and in D2O) and investigated by ESR spectroscopy. Theoretical calculations and deuterium substitution at C8–H and N6–H in dAdo aid in our assignments of structure. We find the pKa value of A•+ in this system to be ca. 8 at 150 K in seeming contradiction to the accepted value of ≤ 1 at ambient temperature. However, upon thermal annealing to ≥160 K, complete deprotonation of A•+ occurs in dAdo in these glassy systems even at pH ca. 3. A•+ found in (dA)6 at 150 K also deprotonates on thermal annealing. The stability of A•+ at 150 K in these systems is attributed to charge delocalization between stacked bases. Theoretical calculations at various levels (DFT B3LYP/6-31G*, MPWB95, and HF-MP2) predict binding energies for the adenine stacked dimer cation radical of 12 to 16 kcal/mol. Further DFT B3LYP/6-31G* calculations predict that, in aqueous solution, monomeric A•+ should deprotonate spontaneously (a predicted pKa of ca. −0.3 for A•+). However, the charge resonance stabilized dimer AA•+ is predicted to result in a significant barrier to deprotonation and a calculated pKa of ca. 7 for the AA•+ dimer which is 7 pH units higher than the monomer. These theoretical and experimental results suggest that A•+ isolated in solution and A•+ in adenine stacks have highly differing acid–base properties resulting from the stabilization induced by hole delocalization within adenine stacks.

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