Effect of base stacking on the acid-base properties of the adenine cation radical [A•+] in solution:: ESR and DFT studies

In this study, the acid-base properties of the adenine cation radical are investigated by means of experiment and theory. Adenine cation radical (A center dot(+)) is produced by one-electron oxidation of dAdo and of the stacked DNA-oligomer (dA)(6) by Cl(2)center dot(-) in aqueous glass (7.5 M LiCl in H(2)O and in D(2)O) 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 pK(a) value of A center dot(+) 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 center dot(+) occurs in dAdo in these glassy systems even at pH ca. 3. A center dot(+) found in (dA)(6) at 150 K also deprotonates on thermal annealing. The stability of A center dot(+) at 150 K in these systems is attributed to charge delocalization between stacked bases. Theoretical calculations at various levels (DFT B3LYP/6-31G(star), MPWB95, and HF-MP2) predict binding energies for the adenine stacked dinner cation radical of 12 to 16 kcal/mol. Further DFT B3LYP/6-31G(star) calculations predict that, in aqueous solution, monomeric A center dot(+) should deprotonate spontaneously (a predicted pK(a) of ca. -0.3 for A center dot(+)). However, the charge resonance stabilized dimer AA center dot(+) is predicted to result in a significant barrier to deprotonation and a calculated pK(a) of ca. 7 for the AA center dot(+) dimer which is 7 pH units higher than the monomer. These theoretical and experimental results suggest that A center dot(+) isolated in solution and A center dot(+) in adenine stacks have highly differing acid-base properties resulting from the stabilization induced by hole delocalization within adenine stacks.