Effect of Base Stacking on the Acid−Base Properties of the Adenine Cation Radical [A•+] in Solution: ESR and DFT Studies

Abstract

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 pK_a 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 pK_a 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 pK_a 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.