Kinetics of PhotoinducedElectron Transfer between DNA Bases and Triplet 33′44′-BenzophenoneTetracarboxylic Acid in Aqueous Solution of Different pHs: Proton-CoupledElectron Transfer?

摘要

The kinetics of triplet state quenching of 3,3′,4,4′-benzophenone tetracarboxylic acid (BPTC) by DNA bases adenine, adenosine, thymine, and thymidine has been investigated in aqueous solution using time-resolved laser flash photolysis. The observation of the BPTC ketyl radical anion at λmax = 630 nm indicates that one electron transfer is involved in the quenching reactions. The pH-dependence of the quenching rate constants is measured in detail. As a result, the chemical reactivity of the reactants is assigned. The bimolecular rate constants of the quenching reactions between triplet BPTC and adenine, adenosine, thymine, and thymidine are kq = 2.3 × 10⁹ (4.7 < pH < 9.9), kq = 4.0 × 10⁹ (3.5 < pH < 4.7), kq = 1.0 × 10⁹ (4.7 < pH < 9.9), and kq = 4.0 × 10⁸ M^–1 s^–1 (4.7 < pH < 9.8), respectively. Moreover, it reveals that in strong basic medium (pH = 12.0) a keto–enol tautomerism of thymine inhibits its reaction with triplet BPTC. Such a behavior is not possible for thymidine because of its deoxyribose group. In addition, the pH-dependenceof the apparent electrochemical standard potential of thymine in aqueoussolution was investigated by cyclic voltammetry. The ΔE/ΔpH ≈ −59 mV/pH result is characteristicof proton-coupled electron transfer. This behavior, together withthe kinetic analysis, leads to the conclusion that the quenching reactionsbetween triplet BPTC and thymine involve one proton-coupled electrontransfer.