Kinetics of Photoinduced Electron Transfer between DNA Bases and Triplet 3,3′,4,4′-Benzophenone Tetracarboxylic Acid in Aqueous Solution of Different pH's: Proton-Coupled Electron Transfer? (original) (raw)

2012, The Journal of Physical Chemistry A

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 λ max = 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 k q = 2.3 × 10 9 (4.7 < pH < 9.9), k q = 4.0 × 10 9 (3.5 < pH < 4.7), k q = 1.0 × 10 9 (4.7 < pH < 9.9), and k q = 4.0 × 10 8 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-dependence of the apparent electrochemical standard potential of thymine in aqueous solution was investigated by cyclic voltammetry. The ΔE/ΔpH ≈ −59 mV/pH result is characteristic of proton-coupled electron transfer. This behavior, together with the kinetic analysis, leads to the conclusion that the quenching reactions between triplet BPTC and thymine involve one proton-coupled electron transfer.