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?

Abstract

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-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.

Effect of Amino Group Charge on the Photooxidation Kinetics of Aromatic Amino Acids

“…Earlier, the two ground states pK a1 = 3.2 and pK a2 = 5.2 estimated using UV−vis spectroscopy and the two triplet excited states pK a1 = 2.1 and pK a2 = 4.7 obtained by triplet−triplet absorbance titration have been published. 25 Using the dependences of 1 H and 13 C chemical shifts on pH, we have determined the acidity constants of TCBP in D 2 O: pK a1 * = 2.5 ± 0.1, pK a2 * = 2.7 ± 0.1, pK a3 * = 4.5 ± 0.1, and pK a4 * = 5.2 ± 0.1. Simulating the chemical shift dependences with four pK a values has led to better agreement than that using two pK a values.…”

Kinetics of the Oxidation of Thymine and Thymidine by Triplet 2,2′-Dipyridyl in Aqueous Solutions at Different pH Values

“…In Figure 8, the pH dependence of the apparent standard oxidation potential of thymine is given. 18 It shows a linear variation of E ap,thy 0 with pH characterized by a slope of −55.2 mV per pH unit. This observation indicates a one-proton coupled one-electron transfer (PCET) during the oxidation of thymine.…”

“…On the other hand, the oxidation of thymine, and thymidine by photoexcited dipyridyl, similar to that of nucleobases and their nucleosides by menadion, 17 benzophenone, 18 and riboflavin, 19 provides the susceptible information in the direct effect of ionizing radiation on DNA. The formation of such strong oxidizing intermediates, which may induce transformations of damaged nucleobases and nucleosides, for this reason, has been of great interest for DNA free-radical chemistry.…”

Oxidation of Purine Nucleotides by Triplet 3,3′,4,4′-Benzophenone Tetracarboxylic Acid in Aqueous Solution: pH-Dependence

“…Thus, after the irradiation of a solution containing TCBP, and GMP or AMP, TCBP triplets are generated first, and then quenching occurs. Wavelengths of 590 nm (pH < p K a2 TCBP = 4.9) and 550 nm (pH > p K a2 TCBP = 4.9) 18 were taken to measure the decay of triplet TCBP. Under these conditions, the triplet-excited dye exhibits absorption maxima.…”

“…Under these conditions, the triplet-excited dye exhibits absorption maxima. Since the concentration of TCBP is low (1 × 10 –4 M) and the triplet–triplet annihilation is negligible, the triplet-excited TCBP decay in the absence of a quencher follows the first-order kinetics with k d = (5.5–8) × 10 5 s –1 . , In the presence of a quencher, the reaction between the triplet TCBP and the quencher occurs with the pseudo-first-order rate constant k obs proportional to the concentration of the quencher, [Q]; k = k q obs [Q], where k q obs is a quenching rate constant. Transient absorption decays of 3 TCBP with an increase in concentration of GMP and the corresponding Stern–Volmer relation are shown in Figure a.…”