Lambert C R, Kochevar I E
Department of Dermatology, Harvard Medical School, Boston, MA, USA. Lambert@helix mgh.harvard.edu
Photochem Photobiol. 1997 Jul;66(1):15-25. doi: 10.1111/j.1751-1097.1997.tb03133.x.
The potential for electron transfer quenching of rose bengal triplet (3RB2-) to compete with energy transfer quenching by oxygen was evaluated. Rate constants for oxidative and reductive quenching were measured in buffered aqueous solution, acetonitrile and in small unilamellar liposomes using laser flash photolysis. Biologically relevant quenchers were used that varied widely in structure, reduction potential and charge. Radical ion yields (phi i) were measured by monitoring the absorption of the rose bengal semireduced (RB.3-) and semioxidized (RB.-) radicals. The results in solution were analyzed as a function of the free energy for electron transfer (delta G) calculated using the Weller equation including electrostatic terms. Exothermic oxidative quenching was about 10-fold faster than exothermic reductive quenching in aqueous solution. The quenching rate constants decreased as delta G approached zero in both aqueous and acetonitrile solution. Exceptions to these generalizations were observed that could be rationalized by specific steric or electrostatic effects or by a change in mechanism. The results suggest that electron transfer reactions with some potential quenchers in cells could compete with formation of singlet oxygen [O2(1 delta g)]. Values of phi i were generally greater for reductive quenching and, for oxidative quenching, greater in acetonitrile than in buffer. Electron transfer quenching of 3RB2- in liposomes, below the phase transition temperature was slower than in solution for both lipid-soluble and water-soluble quenchers indicating that these reactions may not compete with formation of O2(1 delta g) during cell photosensitization.
评估了孟加拉玫瑰三重态（3RB2-）的电子转移猝灭与氧的能量转移猝灭相竞争的可能性。使用激光闪光光解在缓冲水溶液、乙腈和小单层脂质体中测量了氧化猝灭和还原猝灭的速率常数。使用了结构、还原电位和电荷差异很大的具有生物学相关性的猝灭剂。通过监测半还原的（RB.3-）和半氧化的（RB.-）孟加拉玫瑰自由基的吸收来测量自由基离子产率（phi i）。将溶液中的结果作为使用包括静电项的韦勒方程计算的电子转移自由能（δG）的函数进行分析。在水溶液中，放热氧化猝灭比放热还原猝灭快约10倍。在水溶液和乙腈溶液中，随着δG接近零，猝灭速率常数均降低。观察到这些一般规律的例外情况，这些例外情况可以通过特定的空间或静电效应或机制变化来合理解释。结果表明，细胞中与某些潜在猝灭剂的电子转移反应可能与单线态氧[O2(1Δg)]的形成相竞争。对于还原猝灭，phi i值通常更大，对于氧化猝灭，在乙腈中的值大于在缓冲液中的值。在脂质体中低于相变温度时，对于脂溶性和水溶性猝灭剂，3RB2-的电子转移猝灭都比在溶液中慢，这表明这些反应在细胞光敏化过程中可能不会与O2(1Δg)的形成相竞争。
