Proton-coupled electron transfer of tyrosine oxidation:: Buffer dependence and parallel mechanisms
Proton-coupled electron transfer of tyrosine oxidation:: Buffer dependence and parallel mechanisms
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DOI：
10.1021/ja073012u
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发表时间：
2007-12-19
影响因子：
15
通讯作者：
Hammarstrom, Leif
中科院分区：
文献类型：
--
作者：
Irebo, Tania;Reece, Steven Y.;Hammarstrom, Leif
研究了酪氨酸与金属配合物的质子耦合电子转移（PCET）。该反应由金属络合物的激光闪光激发引起，并且PCET是双向的，电子转移到激发的或闪光淬灭的氧化的金属络合物，质子转移到溶液中的水或添加的缓冲液。我们发现了三种不同的PCET机制之间的竞争：（1）以水作为质子受体的协同PCET，其确实显示了如先前报道的pH依赖性（Sjodin，M.; Styring，S.; Akermark，B.;孙湖;哈马斯特伦湖J. Am. Soc.2000，122，3932）;（2）逐步电子转移-质子转移（ETPT），其不依赖于pH;（3）缓冲液辅助的协同PCET。反应2的相对重要性随着氧化剂强度的增加而增加，而反应1的相对重要性随着pH值的增加而增加。在较高的缓冲浓度下，反应3变得重要，并且速率遵循预期的对缓冲碱浓度的一阶依赖性。最重要的是，反应1的pH依赖性，在log k与pH的图中斜率为0.4-0.5，与缓冲液无关，并且不能用简单的一级依赖于[OH-]，[H3 O +]或缓冲物质的反应方案来解释。
The proton-coupled electron transfer (PCET) from tyrosine covalently linked to a metal complex has been studied. The reaction was induced by laser flash excitation of the metal complex, and PCET was bidirectional, with electron transfer to the excited or flash-quenched oxidized metal complex and proton transfer to water or added buffers in the solution. We found a competition between three different PCET mechanisms: (1) A concerted PCET with water as the proton acceptor, which indeed shows a pH-dependence as earlier reported (Sjodin, M.; Styring, S.; Akermark, B.; Sun, L.; Hammarstrom, L. J. Am. Chem. Soc. 2000,122, 3932); (2) a stepwise electron transfer-proton transfer (ETPT) that is pH-independent; (3) a buffer-assisted concerted PCET. The relative importance of reaction 2 increases with oxidant strength, while that of reaction 1 increases with pH. At higher buffer concentrations reaction 3 becomes important, and the rate follows the expected first-order dependence on the concentration of the buffer base. Most importantly, the pH-dependence of reaction 1, with a slope of 0.4-0.5 in a plot of log k vs pH, is independent of buffer and cannot be explained by reaction schemes with simple first-order dependencies on [OH-], [H3O+], or buffer species.