Scanning electrochemical microscopic study of electron transfer at the oil/water interface as a biomembrane model

油水界面上的电子转移对于理解生物膜上的电子转移过程至关重要，其中包括呼吸电子传递反应和天然抗氧化剂（如维生素C和维生素e）的协同抗氧化活性。本研究采用扫描电化学显微镜（SECM）研究了生物相关化合物的界面电子转移反应机制。在2004财政年度，SECM被用于确定四苯基卟啉锌自由基阳离子与天然抗坏血酸（维生素C）和绿原酸在硝基苯/水和苯/水界面上的电子转移速率常数。测定的表观速率常数与抗氧化剂浓度的平方根成正比。这种依赖关系既不能用界面上非均相电子转移的“电子转移机制”来解释，也不能用涉及自由基阳离子向水相分布的“离子转移机制”来解释。然而，这种依赖关系可以用一种新的机制来解释，即在抗氧化剂部分分布到油相后，电子转移应该发生在油相。2005财年，利用SECM研究了油/水界面的非均相（即真）电子转移机理。十甲基二茂铁和六氰高铁在油/水界面上的电子转移速率常数（κ）依赖于电子转移反应的吉布斯自由能（ΔG），但没有表现出马库斯理论所期望的抛物线依赖性。速率常数有一个上限，这支持了我们之前的理论，即考虑了氧化还原物质在油/水界面的“微观”扩散。

Electron transfer at the oil/water interface is important for understanding electron transfer processes at biomembranes, which include respiratory electron transport reactions and synergistic antioxidant activities of natural antioxidants such as vitamin C and vitamin E. In this study scanning electrochemical microscopy (SECM) was employed to examine the reaction mechanisms of interfacial electron transfer reactions for biologically relevant compounds.In the 2004 fiscal year, SECM was used to determine rate constants for the electron transfer between zinc tetraphenylporphyrin radical cation and natural antioxidants including ascorbic acid (vitamin C) and chlorogenic acid at nitrobenzene/water and benzene/water interfaces. The apparent rate constants determined were proportional to the square root of the antioxidant concentration. This dependence could not be elucidated in terms of either the "electron-transfer mechanism" due to heterogeneous electron transfer at the interface or the "ion-transfer mechanism" involving the distribution of the radical cation to the aqueous phase. However, the dependence could be well explained by a new mechanism, in which the electron transfer should occur in the oil phase after the partial distribution of the antioxidant to the oil phase.In the 2005 fiscal year, SECM was employed to study the mechanism of heterogeneous (i.e., true) electron transfer at oil/water interfaces. The rate constants (κ) determined for the electron transfer between decamethylferrocene and hexacyanoferrate at oil/water interfaces depended on the Gibbs free energy (ΔG) of the electron transfer reaction, but did not show the parabolic dependence expected from the Marcus theory. There was an upper limit of the rate constant, which supported our previous theory in which the "microscopic" diffusion of redox species at the oil/water interface was taken into consideration.