Hirst J, Armstrong F A
Inorganic Chemistry Laboratory, Oxford, England.
Anal Chem. 1998 Dec 1;70(23):5062-71. doi: 10.1021/ac980557l.
The rapid electron-exchange characteristics of metalloproteins adsorbed at a pyrolytic graphite "edge" electrode have been studied by analog dc cyclic voltammetry at scan rates up to 3000 V s-1. The voltammetry of four proteins, azurin (a "blue" copper protein) and three 7Fe ferredoxins, reveals oxidation and reduction peaks that display only modest increases in width and peak separation as the scan rate is raised. This is indicative of a substantially homogeneous population of noninteracting centers which undergo rapid electron exchange with the electrode. Both the Butler--Volmer and Marcus models have been tested. The electrochemical kinetics, as reflected by k0 (the rate at zero overpotential), are too fast to allow the determination of reorganization energies by this method. Nonetheless, the rapid and energetically coherent nature of the electron transfer enables the cyclic oxidation and reduction of protein redox centers to be examined on a time scale sufficiently short to recognize coupled processes occurring in the millisecond time domain, which are characteristic of the protein under investigation. Two of the ferredoxins display increasingly asymmetric voltammetry as the scan rate is increased, which is attributed to the coupling of electron transfer to conformational (or orientational) changes. For azurin, the use of higher electrolyte concentrations enables studies to be made at scan rates up to 3000 V s-1, from which a standard electron-transfer rate constant in the region of 5000 s-1 is obtained. At these high scan rates, azurin still shows very symmetrical voltammograms but with peak shapes displaying a more gradual decrease in current, at increasing overpotential, than is predicted using realistic values of the reorganization energy. The ability to measure even faster rate constants and access coupled reactions occurring in shorter time domains is likely to be limited by complex processes occurring on the graphite surface.
通过模拟直流循环伏安法，在高达3000 V s-1的扫描速率下研究了吸附在热解石墨“边缘”电极上的金属蛋白的快速电子交换特性。四种蛋白质（即天青蛋白（一种“蓝色”铜蛋白）和三种7Fe铁氧化还原蛋白）的伏安法显示，随着扫描速率的提高，氧化峰和还原峰的宽度和峰间距仅适度增加。这表明存在大量基本均匀的非相互作用中心，它们与电极进行快速电子交换。巴特勒-沃尔默模型和马库斯模型均已得到检验。由k0（零过电位时的速率）反映的电化学动力学太快，无法用这种方法确定重组能。尽管如此，电子转移的快速性和能量相干性使得能够在足够短的时间尺度上研究蛋白质氧化还原中心的循环氧化和还原，从而识别出在毫秒时域内发生的耦合过程，这是所研究蛋白质的特征。随着扫描速率的增加，其中两种铁氧化还原蛋白的伏安法显示出越来越不对称的情况，这归因于电子转移与构象（或取向）变化的耦合。对于天青蛋白，使用更高的电解质浓度能够在高达3000 V s-1的扫描速率下进行研究，由此获得了约5000 s-1的标准电子转移速率常数。在这些高扫描速率下，天青蛋白仍显示出非常对称的伏安图，但与使用重组能的实际值预测的情况相比，在过电位增加时，峰形显示电流下降更为平缓。测量更快的速率常数以及研究在更短时间域内发生的耦合反应的能力可能会受到石墨表面发生的复杂过程的限制。
