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Observation of Electron Transfer between a Silver Electrode and Putidaredoxin Using Electromodulated Reflectance Spectroscopy

This paper reports measurements of the redox reaction rate for the {2Fe-2S} protein puditaredoxin (Pdx) on bare and bekanamycin-modified silver electrodes. The electron turnover rates were determined using electroreflectance (ER) and cyclic voltammetry (CV). The measurements were analyzed with two kinetic models: (1) strongly adsorbed protein layer and (2) freely diffusing protein. For the adsorbed layer model, the rates obtained with ER were almost two orders of magnitude greater than the rates obtained with CV. For the freely diffusing model, the rates determined by the two methods were similar, suggesting that this kinetic model is more appropriate. Modification of the Ag electrode with bekanamycin resulted in a negative 0.1-V shift of the Pdx formal potential, but showed only a small increase in the observed redox reaction rates. The measured rate constants were compared with the predictions of the Marcus theory of electron transfer.