Scanning Optical Spectroelectrochemistry: Applications in Protein Redox Potential Measurements

The accurate measurement of redox potentials of small molecules is a relatively straightforward task using electrochemical methods such as cyclic voltammetry. However, proteins, in most cases, are not amenable to the same approach due to slow heterogeneous electron transfer and the possibility of denaturing at the electrode surface. This necessitates the use of small molecular weight redox mediators to facilitate electron transfer. This leads to spectroelectrochemical techniques where the applied electrochemical potential is coupled to a spectroscopic signal of the protein. Traditionally this is done at different applied (fixed) potentials akin to an electrochemical titration, but the time required for electrochemical equilibrium to be established, and its consistent application, are major sources of experimental error. Here we have utilised a continuously scanning potential synchronised with time-resolved UV-vis spectroscopy to provide an automated approach that can be used to measure protein redox potentials accurately in an expedient manner. The test cases are the heme proteins cytochrome c and myoglobin. The scope and limitations of the method are discussed.