Spectroscopic investigation of flavoproteins: Mechanistic differences between (electro)chemical and photochemical reduction and oxidation

The spectroelectrochemical investigation of proteins is an important method to study their redox properties and to elucidate protein ET pathways. Such studies are often more complicated for proteins than for small molecules because the redox-active cofactor is not easily accessible and/or the proteins are quite sensitive. For the determination of redox potentials it is necessary to work under equilibrium conditions which can be achieved by the presence of redox mediators. The proteins can be reduced and oxidized by chemical, electrochemical, and photochemical methods, each of which are briefly discussed. Differences in the mechanism of chemical and photochemical reduction of flavoproteins are pointed out. Photochemical reduction by irradiation with blue-light in the presence of EDTA can lead to the formation of flavoprotein semiquinone radicals which otherwise could not be observed.