Blue-light photodegradation of ferricyanide under protein relevant conditions

Ferricyanide is commonly used as a reoxidant in photochemical studies of redox proteins including cytochromes, photosystem II and flavoproteins. A low-spin d⁵ complex, [Fe(III)(CN)₆]^3- is a powerful electron acceptor which efficiently reoxidises photo-generated radical species. Unfortunately, ferricyanide itself absorbs strongly in the blue and a better understanding of its own photochemistry is required. Here, we present a combined UV/Vis and infrared spectroscopic study of the blue light photo-induced degradation of ferricyanide under conditions commonly employed in photochemical studies of proteins. Clear differences are observed in the photochemistry in pure water, Tris buffer and 20% glycerol solution, which are interpreted in terms of solvent-ligand exchange and ligand to metal charge transfer. The implications for photochemical studies of proteins employing ferricyanide as a reoxidant are discussed.