Photochemically induced electron transfer

Biochemical reactions involving electron transfer between substrates or enz yme cofactors are both common and physiologically important; they have been studied by means of a variety of techniques. In this paper we review the a pplication of photochemical methods to the study of intramolecular electron transfer in hemoproteins, thus selecting a small, well-defined sector of t his otherwise enormous field. Photoexcitation of the heme populates short-l ived excited states which decay by thermal conversion and do not usually tr ansfer electrons, even when a suitable electron acceptor is readily availab le, e.g., in the form of a second oxidized heme group in the same protein; because of this, the experimental setup demands some manipulation of the he moprotein. In this paper we review three approaches that have been studied in detail: (i) the covalent conjugation to the protein moiety of an organic ruthenium complex, which serves as the photoexcitable electron donor tin t his case the heme acts as the electron acceptor); (ii) the replacement of t he heme group with a phosphorescent metal-substituted porphyrin, which on p hotoexcitation populates long-lived excited states, capable of acting as el ectron donors (clearly the protein must contain some other cofactor acting as the electron acceptor, most often a second heme group in the oxidized st ate); (iii) the combination of the reduced heme with CO (the photochemical breakdown of the iron-CO bond yields transiently the ground-state reduced h eme which is able to transfer one electron (or a fraction of it) to an oxid ized electron acceptor in the protein; this method uses a "mixed-valence hy brid" state of the redox active hemoprotein and has the great advantage of populating on photoexcitation an electron donor at physiological redox pote ntial). (C) 2001 Academic Press.