Photochemically induced electron transfer.
Biochemical reactions involving electron transfer between substrates or enzyme cofactors are both common and physiologically important; they have been studied by means of a variety of techniques. In this paper we review the application of photochemical methods to the study of intramolecular electron transfer in hemoproteins, thus selecting a small, well-defined sector of this otherwise enormous field. Photoexcitation of the heme populates short-lived excited states which decay by thermal conversion and do not usually transfer electrons, even when a suitable electron acceptor is readily available, 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 hemoprotein. 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 (in this case the heme acts as the electron acceptor); (ii) the replacement of the heme group with a phosphorescent metal-substituted porphyrin, which on photoexcitation populates long-lived excited states, capable of acting as electron donors (clearly the protein must contain some other cofactor acting as the electron acceptor, most often a second heme group in the oxidized state); (iii) the combination of the reduced heme with CO (the photochemical breakdown of the iron-CO bond yields transiently the ground-state reduced heme which is able to transfer one electron (or a fraction of it) to an oxidized electron acceptor in the protein; this method uses a "mixed-valence hybrid" state of the redox active hemoprotein and has the great advantage of populating on photoexcitation an electron donor at physiological redox potential).
Bellelli
A
A
Department of Biochemical Sciences, University of Rome La Sapienza, and CNR Center of Molecular Biology, P.le A. Moro, 5, Rome, 00185, Italy.
Brunori
M
M
Brzezinski
P
P
Wilson
M T
MT
D001703:Biophysics / Q000379:methods*; D003574:Cytochrome c Group / Q000737:chemistry; D004579:Electron Transport; D003576:Electron Transport Complex IV / Q000737:chemistry; D004583:Electrons; D006418:Heme / Q000737:chemistry; D007700:Kinetics; D008024:Ligands; D008027:Light; D008956:Models, Chemical; D009572:Nitrite Reductases / Q000737:chemistry; D010084:Oxidation-Reduction; D010777:Photochemistry / Q000379:methods*; D012428:Ruthenium / Q000737:chemistry; D013816:Thermodynamics; D013997:Time Factors; D015032:Zinc / Q000737:chemistry
D003574:Cytochrome c Group; D008024:Ligands; D006418:Heme; D012428:Ruthenium; D009572:Nitrite Reductases; D003576:Electron Transport Complex IV; D015032:Zinc