Electron Transfer Through Macromolecular Systems

Electron transfer between metal complexes which can be in intimate contact has been the subject of systematic study for about four decades. A major conclusion of the vast amount of work which has been done with intermolecular reactions of ordinary metal complexes is that the reactions are adiabatic, or nearly so (i.e., the only barriers to the reactions are the work of bringing the reagents into contact and the work of exciting them to the isoergic state, which is the configuration reached after the nuclei have readjusted so that the energy of the system is independent of the alternate sites the electron occupies). In adiabatic transfer, the rate of chemical change does not depend on the frequency of electron transfer between the two sites in the isoergic state. The measurement of the rate of electron transfer over large distances, especially when the intervening matter is made up of protein, has been a matter of great interest. At present, it is a very active field of investigation and several different methods for making such measurements have been introduced. The results obtained with one such method, developed by S. S. Isied in 1973, are emphasized. A key feature of the method is that reactions are studied in the intramolecular mode. This is a great simplification because the work of assembling the precursor complex is no longer a factor, and the interesting effects which arise from nonadiabatic behavior are more directly exposed. The method was first applied to simple bridging groups such as 4,4′-bipyridine, which tie the metal-containing moieties (NH₃)₅Co(III) and (NH₃)₅ Ru(III) together. An external reducing agent reduces Ru(III) in preference to Co(III), and the subsequent chemical change, which involves reduction of Co(III) by Ru(II) by an intramolecular process, can be followed spectrophotometrically. The work with these simple bridging ligands showed that unless measures are taken to uncouple the two centers electronically, electron transfer in these systems is adiabatic, a conclusion confirmed by studies of the properties of mixed valence molecules with the same bridging groups. Isied has gone on to study electron transfer through polyprolines using the same general kind of technique. Even with the simplest bridging group of the series, the reactions are nonadiabatic. They become quite slow as the length of the polypeptide chain increases, and with longer chains a conformation change in which the metal centers are brought closer together precedes electron transfer. A similar technique has also been applied by Isied and others to studying the rate of electron transfer between the iron center of cytochrome C and a ruthenium complex attached to a histidine diametrically opposite the heme group.