NEW PERSPECTIVES ON LONG-RANGE ELECTRON-TRANSFER IN CONFORMATIONALLY ORGANIZED PEPTIDES AND ELECTRON-TRANSFER PROTEINS - AN EXPERIMENTAL APPROACH

Intramolecular electron-transfer studies across a series of peptides ranging from dipeptides to longer peptides with secondary structure (such as polyproline II and a 17-amino acid a helix) have been carried out. Metal ammine and bipyridine complexes have been used as donors and acceptors in these studies. These studies show that the rate of electron transfer is sensitive to the peptide structure and conformation, even for dipeptide bridges. For peptides with secondary structure, the connectivity of the donor and acceptor to the peptide is also important for the observation of long-range electron transfer. For example, for (bpy)2Ru(II)L(Pro)n- apyRu(III) (NH3)5 (n=9) (bpy=2,2' bipyridine, L=4-carboxy-4'-methyl-2,2'-bipyridine, apy=4-aminopyridine), an electron transfer rate 2X10(4) s-1 was observed, while intramolecular electron transfer could not be observed for the a helix bridge in (bpy)2Ru(II)[alpha-helical peptide]-(His)2Ru(III) (NH3)4 (alpha-helical peptide=Ala-Glu-(Ala)3Lys-Glu-(Ala)3Lys-His(Ala)3His-Ala). Comparative intramolecular electron-transfer experiments were also conducted with two cytochrome c derivatives: one modified at His 33 by [-Ru(NH3)4isn] and one modified at Met 65 by [-Fe(CN)5]. Although the His 33 and Met 65 sites are located at similar distances from the heme, and the two metal complexes possess similar reorganization energies and driving force, different rates of electron transfer varying by about 1000 were observed for the electron transfer from the heme to the metal complex. The experiments presented show that the shortest through-space distance is not always the most important determinent of the rate of electron transfer, and other factors such as the peptide structure and conformation and the connectivity of the donor and acceptor to the peptide bridge are very important.