Photoinduced electron-transfer along alpha-helical and coiled-coil metallopeptides

A peptide-based electron-transfer system has been designed in which the specific positions of redox-active metal complexes appended to either an alpha-helix, or an alpha-helical coiled-coil, can be reversed to test the effect of the helix dipole in controlling photoinduced electron-transfer rates. Two 30-residue apopeptides were prepared having the following sequences: (I) Ac-K-(IEALEGK)(ICALEGK)(IEALEHK)(IEALEGK)-G-amide, and (II) Ac-K-(IEALEGK)(IHALEGK)-(IEALECK)(IEALEGK)-G-amide. Each apopeptide was reacted first with [Ru(bpy)2(phen-ClAc)]2+, where bpy = 2,2'-bipyridine and phen-ClAc = 5-chloroacetamido-1,10-phenanthroline, to attach the ruthenium polypyridyl center to the cysteine side-chain of the polypeptide. The isolated products were then reacted with [Ru(NH3)5(H2O)]2+ to yield the binuclear electron-transfer metallopeptides ET-I and ET-II. In these systems, electron-transfer occurred from the photoexcited ruthenium polypyridyl donor to the pentammine ruthenium (III) acceptor such that the electron-transfer occurred toward the negative end of the helix dipole in ET-I, and toward the positive end in ET-II. Circular dichroism spectroscopy showed that both peptides exist as dimeric alpha-helical coiled-coils in 100 mM phosphate buffer at pH 7, and as monomeric alpha-helices in the lower dielectric solvents 2,2,2-trifluoroethanol, and a 1:1 (v/v) mixture of CH2Cl2 and 2,2,2-trifluoroethanol. The peptides were predominately (i.e., 65-72%) alpha-helical in these solvents. The emission lifetime behavior of ET-I was seen to be identical to that of ET-II in each of the three solvents: no evidence for directional electron-transfer rates was observed. Possible reasons for this behavior are discussed.