Photoinduced electron transfer along alpha-helical and coiled-coil metallopeptides: The absence of directionality.

摘要

A peptide-based electron-transfer system has been designed in which the specific positions of redox-active metal complexes appended to either an α-helix or an α-helical coiled-coil, can be reversed in order to test the effect of the helix dipole in controlling photoinduced electron-transfer rates. The 30-residue apo-peptides were prepared having the following sequences: C10H21 (I): Ac-K-(IEALEGK)(I CALEGK)(IEALEHK)(IEALEGK)-G-amide; H10C21 (II): Ac-K-(IEALEGK)(IHALEGK)(IEALECK)(IEALEGK)-G-amide; C7H21 (III): Ac-K-(IEALECK)(IEALEGK)(IEALE HK)(IEALEGK)-G-amide; H7C21 (IV): Ac-K-(IEALE HK)(IEALEGK)(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, ET-II, ET-III and ET-IV. In these systems, electron-transfer occurred from the photoexcited ruthenium polypyridyl donor to the pentammine ruthenium (III) acceptor such that the electron-transfer occurred towards the negative end of the helix dipole in ET-I and ET-III, and towards the positive end in ET-II and ET-IV. Circular dichroism spectroscopy showed that all peptides exist as dimeric α-helical coiled-coils in 100 mM phosphate buffer at pH 7, and as monomeric α-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%) α-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. Likewise, the ET-III and ET-IV metallopeptides showed identical kinetics in 100 mM phosphate buffer at pH 7. Thus, no evidence for directional electron-transfer rates was observed for the current metallopeptide system.