Photoinduced electron transfer in alpha-helical polypeptides: dependence on conformation and electron donor-acceptor distance.

Long distance electron transfer in proteins is a multiple-pathway process whose kinetics is modulated by the dynamics of flexible peptide chains. Such complexity can be observed even in relatively simple systems, eg. donor bridge acceptor, where the bridge is a polypeptide alpha-helix. We have investigated a series of 24-residue helical polypeptides that exist as monomers in water alcohol media. The principal chromophore and electron acceptor, a pyrene moiety, is connected to the N-terminus via a flexible linker. The electron donor, a tryptophan residue, was placed various distances away from the pyrene-labeled terminus. Time-resolved emission spectroscopy, associated with the fluorescent pendant, pyrene, was employed to study the photoinduced electron-transfer kinetics for the polypeptide analogs. Mechanisms involving only through-bond pathways could not account for the pattern of measured fast charge-separation rates. When the electron donor was placed far enough from the acceptor (i.e. at least six residues apart), a decrease in the electron-transfer rates with the donor acceptor distance was observed. The emission decays for polypeptides with the electron donor exhibited complex behavior and could not be fit using a single-exponential function. For the treatment of the time-resolved data, a multi-exponential model was developed that is based on the assumption of a Gaussian distribution of the classical electronic coupling beta values among the conformers responsible for the observed electron-transfer processes. This approach proved to be informative because, in addition to the mean values of the electron-transfer rate constants, the widths of the distributions of these rates illustrate the size of the conformational space explored by the flexible chains that provide pathways for electron transfer.