Electric Field Effects on Electron Transfer Rates in Dichromophoric Peptides:  The Effect of Helix Unfolding

The effect of helix unfolding on the rates of photoinduced electron transfer in model dichromophoric peptides was investigated. Two α-helical peptides, 1 and 2, having an alternating Ala-Aib backbone and differing only in the position of an appended electron donor (N,N-dimethylaniline) and an appended photoexcited electron acceptor (pyrene) relative to the electric field generated by the helix, had shown a difference in photoinduced electron transfer rates which had been ascribed to a helix dipole effect. Upon denaturation by protic solvents (EtOH, MeOH, H2O, CF3CH2OH) or guanidinium, the observed electron transfer rates in 1 and 2 became identical. The helix unfolding was studied by circular dichroism (CD) analysis. A second pair of model oligopeptides, 3 and 4, analogous to 1 and 2 but having l-proline (Pro) instead of α-methylalanine (Aib) incorporated into the backbone, were prepared in order to study unfolded peptides in low dielectric constant solvents. The CD, NMR, and steady-state fluorescence spectra in a variety of solvents establish that one of the chromophores experiences a different local environment in 3 than in 4 and that the two peptides have different average conformations.