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 tr ansfer in model dichromophoric peptides was investigated, Two ct-helic al peptides, 1 and 2, having an alternating Ala-Aib backbone and diffe ring only in the position of an appended electron donor (N,N-dimethyla niline) and an appended photoexcited electron acceptor (pyrene) relati ve to the electric field generated by the helix, had shown a differenc e 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 circul ar dichroism (CD) analysis. A second pair of model oligopeptides, 3 an d 4, analogous to 1 and 2 but having L-proline (Pro) instead of alpha- methylalanine (Aib) incorporated into the backbone, were prepared in o rder to study unfolded peptides in low dielectric constant solvents, T he CD, NMR, and steady-state fluorescence spectra in a variety of solv ents establish that one of the chromophores experiences a different lo cal environment in 3 than in 4 and that the two peptides have differen t average conformations.