Understanding Electron Transfer across Negatively-Charged Aib Oligopeptides.

The physicochem. effects modulating the conformational behavior and the rate of intramol. dissociative electron transfer in peroxide peptides Pht-C(Me)2CO-[NHC(Me)2CO]n-O-O-CMe3 (I; n = 0-3; Pht = phthalimido) have been studied by an integrated d. functional/continuum solvent model. The authors found that three different orientations of the phthalimide ring are possible, labeled (phi)hel, (Phi)C7, and (Phi)pII. In the condensed phase, they are very close in energy when the system is neutral and short. When the peptide chain length increases and the system is neg. charged, (phi)hel becomes instead the most stable conformer. The calcns. confirm that the 310-helix is the most stable secondary structure for the peptide bridge. However, upon charge injection in the phthalimide end of I (n = 3), the peptide bridge can adopt an a-helix conformation as well. The study of the dependence of the frontier orbitals on the length and on the conformation of the peptide bridge (in agreement with exptl. indications) suggests that for I (n = 3) the process could be influenced by a 310/a-helix conformational transition of the peptide chain.