Distance Dependence of Electron Transfer through Conformationally Constrained Peptides

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Understanding and the ability of peptides to mediate long-range electron tunneling in proteins is an evergreen topic. In this context, the study of electron transfer (ET) through model peptides has been particularly useful, especially to assess the occurrence (or competition) of the superexchange and sequential hopping mechanisms as a function of the structure and length of the peptide bridge. In previous studies [1,2], we stressed the importance of dealing with peptides with a well-defined and rigid structure to properly correlate kinetic and structural data. This structural control was made possible by using model peptide bridges based on oligomers of the α-aminoisobutyric acid (Aib) residue. The Aib unit is characterized by a marked steric hindrance at the α-carbon, and this allows the preparation of very rigid peptides. Rigidity is ensured by a framework of intramolecular C=O···H-N hydrogen bonds. Consequently, these peptides possess a strong dipole moment along the main axis [3]. In previous studies, we used cyclic voltammetry to study the dissociative ETs in two series of Donor − Aib-peptide − Acceptor systems in which we changed the donor group, whereas the acceptor was kept constant (a dialkyl peroxide moiety, resulting in the cleavage of the O-O peroxide bond [4]). In this communication, we describe the outcome of dissociative ETs carried out using a new series of Donor − Aib-peptide − Acceptor system in which the acceptor was a perbenzoate moiety [5]. This change also affects the dissociative ET driving force. The same peptide-length dependent dissociative ETs were tested for a possible solvent effect.