Insights into the Distance Dependence of Electron Transfer through Conformationally Constrained Peptides

Understanding the distance dependence of electron transfer (ET) through peptides is a topic of continuous interest. We studied a series of donor-bridge-acceptor (D-B-A) systems. The peptide bridges (B) were oligomers of the strongly helicogenic alpha-aminoisobutyric acid, and were prepared in different lengths to form 0, 1, 2, 3, 4, or 5 intramolecular H-bonds. D was a phthalimide group and A was a perester. The study was carried out in DMF, acetonitrile, and dichloromethane. Cyclic voltammetry (CV) and convolution analysis of the model donors and acceptor yielded the relevant thermodynamic information, whereas homogenous redox catalysis provided information on the competitive intermolecular ET. CV analysis of the D-B-A systems showed that, independently of the solvent used, the intramolecular ET occurs by a superexchange mechanism strongly affected by intramolecular H-bonds and the extent of conjugation of the peptide bridge with D and A.