This contribution describes a study on photoinduced intramolecular electron transfer mediated by an amide bond. Protein-mediated electron-transfer processes sustain numerous redox functions in biological systems, such as respiration and photosynthesis. Long-range charge transduction in proteins and polypeptides occurs via a multi-path tunneling mechanism where the pathways include covalent-bonds, hydrogen bonds and through-space interactions. Theoretically, amide (or peptide) bonds, despite their partial pi-conjugation, are treated equivalently to sigma-bonds. We address this issue by preparing a dyad comprised of an electron donor and acceptor covalently attached by an amide bond. We chose a quaterthiophene derivative (T4) and an anthraquinone derivative (AQ) for an electron donor and acceptor, respectively (Figure 1). The spectral features of the dyad conjugate (T4-AQ), did not indicate the presence of a strong electron coupling across the amide bond. The rates of intramolecular photoinduced electron transfer in various solvents, which we measured, ranged from 0 to more than 10¹¹s^-1. From spectroscopic and electrochemical data we estimated that the charge transfer driving forces between T4 and AQ range from about -1.1 to 1.2eV for solvents with varying polarities. From the solvent dependence of the rate constants, we determined the rate distance-decay parameter, β, for the amide linker. The obtained value of β for the amide linker (~ 0.8 Å^-1) is smaller than the value of β reported for saturated hydrocarbons.

Figure 1