This contribution describes a study on photoinduced intramolecular charge transfer mediated by an amide bond. Protein-mediated electron-transfer processes sustain numerous redox functions in the biological systems, such as respiration and photosynthesis. Long-range charge transduction in proteins and polypeptides occurs via a mltipath tunneling mechanism where the largest contribution is through sigma-bond pathways. Theoretically, amide (or peptide) bonds, despite their partial pi-conjugation, are usually treated equivalently to sigma-bonds. We address this issue by preparing a dyad comprised of an electron donor and acceptor covalently attached via an amide bond. For an electron donor and a principal chromophore, we chose a quarterthiophene derivative (T4), and for an electron acceptor - an anthraquinone derivative (AQ). The spectral features of the dyad conjugate, T4-AQ, did not indicate the presence of a strong electronic coupling between the donor and the acceptor across the amide bond. This finding allowed us to apply the semi-classical transition-state theory for diabatic processes for analysis of the charge transfer process across the peptide bond. The rates of intramolecular photoinduced electron transfer in various media, which we measured, ranged from 0 to > 10^11 s^(- 1). From spectroscopic and electrochemical data we estimated that the charge-transfer driving forces between T4 and AQ range from about - 1 to - 1.5 eV for media with different polarities. Because of the large driving forces, the system we investigated behaved in accordance with the kinetics expected for the tip of the Marcus curve and the inverted Marcus region. We observed shutting down of the charge transfer when the external reorganization energy decreased under about 0.1 eV. From the charge-transfer data, we estimated the coupling across the amide bond.

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