Intramolecular electron transfer in cyclopeptide involving tryptophan and tyrosine and corrected two-sphere model for solvent reorganization energy

Theoretical investigation on the intramolecular electron transfer between the side chains of tryptophan and tyrosine in the cyclopeptide has been performed. After geometric optimization and the double-well potential construction, reorganization energy and reaction energy difference of electron transfer are obtained. Furthermore, a corrected new two-sphere model for solvent reorganization energy, which differs from the conventional one, is deduced. For the double-well potential construction, the linear reaction coordinate approximation has been adopted. The electron transfer rate constant for the model system after deprotonation is estimated, kET = 9.1 × 108 s−1 and it is in agreement with the experimental observations. Also, the dependence of the rate constant on pH is simply discussed.