A Study On The Electron Transfer Of Dipeptide And Solvent Effect

Electron transfer (ET) reactions are of great importance to nearly every subdiscipline of Chemistry. In this dissertation, within the framework of the transition state theory, we have discussed the electron transfer in indole, phenol, tryptophan, tyrosine and dipeptide by means of the classical trajectory approach and the two-state model. In the determination of the nonadiabatic electron transfer dynamic parameters, the AMI method and ab initio calculations have been employed. We have investigated the inner reorganization energy and reaction energy difference between indole side chain and phenol side chain of the peptides involving tryptophan and tyrosine by the means of HF/6-31++G*. Theoretical investigation on the electron transfer between tryptophan and tyrosine has been performed at the level of HF/6-31G and CASSCF/6-31G. The solvent effect has been considered by means of the conductor-like screening model (COSMO). After geometric optimizations of the isolated donor and acceptor, inner reorganization energy and reaction energy difference of the electron transfer have been obtained. For comparison, the ionization potentials are calculated for tryptophan and tyrosine, employing Koopmanns theorem and CASSCF/6-31G method. The transition energy from the ground state to the lowest excited state of these two amino acids has also been calculated. Theoreticalresults give good explanations on the experimental phenomena that N3* can preferably oxide the side chain of tryptophan residue and then the electron transfer from tyrosine residue to tryptophan residue follows in peptides involving tryptophan and tyrosine. In the double-well potential construction of the intermolecular electron transfer in dipeptide, the geometries of the isolated donor, acceptor, and bridge have been optimized by using ab initio calculation at the level of UHF/6-31G. We have used an electron-localized initial guess to induce the SCF calculation of the whole molecule. It has been found that the crossing of the diabatic potential energy surface appears as long as R takes a value of about -0.217, which implies that the gas-phase occurs in inverted region. In the calculation of the electron transfer matrix element V AB, by using the variation principle and the induced SCF calculation at the crossing of the diabatic potential energy surface, we have determined the value of electron transfer matrix element at the level of UHF/6-31G. All the results are satisfactory. The investigation of electron transfer reaction in biologic system is very significant in discussing the physiological mechanism.