Theoretical Study Of The Dynamics Of Electron Transfer In Biomolecular System

This work includes five chapters. The fundamentals of electron transfer developed during the recent half-century are summarized in chapter 1, and the factors which affect the rate constant of electron transfer reaction are discussed based on the classical Marcus's theory. The computational approaches of these factors are also shown in chapter 1. In chapter 2 to chapter 5, the electron transfer reactions in some biomolecular systems are theoretically investigated. Among these theoretical studies, the mechanisms of both adiabatic and nonadiabatic thermal electron transfer reactions as well as the photoinduced electron transfer reactions are discussed by investigating the energy relationship, the interaction between reaction molecules, and the reaction rate constant.The mechanism of the oxidation of ?-carbon-centered radical by O2 has been theoretically presented in chapter 2. It has been found that the polar solvent effect makes the activation energy barrier lower remarkably, and the reaction heat of the endothermal reaction in aqueous solution becomes nearly half of that in the gas phase. The electron transfer reaction in the oxidation of ?-carbon-centered amino radical has been found to be difficult to occur in the gas phase, however the polar solvent fluctuation will propel the reaction to occur at near the product state with a certain probability. This electron transfer reaction is judged as an adiabatic one accordinting to the values of the electronic coupling matrix element, and the reaction rate constant has been calculated to be 7.1(10-12 M-1.s-1 in aqueous solution using the Marcus' adiabatic electron transfer theory. The theoretical study shows that this model electron transfer reaction can occur thermodynamically in polar solvent, but the rate is predicted very slow.Intramolecular and intermolecular electron transfer reactions in polypeptide involving tyrosine and tryptophan have been theoretically investigated and presented in chapter 3. Three different mechanisms and the solvent effect of the electron transfer reaction between indol moiety and phenol moiety of the model system corresponding to the dipeptide of tryptophan and tyrosine have been studied by energy calculations. It has been found that: (1) the electron transfer reaction can hardly occur in gas-phase; (2) the electron transfer reaction in the doubly deprotonated radical anion system can occur spontaneously with the transition state being near the reaction state in solution, and this reaction is exothermic; (3) the third mechanism found is the multi-step one. The theoretical results show that the reaction pathway in which proton transfer reaction occurs first, and then electron transfer occurs is predominant in aqueous solution. On the other hand, the study of coupling matrix element of electron transfer in model systems InH-(CH2)n-PhoH (n=2~5) shows that the coupling matrix element (Vrp) decreases exponentially with increasing central separation distance of the electron donor and acceptor. The conformational effect of the dipeptide TrpH-TyrOH has been also investigated in this chapter. The theoretical investigations reveal that the separation distance of donor and acceptor is different in different conformations, so as to affect the magnitude of the coupling matrix element, and the slightly change of the bridge will also affect the rate constant of the electron transfer reaction in dipeptide.In chapter 4, the the photoinduced electron transfer reactions between tryptophan and 4-nitroquinoline-1-oxide (4NQO), and that between the purine bases of DNA and 4NQO have been theoretically investigated using Hartree-Fock (HF) and the complete active space SCF (CASSCF) method, in order to understand the carcinogenicity of 4NQO. The results of theoretical study show that (1) two kinds of hydrogen-bonding interaction exist in the complexes composed of 4NQO and tryptophan or purine bases, one is strong and the other is weak; (2) the first and the second singlet excited states and the first triplet excited state of the most stable ...