Hydroxyl And Hydrogen Free Radical Addition Of At Proton Transfer And Amino Acid Residues Auxiliary Hno Proton Transfer In The State Of Theoretical Research

The hydrogen bonds play an important role in the structure and stability of molecular and biomolecular complexes because many chemical, physical and biological processes involve the formation and/or breaking of this kind of bonds. As one of the simplest and the most fundamental phenomena in the tautometric equilibria and oxidation-reduction reaction, intramolecular or intermolecular proton transfers (PTs) also play an important role in many chemical and biochemical processes. The investigations of the hydrogen bonding and proton transfers have attracted considerable attentions over the years theoretically and experimentally.In this paper, three proton transfer processes have been theoretically studied. They were, the PT processes between A-T base pair under the effect of the OH radical and H radical addition and the PT process of HNO with the assistance of amino acid residues while the water molecules exist or not. In order to investigate these three PT processes, the B3LYP method within the framework of the density functional theory has been used with the 6-31G^* or 6-311++G^** basis sets using the Gaussian03 program. The influences of OH radical addition and H radical addition on the PT processes of A-T base pairs have been studied, as well as the influences of the amino acid residues on the tautomerism of HNO.Firstly, the influences of the OH radical addition on the structure, energies, and activation barriers of the proton transfer for A-T base pair have been investigated systematically. Analyses of the results indicate that five rational complexes exist, in which the ^8OHA-Tå'ŒA-T^6OH are more stable than the other complexes. Compared with the isolated PT process of A-T base pairs, the formation of the five complexes are helpful to the proton transfer and the assisting effect of the ^4OHA-T is the best.Secondly, the influences of H radical addition on the structure, energies, and reaction energies of the proton transfer for A-T base pairs have been investigated. With the structure optimization and further frequence analyses, 11 rational complexes have been found on the potential energy surface. As a result, the proton transfer of A-T7 and A-T9 can't happen upon hydrogen addition. For the other nine complexes, the barriers for the PT processes have decreased more or less, suggesting that the formations of nine complexes are helpful for the PT processes. In the end, the PT processes of HNO assisted by amino acid residues in the presence orabsence H₂O molecules have been studied. Compared with the isolated PT process of monomer HNO, the assistances of different amino acid residues make the barrier of PT processes decrease by about 32-38 kcal.mol^-1, showing that assisting effects of the amino acid residues are very good. When one or two water molecules are introduced, the barrier heights have been further decreased. It proves that the water molecules can further strengthen the assisting effect of the amino acid residues.In the processes of structure optimization of the three systems, all the various intermediates resulting from the possible stepwise mechanism have been investigated. As a result, all such optimizations led back to the corresponding reactants or products. What is more, in each PT process, the reactant and its corresponding product are separated by a single transition state possessing the unique imaginary frequency. Thus, the PT processes in this study share the concerted mechanism other than the stepwise mechanism.