Groups Assisted In The Hnx / Hxn (x = O, S) System, Proton Transfer Mechanism In Theoretical Research

As one of the simplest and the most fundamental phenomena in the tautometric equilibria and oxidation-reduction reactions, intramolecular or intermolecular proton transfers (PTs) play an important role in many chemical and biochemical process. Elements within and between proton transfer widespread elements in photosynthesis chain, respiratory chain, enzymatic catalysis response of the cell metabolism process, a driving force behind cell communication, the transmission of information to determine the direction, procedures and biological information to energy quantum coding and transmission mode. Proton transfer between elements in the flow of information transmission and dissemination of information plays an important biological role. Proton transfer along the hydrogen bond for the transmission of biological elements, such transfers were considered in the biological system is the process of energy transmission and electronic transfer of the same importance. The role of hydrogen bonds in the structure and stability of molecular and biomolecular complexes are very important because many chemical, physical and biological processes involve the formation and/or breaking of this kind of bonds. The investigation of the hydrogen bonding has attracted considerable attention over the years. Much research has been done examining hydrogen bonding by studying weakly bound complexes.In this paper, we choose these two typical systems, HNS/HSN and HNO/HON, to theoretically investigate the intramolecular or intermolecular proton transfers (PTs) isomerization under the assistant of water molecular or the seven simulated acid residues compounds using the density functional theory. Geometries and harmonic vibrational frequencies of the considered structures have been obtained by using the nonlocal hybrid three-parameter B3LYP density functional approach at 6-311++G** bases set level. Their interaction energies for various isomers have been calculated at the same method. To build up thecorrelations among the optimized stable structures and the transition states, intrinsic reaction coordinate calculations are carried out. We have also analyzed the character of the hydrogen bond and calculated the energies of the hydrogen bond formed between the monomers.Through the investigation of our selected systems, we have found that:1. water-catalyzed pathways of the HNS/HSN isomerization. Two modes are considered to model the catalytic effect of m water molecules: (1) water molecule(s) directly participate in forming a proton transfer lpop with HNO/HON species and (2) water molecules are out-of-loop, modeling the outer-sphere water effect from the other water molecules directly H-bonded to the loop (referred to as out-of-loop waters). The analyses indicate that three water molecules directly participate in forming a proton transfer loop is most favorable for the proton transfer, and the increasing out-of-loop water molecules does not significantly change these energy quantities for each of the hydration cases.2. We have chosen seven kinds of compounds (CH3COOH, CH3CONH2, CH3-NH-C(NH2)-NH, CH3NH2, CH3OH, CH3SH, and CH3-C6H4-OH) to simulate amino acid catalyzed pathways for the HNS isomerization. In the seven compounds, the combining of the carboxyl of the CH3COOH is most favorable for the proton transfer of HNS/HSN. In this proton transfer process, it must surmount a energy barrier of 20.73 kcal/mol for the transform from HNS to HSN, and the reverse proton transfer only have a energy barrier of 2.19 kcalmol"1. It is endothermic 18.55 kcalmol'1 for this proton transfer process.3. We have chosen seven kinds of compounds (CH3COOH, CH3CONH2, CH3-NH-C(NH2)-NH, CH3NH2, CH3OH, CH3SH, and CH3-C6H4-OH.) to simulate 21 amino acid catalyzed pathways for the HNO isomerization. (1) The forward barrier, the backward barrier and the heat of the triplet state are reduced by 30.03, -0.05 and 30.10 kcal/mol compared with the singlet state 1R (lP)(72.44/32.29kcal/mol). Therefore, the proton-transfer of 3R/3P is easier than R/P. (2) We compare the assisting effect of the analogues of 21 aminoacids in the HNO/HON isomerization on the singlet and triplet excited state PES. For the singlet state, the amino of methylamine is the most favorable assistant group for HNO/HON proton transfer isomerization. For the triplet state PES, the PT isomerization of HNO/HON with the assisting -COOH of the CH3COOH molecule is optimal. (3) With the same assistant group, the PT isomerization of the HNO/HON on the triplet state PES is easier than that on the singlet state PES.