Natural Amino Acid Ion Quantum Chemistry Research

In large quantities of published results, researches on the dimensional structure of the amino acids mainly aimed at one of or some of the natural amino acids instead of the systematic researches of the common and special characters of all the natural amino acids. In this thesis, the hydrogen bond structure in the amino acids was calculated by optimizing the 20 crude amino acids positive ions and negative ions and zwitterion with ab inito calculations, and the effects of the side-chain were discussed as well. Some new results were achieved as follows:1) In the positive ions, N1-H2...03 hydrogen bond forms between the nitrogen atom of -NH2 and the oxygen atom of the carboxyl .With the N1-H2...03 hydrogen bond , N1, H2,03, C4, C5 form a five-numbered ring. The hydrogen bonds are not affected by the side-chains except Ser and Thr. There is a second hydrogen bond N1-H8...07 in Ser and Thr which result in the differences of the hydrogen bonds in them.2) In the negative ions, N1-H2...03 hydrogen bond forms between the nitrogen atom of a -NH2 and the oxygen atom of the carboxyl. Most of the hydrogen bonds are affected by the side chains. We classified the 20 natural amino acids according to the different effects : a) the amino acids consists of only N1-H2...03: Gly, Ala, Val, Ile, Leu, Met, Arg, Cys, His, Lys, Ser, Thr, Pro; b) the amino acids consists of N1-H2...03 and another C-H...0 hydrogen bond : Phe, Trp, Tyr, Glu, Gln; c) the amino acids consists of N1-H2...03 and 0-H... 0 or N-H...0: Asp and Asn.3) In the zwitterion , the 03-H2...N1 Hydrogen bond forms and the side chain hardly affects the hydrogen bond.4) The intensity of the hydrogen bonds weaken and the effects of the atoms of side chains reduce from positive ions, negative ions to zwitterions.Basing on the optimized structures, vibration frequencies were calculated. The correlative infrared vibration modes of the hydrogen bond were found, which proved the correctness of the results above. In addition, ab initio method and DFT method were compared by the computing the 1H-NMR of amino acid zwitterions.