Theoretical Studies Of The Interations Between Amino Acids And Solvents

The intermolecular interactions of three different binary systems were investigated with DFT and Ab initio methods, respectively. According to the calculation results of all these complexes, we have taken the systematically analysis to weak hydrogen bonding about the principle and mechanism.The B3LYP/6-31G* and MP2/6-31G* methods were performed on complexes of DMF with xylenes (o-, or m- or p-xylene), and seven stable configurations were obtained with no imaginary frequencies. The results indicated the presence of double C–H…O hydrogen bonds between DMF and xylenes in these conformers and the interaction energies of hydrogen bonding of the DMF–xylene systems decreased in the following sequence: DMF–o-xylene: a1 > DMF–m-xylene: b1 > DMF–p-xylene: c1.Hydrogen-bonded complexes between amino acids (glycine, alanine and leucine) and DMF in gas phase have been carried out at the B3LYP/6-311++g** theory level. The structures, binding energy, stretching frequency and bond characteristics of the mentioned complexes were calculated. The NH2 and COOH groups of amino acids form some different types of hydrogen bonds with DMF molecular, as well as alkyl sidechains. The nearly linear OH…O and NH…O contacts are stronger than a conventional hydrogen bond interaction with their H…O separation between 1.74 and 2.16 ?. The observed weaker CH…O H-bond results in a blue shift in the CH stretching frequency.Further studies on hydrogen-bonded complexes between amino acids (glycine, alanine and leucine) and urea in gas phase have been carried out at the B3LYP/6-311++g** theory level. The NH2 and COOH groups of amino acids can form several different types of hydrogen bonds with urea molecular, as well as CαH and alkyl sidechains. The OH…O contact is the strongest hydrogen bond interaction with H…O separation around 1.65 ? and its relavant angle close to 176°. The closely linear amide hydrogen bonds NH…O and OH…N strongly stabilize the amino acid-urea complex with H…O separation between 1.89 and 2.38 ?. The weaker CH…O/N H-bonds are also discussed as significant interaction in biological systems involving amino acids.