Hydrogen-bonding interaction in a complex of amino acid with urea studied by DFT calculations

Theoretical studies on hydrogen-bonded complexes between amino acids (glycine, alanine, and leucine) and urea in gas phase have been carried out using density functional theory (DFT) and ab initio methods at the B3LYP/6-311++g** and MP2/6-311++g** theory levels. The structures, binding energy, Chelpg (charges from electrostatic potentials using a grid-based method) charge distribution, and bond characteristics of the mentioned complexes were calculated. Urea is a good H-bond donor and an excellent receptor for highly electronegative atoms like O and N, through the formation of two or more hydrogen bonds. The NH2 and COOH groups of amino acids can form several different types of H-bonds with urea molecular, as well as (CH)-H-alpha and alkyl side chains. The calculated high binding energy also suggests multiple H-bonds formed in one complex. The OH center dot center dot center dot O contact is the strongest hydrogen bond interaction with H center dot center dot center dot O separation around 1.65 A... and its relevant angle close to 176A degrees. The closely linear amide H-bonds NH center dot center dot center dot O and OH center dot center dot center dot N strongly stabilize the amino acid-urea complex with H center dot center dot center dot O separation between 1.89 and 2.38 A.... The weaker CH center dot center dot center dot O/N H-bonds are also discussed as significant interaction in biological systems involving amino acids.