Interaction of alanine with small water clusters; Ala-(H2O)n (n = 1, 2 and 3): A density functional study

Density functional theory (DFT) calculations for the hydrogen bond interaction between alanine (Ala) and water clusters Ala-(H2O)n (n = 1, 2 and 3) have been carried at B3LYP/6-31G(d) level of theory. The optimized Ala-(H2O)n complexes are solvated in aqueous medium using polarizable continuum model (PCM) to see the effect of bulk water interaction on Ala-(H2O)n complexes. Structural parameters and stabilities of the Ala-(H2O)n (n = 1, 2 and 3) conformers have been discussed in terms of relative stable energies. The Ala-(H2O)n complexes, A [Ala + (H2O)], C [Ala + (H2O)2] and E [Ala + (H2O)3], where the water molecule is attached only with -COOH group of Ala molecule, are the most stable conformers in gas phase and aqueous medium for n = 1, 2 and 3 clusters. However, the Ala-(H2O)n complexes, where water molecules are attached through both, -COOH and -NH2 groups of Ala are less stable. Thus, the conformers, A, C and E are the most favorable conformers in gas phase and aqueous medium. The clusters with three water molecules are dominated by strong hydrogen bond interactions over one and two water molecules. The strength of hydrogen bond interaction in Ala-(H2O)n complexes are increasing on going from gas phase to aqueous medium. All Ala-(H2O)n complexes are shown thermodynamically stable with respect to separate monomers in gas phase and aqueous medium. Potential energy curves are drawn for the conformers, A, C and E and found that the depth of the potential well increases upon the addition of the water molecules in Ala-(H2O)n complexes.