The influences of water solvent on the structures and stabilities of Na+-AD conformers

The influences of water solvent on the structures and stabilities of the complex ion conformers formed by the coordination of alanine dipeptide (AD) and Na+ have been investigated using supramolecular and polarizable continuum solvation models at the level of B3LYP/6-311++G**, respectively; 12 monohydrated and 12 dihydrated structures of Na+-AD complex ion were obtained after full geometrical optimization. The results showed that H2O molecules easily bind with Na+ of Na+-AD complex ion, forming an ion-lone pair interaction with the Na-O bond length of 2.1-2.3 A. Besides, H2O molecules also can form hydrogen bonds OW-HW...O(1), OW-HW...O(2), N(1)-H(1)...OW or N(2)-H(2)...OW with O or N groups of the Na+-AD backbone. The most stable gaseous bidentate conformer C7AB of Na+-AD is still the most stable one in the solvent of water. However, the structure of the most unstable gaseous conformer alpha'B of Na+-AD collapses under the attack of H2O molecules and changes into C7AB conformation. Computations with IEFPCM solvation model of self-consistent reaction field theory give that aqueous C5A is more stable than C7eqB and that the stabilization energies of water solvent on monodentate conformers of Na+-AD complex ion (about 272-294 kJ/mol) are more than those on bidentate ones (about 243 kJ/mol).