Potential energy surfaces for the gas-phase interaction between alpha-alanine and alkali metal Ions (Li+, Na+, K+). A density functional study.

The conformations of alpha-alanine and those of the eight most stable adducts that it forms with lithium, sodium, and potassium cations were studied by means of the density functional theory using the hybrid B3LYP exchange correlation potential and the 6-311++G basis set. Minima and transition states characterizing the energetic paths for the interaction of the three metal ions with the free amino acid were explored in detail, and the results show that they are almost the same in the cases of lithium and sodium ions. At the absolute minimum, the Li+ and Na+ cations appear to be contemporaneously linked to the carbonyl oxygen and nitrogen atoms of alpha-alanine. Slight differences were found for potassium; the most stable adduct has the cation coordinated to both oxygen atoms of carboxyl function. The influence of the low-energy conformers in the determination of the gas-phase absolute affinities is demonstrated.