Interaction of Li+, Na+, and K+ with the Proline Amino Acid. Complexation Modes, Potential Energy Profiles, and Metal Ion Affinities

The stationary points characterizing the potential energy profiles of the complexation process of the proline unusual α-amino acid with the alkali metal ions M + (Li +, Na + and K + ) were investigated by density functional theory using the B3LYP hybrid potential and the 6-311++G** basis set. Different types of M + coordinations on several proline conformers were considered. Results show that Li +, Na +, and K + cations bind very similarly to the proline. In the M + -proline lowest-energy conformer, the cation appears always coordinated to both the oxygen atoms of the zwitterionic form of the amino acid which presents the up ring pucker. The complexes obtained by the interaction of the alkali ions with both the carbonyl oxygen and nitrogen of the α-amino acid were found, in all cases, at energy values not too far from those of the global minima. Absolute metal ion affinities were computed using various exchange-correlation functionals.