DFT STUDY ON GAS-PHASE INTERACTION BETWEEN HISTIDINE AND ALKALI METAL IONS (Li+, Na+, K+); AND INFLUENCE OF THESE IONS ON HISTIDINE ACIDITY.

The gas-phase metal affinities of histidine Li+, Na+ and K+ ions have been determined theoretically employing the hybrid B3LYP exchange–correlation functional and using 6-311++G** basis sets. All computations indicate that the metal ion affinity decreases on going from Li+ to Na+ and K+ for the considered amino acid. Different types of M+ coordinations on several histidine conformers/tautomers were considered in detail. The optimized structures indicate that Li+ and Na+ prefer a tri-dentate coordination, bonding with a nitrogen atom of imidazole ring (Nτ), –NH2, and an oxygen atom of a carbonyl, while in the K+-histidine lowest-energy conformer, the cation appears to be bi-coordinated to both oxygen atoms of the zwitterionic form by the energy values not too far from that of tri-coordination. We also performed the DFT calculations for proton dissociation energy of histidine both in the presence and absence of alkali metal ions. Our results also reveal that the acidity of histidine dramatically increases upon metal ion complexation. [ABSTRACT FROM AUTHOR]