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In view of better understanding interactions of amino acids and peptides with metallic cations in the isolated state, the model system histidine–M2+ (M2+ = Mg2+, Ca2+, Zn2+) has been studied theoretically. The computations have been performed with the help of the density functional theory (DFT) and the B3LYP functional. The extended basis set was the standard 6-311++G**. All the molecular complexes obtained by the interaction between several energetically low-lying tautomers/conformers/zwitterions of histidine and the cations on different binding sites were considered. Our study shows that complexes of histidine with Mg2+, Ca2+, Zn2+ are rather similar. In the isolated state, the most stable form corresponds to a tridentate complex in which the cation interacts with oxygen and two nitrogen atoms: one from the terminal NH2 and one from the imidazole ring. All computations indicate that the metal ion affinity (MIA) decreases on going from Zn2+ to Mg2+ and Ca2+, for the considered amino acid. This indicates that histidine prefers to bind to the transition metal cation rather than alkali earth metals. The influence of theses cations on the acidity of histidine were also considered. As expected, upon metal complexation, proton dissociation of histidine becomes much more favorable, that is, its acidity becomes much less endothermic. [ABSTRACT FROM AUTHOR]

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]