Effect of metal ions (Li+, Na+, K+, Mg2+, Ca2+, Ni2+, Cu2+ and Zn2+) and water coordination on the structure and properties of l-histidine and zwitterionic l-histidine
Interactions between metal ions and amino acids are common both in solution and in the gas phase. The effect of metal ions and water on the structure of L-histidine is examined. The effect of metal ions (Li+, Na+, K+, Mg2+, Ca2+, Ni2+, Cu2+ and Zn2+) and water on structures of His·M(H2O)m, m=0.1 complexes have been determined theoretically employing density functional theories using extended basis sets. Of the five stable complexes investigated the relative stability of the gas-phase complexes computed with DFT methods (with one exception of K+ systems) suggest metallic complexes of the neutral L-histidine to be the most stable species. The calculations of monohydrated systems show that even one water molecule has a profound effect on the relative stability of individual complexes. Proton dissociation enthalpies and Gibbs energies of L-histidine in the presence of the metal cations Li+, Na+, K+, Mg2+, Ca2+, Ni2+, Cu2+ and Zn2+ were also computed. Its gas-phase acidity considerably increases upon chelation. Of the Lewis acids investigated, the strongest affinity to L-histidine is exhibited by the Cu2+ cation. The computed Gibbs energies ΔG are negative, span a rather broad energy interval (from -130 to -1,300 kJ/mol), and upon hydration are appreciably lowered.