Structural stabilities of metalated histidines in gas phase and existence of gaseous zwitterionic histidine conformers

A systematic search of the various structures of metalated histidine bound to Li+, Na+, K+, Mg2+, and Ca2+ has been performed. The metal binding energies of the gaseous phase of these histidines have been determined at the B3LYP/6-311 + G(2df,p) level. All computational results indicate that binding energies decrease with increasing size of monovalent and divalent cations and moreover values for divalent metal cations are much higher than for monovalent ones. Also, for all cations except K+, charge solvated (CS) structures of histidine are preferentially more stable compared to their respective salt bridge (SB) structures, even though for potassium cations, the CS structure is 0.9 kcal/mole less stable than the SB structure. The tridenated structures are the most stable for Li +, Na+, Mg2+ and Ca2+ metal histidine complexes, while the bidenate structure is the most stable for the potassium histidine complex. Furthermore, some characteristic vibrational assignments for the most stable tridenated and bidenated structures are described, where remarkable shifting of frequencies are noticed. Thermodynamical properties such as enthalpies and the free energy for different metal histidine complexes are also presented. In addition, a systematic search of zwitterionic histidine conformers in the gas phase is performed with the finding of seven locally stable zwitterions in three tautomeric forms.