INTERACTION OF THE TAUTOMERIC STATES OF HISTIDINE WITHCuANDZnMETAL IONS – A THEORETICAL STUDY

A density functional study on the interaction of Cu2+and Zn2+metal ions at the predominant positions of Nτ– H and Nπ– H histidine tautomers has been performed. The fully optimized energy of the isolated histidine tautomers at B3LYP/6-311++G** level of theory depicts Nτ– H tautomer to be much stable compared to that of the Nπ– H tautomer. The interaction of metal ions forms bidentate and tridentate complexes with Nτ– H tautomer, while it is absent in the case of Nπ– Htautomer emphasizing the role of former in structural determination of liganated proteins. The Zn2+ion induces a barrier-free proton transfer when interacted at the carbonyl position of Nτ– H histidine tautomer necessitating the tautomer to be in the zwitterionic form for complexation. The thermodynamical analysis predicts a blue shift in the NH and CO stretching vibrational frequencies and suggests Nτ– H tautomer to be best suitable for Cu2+and Zn2+ion interactions. The topological and charge transfer studies in concert with frontier molecular orbital (FMO) analysis confirm the covalent interaction, validating the findings based on the geometrical data.