Ab Initio Density Functional Theory Calculations and Vibrational Analysis of Zinc-Bound 4-Methylimidazole as a Model of a Histidine Ligand in Metalloenzymes

Abstract

Histidine is often found as a ligand in metalloenzymes. The imidazole side group has two nitrogen atoms capable of being protonated or of participating in metal binding. Hence, histidine can take on various metal-bound and protonated forms in proteins. Because of its variable structural state, histidine often functions as a key amino acid residue in enzymatic reactions. Raman and IR spectroscopies have been used as powerful methods to investigate the structure of histidine in proteins. In an attempt to establish the Raman and IR markers reflecting the coordinated and protonated states of histidine, we have calculated the optimized geometry and vibrational frequencies of various forms of zinc(II) complexes of 4-methylimidazole (4-MeIm), as a model of a histidine ligand, using the density functional theory (DFT) method. The effects of metal binding on the frequency and N-deuteration shift of the C4C5 stretching mode around 1600 cm-1 and of the C5N1 stretch around 1100 cm-1 were satisfactorily reproduced in ...