Infrared spectroscopic characterization of copper–polyhistidine from 1,800 to 50 cm−1: model systems for copper coordination

Poly(l-histidine) and imidazole in the presence of copper cations have been investigated by means of Fourier transform infrared (IR) spectroscopy in the mid- and far-IR spectral range to establish specific marker bands of the copper-coordination site in metalloproteins as a function of pH as well as the effect of the coordination on the amino acid contributions. Whereas the well-known mid-IR region was specific for the secondary structure of the protein mimics, the far-IR region included contributions from the metal–ligand vibrations. The addition of copper led to secondary structure changes of poly(l-histidine) at neutral and basic pD and to specific shifts of ring vibrations. At pD 9.5 the addition of copper deprotonated the nitrogen atoms of the imidazole ring and the backbone. At neutral pD the copper cations were coordinated by the N3 atom of the imidazole ring. Copper–imidazole vibrations at neutral pD were observed at 154 and 128 cm−1. Signals observed at 313 and 162 cm−1 were assigned to metal–ligand vibrations arising from copper–poly(l-histidine) complexes with a negatively charged imidazole ring.