Structure, Disorder, and Molecular Dynamics in Zn(d,l-histidine)2: EPR of Copper Ion Dopants, X-ray Diffraction, and Calorimetric Studies

Abstract

We report EPR, X-ray crystallographic, and calorimetric measurements in copper-doped and pure Zn(II) bis(d,l-histidino)pentahydrate (C12H26N6O9Zn), to be called Zn(d,l-histidine)2. EPR data in copper-doped single crystals were obtained at 9.8 GHz as a function of the orientation of the magnetic field B at 298 and 80 K, and as a function of temperature (T) for fixed orientations of B. Powder samples were studied at 9.8 and 35 GHz. Steep changes of the EPR spectra with temperature occur around Tc = 268 K. Above Tc we observe spectra arising from a single copper with rhombic g and copper hyperfine A tensors. The resonances display a strong dependence of the line width with the hyperfine component. Below Tc the spectra can be assigned to two different copper ions displaying axial symmetry and related by a C2 rotation. As T increases approaching Tc, they collapse into the high-temperature spectrum. A peak in the differential specific heat of pure Zn(d,l-histidine)2 between 235 and 270 K indicates a transition intrinsic to the host material. Full X-ray structural studies were performed at 293 and at 150 K, above and below Tc. At 293 K our results are similar to those reported before. The low-temperature data show disorder in the water molecules. We interpret the experimental results with a model where the copper atoms hop randomly between different states. This dynamics is related to the fluctuating disorder in the lattice and produces steep changes on the EPR spectra of copper ion dopants. The role of dynamic Jahn−Teller distortions is discussed. A bonding scheme for the copper ions in Zn(d,l-histidine)2 is proposed, and it is compared with that encountered in type 1 blue copper proteins.