Models for Copper Dynamic Behavior in Doped Cadmium dl-Histidine Crystals: Electron Paramagnetic Resonance and Crystallographic Analysis

Electron paramagnetic resonance and crystallographic studies of copper-doped cadmium dl-histidine, abbreviated as CdDLHis, were undertaken to gain further understanding on the relationship between site structure and dynamic behavior in biological model complexes. X-ray diffraction measurements determined the crystal structure of CdDLHis at 100 and 298 K. CdDLHis crystallizes in the monoclinic space group <i>P</i>2₁/<i>c</i> with two cadmium complexes per asymmetric unit. In each complex, the Cd is hexacoordinated to two histidine molecules. Both histidines are l in one complex and d in the other. Additionally, each complex contains multiple waters of varying disorder. Single crystal EPR spectroscopic splitting (<b>g</b>) and copper hyperfine (<b>A</b>^<b>Cu</b>) tensors at room temperature (principal values: <b>g</b> = 2.249, 2.089, 2.050; <b>A</b>^<b>Cu</b> = −453, −30.5, −0.08 MHz) were determined from rotational experiments. Alignments of the tensor directions with the host structure were used to position the copper unpaired d_{<i>x</i>²‑<i>y</i>²} orbital in an approximate plane made by four proposed ligand atoms: the <i>N</i>-imidazole and <i>N</i>-amino of one histidine, and the <i>N</i>-amino and <i>O</i>-carboxyl of the other. Each complex has two such planes related by noncrystallographic symmetry, which make an angle of 65° and have a 1.56 Å distance between their midpoints. These findings are consistent with three interpretations that can adequately explain previous temperature-dependent EPR powder spectra of this system: (1) a local structural distortion (static strain) at the copper site has a temperature dependence significant enough to affect the EPR pattern, (2) the copper can hop between the two sites in each complex at high temperature, and (3) there exists a dynamic Jahn–Teller effect involving the copper ligands