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 P21/c 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 (g) and copper hyperfine (ACu) tensors at room temperature (principal values: g = 2.249, 2.089, 2.050; ACu =
−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 dx2‑y2 orbital in an approximate plane made by four proposed ligand atoms:
the N-imidazole and N-amino of one
histidine, and the N-amino and O-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.