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

Abstract

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 (A(Cu)) tensors at room temperature (principal values: g = 2.249, 2.089, 2.050; A(Cu) = -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 dx(2)-y(2) 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.

Electron Paramagnetic Resonance Characteristics and Crystal Structure of a Tutton Salt Analogue: Copper-Doped Cadmium Creatininium Sulfate

“…The deviations (δ°: 1.6°, 6.0°, and 7.9°) between the g tensor and the metal−OW directions are small and within normal error limits for X-band singlecrystal measurements. 27,29 The tensor axes are consistent with those found in previous studies of Cu 2+ (H 2 O) 6 in Tutton salts. 1,3,7−9 The metal−water bond lengths in Figure 6 are those of the host CdCrnS structure.…”

“…They can also add valuable information on the geometric structure of different states which interconvert and may ultimately help interpret the role of copper motions in the function of important biological molecules [22][23][24][25][26] . In addition, the current study complements our earlier EPR investigations on copper-histidine complexes whose temperature dependent spectra were could be explained as a motional averaging between neighboring metal binding sites [27][28][29] .…”

“…The copper hexahydrate complex is depicted in Figure 6. The deviations (°: 1.6°, 6.0°, 7.9°) between the g tensor and the metal-OW directions are small and within normal error limits for X-band single crystal measurements 27,29 . The tensor axes are consistent with those found in previous studies of Cu 2+ (H2O)6 in Tutton salts 1,3,[7][8][9] .…”

“…The CW-EPR measurements were carried out using a Varian X-band E-109 spectrometer interfaced to a PC and gaussmeter as described in the past [27][28][29] . DPPH (g=2.0036) was used as a g-value reference.…”

“…Some crystals were manually crushed into powders and placed directly in the tubes. The g and copper hyperfine (A Cu ) tensors were determined by a least-square fit of the spectral data to the spin Hamiltonian (Equation 1) using methods outlined before (29).…”