Probing Axial Water Bound to Copper in Tutton Salt Using Single Crystal ¹⁷O‑ESEEM Spectroscopy

Electron spin–echo envelope modulation (ESEEM) signals attributed to axial water bound to Cu²⁺ have been detected and analyzed in Cu­(II)-doped ¹⁷O-water-enriched potassium zinc sulfate hexahydrate (Tutton salt) crystals. The magnetic field orientation dependences of low frequency modulations were measured to fit hyperfine and quadrupole coupling tensors of a ¹⁷O (<i>I</i> = ⁵/₂) nucleus. The hyperfine tensor (<i>A</i>_<i>xx</i>, <i>A</i>_<i>yy</i>, <i>A</i>_<i>zz</i>: 0.13, 0.23, −3.81 MHz) exhibits almost axial symmetry with the largest value directed normal to the metal equatorial plane in the host structure. Comparisons with quantum chemical calculations position this nucleus about 2.3 Å from the copper. The isotropic coupling (−1.15 MHz) is small and reflects the weak axial water interaction with a d_x2‑y2 unshared orbital of copper. The ¹⁷O-water quadrupole interaction parameters (<i>e</i>²<i>qQ</i>/<i>h</i> = 6.4 MHz and η = 0.93) are close to the average of those found in a variety of solid hydrates. In addition, the coupling tensor directions correlate very closely with the O8 water geometry, with the maximum quadrupole direction 3° from the water plane normal, and its minimum coupling about 2° from the H–H direction. In almost all previous magnetic resonance ¹⁷O-water studies, the quadrupole tensor orientation was based on theoretical considerations. This work represents one of the few experimental confirmations of its principal axis frame. When Cu²⁺ dopes into the Tutton salt, a Jahn–Teller distortion interchanges the relative long and intermediate metal O7 and O8 bond lengths of the zinc host. Therefore, only those unit cells containing the impurity conform to the pure copper Tutton structure. This study provides further support for this model. Moreover, coupling interactions from distant H₂¹⁷O such as in the present case have important implications in studies of copper enzymes and proteins where substrates have been proposed to displace weakly bound water in the active site

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

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

Electron Paramagnetic Resonance Spectroscopic Study of Copper Hopping in Doped Bis(l‑histidinato)cadmium Dihydrate

Electron paramagnetic resonance (EPR) spectroscopy was used to study Cu­(II) dynamic behavior in a doped biological model crystal, bis­(l-histidinato)cadmium dihydrate, in order to gain better insight into copper site stability in metalloproteins. Temperature-dependent changes in the low temperature X-band EPR spectra became visible around 100 K and continued up to room temperature. The measured 298 K g-tensor (principal values: 2.17, 2.16, 2.07) and copper hyperfine coupling tensor (principal values: −260, −190, −37 MHz) were similar to the average of the 77 K tensor values pertaining to two neighboring histidine binding sites. The observed temperature dependence was interpreted using Anderson’s theory of motional narrowing, where the magnetic parameters for the different states are averaged as the copper rapidly hops between sites. The EPR pattern was also found to undergo a sharp sigmoidal-shaped, temperature-dependent conversion between two species with a critical temperature <i>T</i>_c ≈ 160 K. The species below <i>T</i>_c hops between the two low temperature site patterns, and the one above <i>T</i>_c represents an average of the molecular spin Hamiltonian coupling tensors of the two 77 K sites. In addition, the low and high temperature species hop between one another, contributing to the dynamic averaging. Spectral simulations using this 4-state model determined a hop rate between the two low temperature sites ν_<i>h</i>4 = 4.5 × 10⁸ s^–1 and between the low and high temperature states ν_<i>h</i>2 = 1.7 × 10⁸ s^–1 at 160 K. An Arrhenius relationship of hop rate and temperature gave energy barriers of Δ<i>E</i>₄ = 389 cm^–1 and Δ<i>E</i>₂ = 656 cm^–1 between the two low temperature sites and between the low and high temperature states, respectively

Electron Paramagnetic Resonance Spectroscopic Study of Copper Hopping in Doped Bis(l‑histidinato)cadmium Dihydrate

Electron paramagnetic resonance (EPR) spectroscopy was used to study Cu­(II) dynamic behavior in a doped biological model crystal, bis­(l-histidinato)cadmium dihydrate, in order to gain better insight into copper site stability in metalloproteins. Temperature-dependent changes in the low temperature X-band EPR spectra became visible around 100 K and continued up to room temperature. The measured 298 K g-tensor (principal values: 2.17, 2.16, 2.07) and copper hyperfine coupling tensor (principal values: −260, −190, −37 MHz) were similar to the average of the 77 K tensor values pertaining to two neighboring histidine binding sites. The observed temperature dependence was interpreted using Anderson’s theory of motional narrowing, where the magnetic parameters for the different states are averaged as the copper rapidly hops between sites. The EPR pattern was also found to undergo a sharp sigmoidal-shaped, temperature-dependent conversion between two species with a critical temperature <i>T</i>_c ≈ 160 K. The species below <i>T</i>_c hops between the two low temperature site patterns, and the one above <i>T</i>_c represents an average of the molecular spin Hamiltonian coupling tensors of the two 77 K sites. In addition, the low and high temperature species hop between one another, contributing to the dynamic averaging. Spectral simulations using this 4-state model determined a hop rate between the two low temperature sites ν_<i>h</i>4 = 4.5 × 10⁸ s^–1 and between the low and high temperature states ν_<i>h</i>2 = 1.7 × 10⁸ s^–1 at 160 K. An Arrhenius relationship of hop rate and temperature gave energy barriers of Δ<i>E</i>₄ = 389 cm^–1 and Δ<i>E</i>₂ = 656 cm^–1 between the two low temperature sites and between the low and high temperature states, respectively