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

Abstract

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 Tc ≈ 160 K. The species below Tc hops between the two low temperature site patterns, and the one above Tc 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 νh4 = 4.5 × 108 s−1 and between the low and high temperature states νh2 = 1.7 × 108 s−1 at 160 K. An Arrhenius relationship of hop rate and temperature gave energy barriers of ΔE4 = 389 cm−1 and ΔE2 = 656 cm−1 between the two low temperature sites, and between the low and high temperature states, respectively.

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] .…”

“…In general, EPR studies of small molecule crystal systems are particularly advantageous because they provide insights into how dynamics are related to the nature of the unpaired electron wave function. 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. − In addition, the current study complements our earlier EPR investigations on copper–histidine complexes whose temperature-dependent spectra could be explained as a motional averaging between neighboring metal-binding sites. − …”

“…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.…”