Department of Chemistry and Physics, State University of New York at Old Westbury, Old Westbury, New York 11568, USA.
J Phys Chem A. 2013 Apr 25;117(16):3414-27. doi: 10.1021/jp401477m. Epub 2013 Apr 12.
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 T(c) ≈ 160 K. The species below T(c) hops between the two low temperature site patterns, and the one above T(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(h4) = 4.5 × 10(8) s(-1) and between the low and high temperature states ν(h2) = 1.7 × 10(8) 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.
电子顺磁共振（EPR）光谱被用于研究掺杂生物模型晶体双（L-组氨酸）镉二水合物中 Cu(II) 的动态行为，以更好地了解金属蛋白酶中铜位点的稳定性。在 100 K 左右和室温下，低温 X 波段 EPR 光谱的温度依赖性变化变得可见。测量的 298 K g 张量（主值：2.17、2.16、2.07）和铜超精细耦合张量（主值：-260、-190、-37 MHz）与两个相邻组氨酸结合位点的平均 77 K 张量值相似。观察到的温度依赖性使用 Anderson 的运动变窄理论进行解释，其中不同状态的磁参数作为铜在位点之间快速跳跃而被平均。EPR 模式还发现经历了尖锐的类正弦形、温度依赖的转换，其中临界温度 T(c)≈160 K。低于 T(c)的物质在两个低温位点之间跳跃，高于 T(c)的物质代表两个 77 K 位点的分子自旋哈密顿耦合张量的平均值。此外，低温和高温物质在彼此之间跳跃，有助于动态平均。使用此 4 状态模型的光谱模拟确定了两个低温位点之间的跳跃速率 ν(h4)=4.5×10(8) s(-1)和低温和高温状态之间的跳跃速率 ν(h2)=1.7×10(8) s(-1)在 160 K。跳跃速率和温度的 Arrhenius 关系给出了两个低温位点之间的能垒ΔE4=389 cm(-1)和低温和高温状态之间的能垒ΔE2=656 cm(-1)。
