Institute of Chemical Kinetics and Combustion, Russian Academy of Sciences, Novosibirsk 630090, Russian Federation.
Department of Physics, Novosibirsk State University, Novosibirsk 630090, Russian Federation.
J Chem Phys. 2017 Aug 14;147(6):064501. doi: 10.1063/1.4997035.
In glassy substances and biological media, dynamical transitions are observed in neutron scattering that manifests itself as deviations of the translational mean-squared displacement, 〈x〉, of hydrogen atoms from harmonic dynamics. In biological media, the deviation occurs at two temperature intervals, at ∼100-150 K and at ∼170-230 K, and it is attributed to the motion of methyl groups in the former case and to the transition from harmonic to anharmonic or diffusive motions in the latter case. In this work, electron spin echo (ESE) spectroscopy-a pulsed version of electron paramagnetic resonance-is applied to study the spin relaxation of nitroxide spin probes and labels introduced in molecular glass former o-terphenyl and in protein lysozyme. The anisotropic contribution to the rate of the two-pulse ESE decay, ΔW, is induced by spin relaxation appearing because of restricted orientational stochastic molecular motion; it is proportional to 〈α〉τ, where 〈α〉 is the mean-squared angle of reorientation of the nitroxide molecule around the equilibrium position and τ is the correlation time of reorientation. The ESE time window allows us to study motions with τ < 10 s. For glassy o-terphenyl, the 〈α〉τ temperature dependence shows a transition near 240 K, which is in agreement with the literature data on 〈x〉. For spin probes of essentially different size, the obtained data were found to be close, which evidences that motion is cooperative, involving a nanocluster of several neighboring molecules. For the dry lysozyme, the 〈α〉τ values below 260 K were found to linearly depend on the temperature in the same way as it was observed in neutron scattering for 〈x〉. As spin relaxation is influenced only by stochastic motion, the harmonic motions seen in ESE must be overdamped. In the hydrated lysozyme, ESE data show transitions near 130 K for all nitroxides, near 160 K for the probe located in the hydration layer, and near 180 K for the label in the protein interior. For this system, the two latter transitions are not observed in neutron scattering. The ESE-detected transitions are suggested to be related with water dynamics in the nearest hydration shell: with water glass transition near 130 K and with the onset of overall water molecular reorientations near 180 K; the disagreement with neutron scattering is ascribed to the larger time window for ESE-detected motions.
在玻璃状物质和生物介质中，观察到中子散射中的动力学转变，表现为氢原子的平移均方位移〈x〉偏离谐波动力学。在生物介质中，这种偏差发生在两个温度区间，在约 100-150 K 和约 170-230 K，前者归因于甲基基团的运动，后者归因于从谐波到非谐波或扩散运动的转变。在这项工作中，电子自旋回波（ESE）光谱——电子顺磁共振的脉冲版本——被应用于研究氮氧自由基自旋探针和标签在分子玻璃形成剂邻三联苯和蛋白质溶菌酶中的自旋弛豫。双脉冲 ESE 衰减的各向异性贡献ΔW 是由受限的各向同性随机分子运动引起的自旋弛豫引起的；它与〈α〉τ成正比，其中〈α〉是氮氧自由基分子围绕平衡位置的均方角重取向，τ是重取向的相关时间。ESE 时间窗口允许我们研究 τ<10 s 的运动。对于玻璃状邻三联苯，〈α〉τ的温度依赖性在 240 K 附近出现转变，这与文献中关于〈x〉的报道一致。对于大小基本不同的自旋探针，得到的数据非常接近，这表明运动是协同的，涉及几个相邻分子的纳米团簇。对于干燥的溶菌酶，在 260 K 以下，〈α〉τ值以与中子散射中观察到的〈x〉相同的方式线性依赖于温度。由于自旋弛豫仅受随机运动的影响，因此在 ESE 中观察到的谐波运动必须是过阻尼的。在水合溶菌酶中，所有氮氧自由基的 ESE 数据在 130 K 附近显示转变，位于水合层的探针在 160 K 附近显示转变，位于蛋白质内部的标签在 180 K 附近显示转变。对于这个系统，后两个转变在中子散射中没有观察到。ESE 检测到的转变被认为与最近水合壳层中的水动力学有关：在 130 K 附近发生水玻璃化转变，在 180 K 附近发生整体水分子重取向的开始；与中子散射的不一致归因于 ESE 检测到的运动的更大时间窗口。
