CNR-IPCF, Consiglio Nazionale delle Ricerche, Istituto per i Processi Chimico-Fisici, c/o Dipartimento di Fisica, Largo Bruno Pontecorvo 3, I-56127 Pisa, Italy.
J Phys Chem B. 2012 Feb 16;116(6):1745-57. doi: 10.1021/jp2057892. Epub 2012 Feb 3.
Mössbauer spectroscopy and neutron scattering measurements on proteins embedded in solvents including water and aqueous mixtures have emphasized the observation of the distinctive temperature dependence of the atomic mean square displacements, <u(2)>, commonly referred to as the dynamic transition at some temperature T(d). At low temperatures, <u(2)> increases slowly, but it assumes stronger temperature dependence after crossing T(d), which depends on the time/frequency resolution of the spectrometer. Various authors have made connection of the dynamics of solvated proteins, including the dynamic transition to that of glass-forming substances. Notwithstanding, no connection is made to the similar change of temperature dependence of <u(2)> obtained by quasielastic neutron scattering when crossing the glass transition temperature T(g), generally observed in inorganic, organic, and polymeric glass-formers. Evidences are presented here to show that such a change of the temperature dependence of <u(2)> from neutron scattering at T(g) is present in hydrated or solvated proteins, as well as in the solvent used, unsurprisingly since the latter is just another organic glass-former. If unaware of the existence of such a crossover of <u(2)> at T(g), and if present, it can be mistaken as the dynamic transition at T(d) with the ill consequences of underestimating T(d) by the lower value T(g) and confusing the identification of the origin of the dynamic transition. The <u(2)> obtained by neutron scattering at not so low temperatures has contributions from the dissipation of molecules while caged by the anharmonic intermolecular potential at times before dissolution of cages by the onset of the Johari-Goldstein β-relaxation or of the merged α-β relaxation. The universal change of <u(2)> at T(g) of glass-formers, independent of the spectrometer resolution, had been rationalized by sensitivity to change in volume and entropy of the dissipation of the caged molecules and its contribution to <u(2)>. The same rationalization applies to hydrated and solvated proteins for the observed change of <u(2)> at T(g).
在包括水和水混合物在内的溶剂中嵌入蛋白质的穆斯堡尔光谱和中子散射测量强调了观察到原子均方位移<u(2)>的独特温度依赖性，通常称为在某个温度 T(d)下的动态转变。在低温下，<u(2)>缓慢增加，但在穿过 T(d)后，它呈现出更强的温度依赖性，这取决于光谱仪的时间/频率分辨率。许多作者已经将溶剂化蛋白质的动力学，包括动态转变，与玻璃形成物质的动力学联系起来。然而，当穿过玻璃化转变温度 T(g)时，在由准弹性中子散射获得的<u(2)>的温度依赖性相似变化方面，没有任何联系，这种变化通常在无机、有机和聚合物玻璃形成剂中观察到。本文提出了证据表明，在水合或溶剂化蛋白质中以及在溶剂中存在这种<u(2)>的温度依赖性的变化，这并不奇怪，因为后者只是另一种有机玻璃形成剂。如果不知道在 T(g)处存在这种<u(2)>的交叉，并且如果存在，它可能会被误认为是在 T(d)处的动态转变，从而导致低估 T(d)值为 T(g)，并混淆对动态转变起源的识别。在不那么低的温度下由中子散射获得的<u(2)>，在分子被非谐分子间势束缚时，在笼的溶解之前，会有分子的耗散贡献，直到乔哈里-戈尔登斯坦β弛豫或合并的α-β弛豫开始溶解笼子。在玻璃形成剂中，在不依赖于光谱仪分辨率的情况下，<u(2)>的普遍变化归因于对笼中分子耗散的体积和熵变化的敏感性及其对<u(2)>的贡献。对于在 T(g)处观察到的<u(2)>的变化，对于水合和溶剂化蛋白质，也适用相同的合理化。
