Dipartimento di Fisica, Università di Pisa, Largo Bruno Pontecorvo 3, I-56127, Pisa, Italy.
J Chem Phys. 2013 Jun 21;138(23):235102. doi: 10.1063/1.4810752.
The question whether the dynamics of hydrated proteins changes with temperature on crossing the glass transition temperature like that found in conventional glassformers is an interesting one. Recently, we have shown that a change of temperature dependence of the mean square displacement (MSD) at Tg is present in proteins solvated with bioprotectants, such as sugars or glycerol with or without the addition of water, coexisting with the dynamic transition at a higher temperature Td. The dynamical change at Tg is similar to that in conventional glassformers at sufficiently short times and low enough temperatures, where molecules are mutually caged by the intermolecular potential. This is a general and fundamental property of glassformers which is always observed at or near Tg independent of the energy resolution of the spectrometer, and is also the basis of the dynamical change of solvated proteins at Tg. When proteins are solvated with bioprotectants they show higher Tg and Td than the proteins hydrated by water alone, due to the stabilizing action of excipients, thus the observation of the change of T-dependence of the MSD at Tg is unobstructed by the methyl-group rotation contribution at lower temperatures [S. Capaccioli, K. L. Ngai, S. Ancherbak, and A. Paciaroni, J. Phys. Chem. B 116, 1745 (2012)]. On the other hand, in the case of proteins hydrated by water alone unambiguous evidence of the break at Tg is hard to find, because of their lower Tg and Td. Notwithstanding, in this paper, we provide evidence for the change at Tg of the T-dependence of proteins hydrated by pure water. This evidence turns out from (i) neutron scattering experimental investigations where the sample has been manipulated by either full or partial deuteration to suppress the methyl-group rotation contribution, and (ii) neutron scattering experimental investigations where the energy resolution is such that only motions with characteristic times shorter than 15 ps can be sensed, thus shifting the onset of both the methyl-group rotation and the dynamic transition contribution to higher temperatures. We propose that, in general, coexistence of the break of the elastic intensity or the MSD at Tg with the dynamic transition at Td in hydrated and solvated proteins. Recognition of this fact helps to remove inconsistency and conundrum encountered in interpreting data of hydrated proteins that thwart progress in understanding the origin of the dynamic transition.
水合蛋白质的动力学是否会像传统玻璃形成体那样随着温度跨越玻璃化转变温度而发生变化，这是一个有趣的问题。最近，我们已经表明，在含有生物保护剂（例如糖或甘油）的蛋白质中，即使在没有添加水的情况下，随着温度的升高，均方位移（MSD）的温度依赖性也会发生变化，这与更高温度 Td 下的动态转变并存。在足够短的时间和足够低的温度下，Tg 处的动力学变化与传统玻璃形成体相似，在这些条件下，分子被分子间势能相互困住。这是玻璃形成体的一个普遍而基本的性质，无论光谱仪的能量分辨率如何，都会在 Tg 或其附近观察到，也是 Tg 下水合蛋白质动力学变化的基础。当蛋白质被生物保护剂水合时，由于赋形剂的稳定作用，它们的 Tg 和 Td 会高于单独用水水合的蛋白质，因此在较低温度下，通过甲基旋转贡献来观察 MSD 对 T 的依赖性的变化不会受阻 [S. Capaccioli、K. L. Ngai、S. Ancherbak 和 A. Paciaroni，J. Phys. Chem. B 116，1745（2012）]。另一方面，在单独用水水合的蛋白质的情况下，由于 Tg 和 Td 较低，很难找到 Tg 处的明显证据。尽管如此，在本文中，我们提供了水合蛋白质 Tg 处 T 依赖性变化的证据。这些证据来自（i）中子散射实验研究，其中通过完全或部分氘化来操纵样品以抑制甲基旋转贡献，以及（ii）中子散射实验研究，其中能量分辨率使得只能感知到特征时间短于 15 ps 的运动，从而将甲基旋转和动态转变贡献的开始都推向更高的温度。我们提出，一般来说，在水合和溶剂化蛋白质中，Tg 处弹性强度或 MSD 的断裂与 Td 处的动态转变共存。认识到这一事实有助于消除在解释水合蛋白质数据时遇到的不一致和难题，这些难题阻碍了对动态转变起源的理解。
