NMR at low and ultralow temperatures.
NMR at low and ultralow temperatures.
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DOI：
10.1021/ar300358z
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发表时间：
2013-09-17
影响因子：
18.3
通讯作者：
Tycko R
中科院分区：
文献类型：
--
作者：
Tycko R
低温下的固态核磁共振（NMR）测量在物理科学中已经很常见了很多年，并且在生物分子系统的研究中变得越来越重要。本文回顾了我的实验室从20世纪90年代初开始的一系列不同的项目，这些项目说明了低温固态核磁共振的动机，从测量中获得的信息类型，以及未来研究的可能方向。这些项目包括物理和生物系统的核磁共振研究，在低温（用氮气冷却，温度低于77 K）和极低温（用氦气冷却，温度低于77 K）下进行，并在有或没有魔角旋转（MAS）的情况下进行。在物理系统的核磁共振研究中，主要动机是研究仅在低温下发生的现象。我实验室的两个例子是研究低温下固体C60中的分子旋转和取向有序，以及研究半导体量子阱中二维受限电子系统中不寻常的电子态（称为skyrmions）。核自旋极化的光抽运促进了量子阱的核磁共振测量，这是一种存在于极低温度下的信号增强现象。在生物分子系统的研究中，低温核磁共振的动机包括抑制分子翻滚（从而允许对可溶性蛋白质进行固态核磁共振测量），抑制构象交换（从而允许定量构象分布），以及在非平衡动力学过程中捕获瞬态中间状态（通过快速冷冻淬火）。对艾滋病相关肽/抗体复合物的固体核磁共振测量，模型蛋白HP35的化学变性状态，以及HP35快速折叠途径中的瞬时中间体说明了这些动机。核磁共振灵敏度一般随样品温度的降低而升高。因此，尽可能地低温是有利的，特别是在冷冻溶液中研究生物分子系统时。然而，生物分子体系的固态核磁共振研究通常需要快速的MAS。一种新颖的MAS NMR探针设计，使用氮气进行样品旋转，冷氦仅用于样品冷却，允许在20-25 K下对生物分子系统进行各种固态NMR测量，其中信号相对于室温下的测量增强了12-15倍。在非常低的温度下，MAS核磁共振也促进了动态核极化（DNP），允许相当大的额外信号增强和大的绝对核磁共振信号幅度，以相对较低的微波功率实现。我的实验室目前的研究是在非常低的温度下开发和利用dnp增强的MAS NMR，例如研究蛋白质折叠和聚集过程中的瞬态中间体，以及研究只能在低浓度下制备的肽/蛋白质复合物。
Solid state nuclear magnetic resonance (NMR) measurements at low temperatures have been common in physical sciences for many years, and are becoming increasingly important in studies of biomolecular systems. This article reviews a diverse set of projects from my laboratory, dating back to the early 1990s, that illustrate the motivations for low-temperature solid state NMR, the types of information that are available from the measurements, and likely directions for future research. These projects include NMR studies of both physical and biological systems, performed at low (cooled with nitrogen, down to 77 K) and very low (cooled with helium, below 77 K) temperatures, and performed with and without magic-angle spinning (MAS). In NMR studies of physical systems, the main motivation is to study phenomena that occur only at low temperatures. Two examples from my laboratory are studies of molecular rotation and an orientational ordering in solid C60 at low temperatures and studies of unusual electronic states, called skyrmions, in two-dimensionally confined electron systems within semiconductor quantum wells. NMR measurements on quantum wells were facilitated by optical pumping of nuclear spin polarizations, a signal enhancement phenomenon that exists at very low temperatures. In studies of biomolecular systems, motivations for low-temperature NMR include suppression of molecular tumbling (thereby permitting solid state NMR measurements on soluble proteins), suppression of conformational exchange (thereby permitting quantitation of conformational distributions), and trapping of transient intermediate states in a non-equilibrium kinetic process (by rapid freeze-quenching). Solid state NMR measurements on AIDS-related peptide/antibody complexes, chemically denatured states of the model protein HP35, and a transient intermediate in the rapid folding pathway of HP35 illustrate these motivations. NMR sensitivity generally increases with decreasing sample temperature. It is therefore advantageous to go as cold as possible, particularly in studies of biomolecular systems in frozen solutions. However, solid state NMR studies of biomolecular systems generally require rapid MAS. A novel MAS NMR probe design that uses nitrogen gas for sample spinning and cold helium only for sample cooling allows a wide variety of solid state NMR measurements to be performed on biomolecular systems at 20-25 K, where signals are enhanced by factors of 12-15 relative to measurements at room temperature. MAS NMR at very low temperatures also facilitates dynamic nuclear polarization (DNP), allowing sizeable additional signal enhancements and large absolute NMR signal amplitudes to be achieved with relatively low microwave powers. Current research in my laboratory seeks to develop and exploit DNP-enhanced MAS NMR at very low temperatures, for example in studies of transient intermediates in protein folding and aggregation processes and studies of peptide/protein complexes that can be prepared only at low concentrations.
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