Laboratory of Chemical Physics, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892-0520, USA.
Protein Sci. 2011 Feb;20(2):229-46. doi: 10.1002/pro.576.
Sparsely populated states of macromolecules, characterized by short lifetimes and high free-energies relative to the predominant ground state, often play a key role in many biological, chemical, and biophysical processes. In this review, we briefly summarize various new developments in NMR spectroscopy that permit these heretofore invisible, sparsely populated states to be detected, characterized, and in some instances visualized. Relaxation dispersion spectroscopy yields detailed kinetic information on processes involving species characterized by distinct chemical shifts with lifetimes in the ∼50 μs-10 ms range and populations as low as 0.5%. In the fast exchange regime (time scale less than ∼250-500 μs), the footprint of sparsely populated states can be observed on paramagnetic relaxation enhancement profiles measured on the resonances of the major species, thereby yielding structural information that is directly related to paramagnetic center-nuclei distances from which it is possible, under suitable circumstances, to compute a structure or ensemble of structures for the minor species. Finally, differential transverse relaxation measurements can be used to detect lifetime broadening effects that directly reflect the unidirectional rates for the conversion of NMR-visible into high-molecular weight NMR-invisible species. Examples of these various approaches are presented.
稀疏分布的大分子态，其特点是寿命短，自由能相对于主要基态高，在许多生物、化学和生物物理过程中经常起着关键作用。在这篇综述中，我们简要总结了 NMR 光谱学的各种新发展，这些新发展使得这些以前看不见的、稀疏分布的状态能够被检测、表征，并在某些情况下可视化。弛豫分散光谱学提供了关于具有不同化学位移、寿命在 50 μs-10 ms 范围内以及种群密度低至 0.5%的物种的详细动力学信息。在快速交换（时间尺度小于 250-500 μs）的情况下，在主要物种共振上测量的顺磁弛豫增强谱上可以观察到稀疏分布状态的足迹，从而提供与顺磁中心-核之间距离直接相关的结构信息，在适当的情况下，可以计算出次要物种的结构或结构集合。最后，差示横向弛豫测量可用于检测寿命展宽效应，该效应直接反映了将 NMR 可见物种转化为高分子量 NMR 不可见物种的单向速率。展示了这些各种方法的示例。
