Long-range distance measurements based on paramagnetic relaxation enhancement (PRE) in NMR, quantification of surface water dynamics near biomacromolecules by Overhauser dynamic nuclear polarization (DNP) and sensitivity enhancement by solid-state DNP all depend on introducing paramagnetic species into an otherwise diamagnetic NMR sample. The species can be introduced by site-directed spin labeling, which offers precise control for positioning the label in the sequence of a biopolymer. However, internal flexibility of the spin label gives rise to dynamic processes that potentially influence PRE and DNP behavior and leads to a spatial distribution of the electron spin even in solid samples. Internal dynamics of spin labels and their static conformational distributions have been studied mainly by electron paramagnetic resonance spectroscopy and molecular dynamics simulations, with a large body of results for the most widely applied methanethiosulfonate spin label MTSL. These results are critically discussed in a unifying picture based on rotameric states of the group that carries the spin label. Deficiencies in our current understanding of dynamics and conformations of spin labeled groups and of their influence on NMR observables are highlighted and directions for further research suggested.

中文翻译：

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氮氧化物自旋标记的构象动力学和分布

基于 NMR 中顺磁弛豫增强 (PRE) 的远程距离测量、Overhauser 动态核极化 (DNP) 对生物大分子附近地表水动力学的量化以及固态 DNP 的灵敏度增强都取决于将顺磁物质引入到其他抗磁 NMR 中样本。该物种可以通过定点自旋标记引入，这提供了在生物聚合物序列中定位标记的精确控制。然而，自旋标签的内部灵活性产生了可能影响 PRE 和 DNP 行为的动态过程，并导致电子自旋的空间分布，即使在固体样品中也是如此。自旋标记的内部动力学及其静态构象分布主要通过电子顺磁共振光谱和分子动力学模拟进行了研究，并获得了大量应用最广泛的甲硫基磺酸盐自旋标记 MTSL 的结果。这些结果在基于带有自旋标签的组的旋转异构状态的统一图片中进行了批判性讨论。强调了我们目前对自旋标记基团的动力学和构象及其对 NMR 可观测值的影响的理解中的不足，并提出了进一步研究的方向。这些结果在基于带有自旋标签的组的旋转异构状态的统一图片中进行了批判性讨论。强调了我们目前对自旋标记基团的动力学和构象及其对 NMR 可观测值的影响的理解中的不足，并提出了进一步研究的方向。这些结果在基于带有自旋标签的组的旋转异构状态的统一图片中进行了批判性讨论。强调了我们目前对自旋标记基团的动力学和构象及其对 NMR 可观测值的影响的理解中的不足，并提出了进一步研究的方向。