ETH Zurich, Physical Chemistry, Vladimir-Prelog-Weg 2, 8093 Zurich, Switzerland.
J Chem Phys. 2018 Jan 28;148(4):045104. doi: 10.1063/1.5013316.
Relaxation in nuclear magnetic resonance (NMR) results from stochastic motions that modulate anisotropic NMR interactions. Therefore, measurement of relaxation-rate constants can be used to characterize molecular-dynamic processes. The motion is often characterized by Markov processes using an auto-correlation function, which is assumed to be a sum of multiple decaying exponentials. We have recently shown that such a model can lead to severe misrepresentation of the real motion, when the real correlation function is more complex than the model. Furthermore, multiple distributions of motion may yield the same set of dynamics data. Therefore, we introduce optimized dynamics "detectors" to characterize motions which are linear combinations of relaxation-rate constants. A detector estimates the average or total amplitude of motion for a range of motional correlation times. The information obtained through the detectors is less specific than information obtained using an explicit model, but this is necessary because the information contained in the relaxation data is ambiguous, if one does not know the correct motional model. On the other hand, if one has a molecular dynamics trajectory, one may calculate the corresponding detector responses, allowing direct comparison to experimental NMR dynamics analysis. We describe how to construct a set of optimized detectors for a given set of relaxation measurements. We then investigate the properties of detectors for a number of different data sets, thus gaining an insight into the actual information content of the NMR data. Finally, we show an example analysis of ubiquitin dynamics data using detectors, using the DIFRATE software.
磁共振（NMR）弛豫源于各向异性 NMR 相互作用的随机运动。因此，弛豫率常数的测量可用于描述分子动力学过程。运动通常通过自相关函数用马尔可夫过程来描述，该函数被假定为多个衰减指数的和。我们最近表明，当实际相关函数比模型更复杂时，这种模型可能会严重歪曲实际运动。此外，运动的多个分布可能会产生相同的动力学数据集。因此，我们引入了优化的动力学“探测器”，以描述由弛豫率常数线性组合而成的运动。探测器估计运动的平均或总幅度，适用于一系列运动相关时间。通过探测器获得的信息不如使用显式模型获得的信息具体，但这是必要的，因为如果不知道正确的运动模型，弛豫数据中包含的信息是模糊的。另一方面，如果有分子动力学轨迹，可以计算相应的探测器响应，从而可以直接与实验 NMR 动力学分析进行比较。我们描述了如何为给定的一组弛豫测量构建一组优化的探测器。然后，我们研究了许多不同数据集的探测器的特性，从而深入了解 NMR 数据的实际信息量。最后，我们使用 DIFRATE 软件，通过探测器展示了对泛素动力学数据的示例分析。
