Department of Chemistry and Biochemistry , The Ohio State University , Columbus , Ohio 43210 , United States.
Biophysics Graduate Program , The Ohio State University , Columbus , Ohio 43210 , United States.
J Am Chem Soc. 2019 Oct 23;141(42):16829-16838. doi: 10.1021/jacs.9b08032. Epub 2019 Oct 14.
NMR spectroscopy is an extraordinarily rich source of quantitative dynamics of proteins in solution using spin relaxation or chemical exchange saturation transfer (CEST) experiments. However, N-CEST measurements require prolonged multidimensional, so-called pseudo-3D HSQC experiments where the pseudo dimension is a radio frequency offset Δω of a weak N saturation field. Nonuniform sampling (NUS) approaches have the potential to significantly speed up these measurements, but they also carry the risk of introducing serious artifacts and the systematic optimization of nonuniform sampling schedules has remained elusive. It is demonstrated here how this challenge can be addressed by using fitted cross-peaks of a reference 2D HSQC experiment as footprints, which are subsequently used to reconstruct cross-peak amplitudes of a pseudo-3D data set as a function of Δω by a linear least-squares fit. It is shown for protein Im7 how the approach can yield highly accurate CEST profiles based on an absolutely minimally sampled (AMS) data set allowing a speed-up of a factor 20-30. Spectrum-specific optimized nonuniform sampling (SONUS) schemes based on the Cramer-Rao lower bound metric were critical to achieve such a performance, revealing also more general properties of optimal sampling schedules. This is the first systematic exploration and optimization of NUS schedules for the dramatic speed-up of quantitative multidimensional NMR measurements that minimize unwanted errors.
NMR 光谱学是一种非常丰富的定量研究溶液中蛋白质动力学的方法，可利用自旋弛豫或化学交换饱和转移（CEST）实验。然而，N-CEST 测量需要长时间的多维，所谓的伪 3D HSQC 实验，其中伪维度是弱 N 饱和场的射频偏移 Δω。非均匀采样（NUS）方法有可能显著加快这些测量速度，但它们也存在引入严重伪影的风险，因此非均匀采样方案的系统优化仍然难以捉摸。本文展示了如何通过使用参考 2D HSQC 实验的拟合交叉峰作为足迹来解决这一挑战，然后通过线性最小二乘法拟合，将其用于根据 Δω 重建伪 3D 数据集的交叉峰幅度。本文以 Im7 蛋白为例，展示了该方法如何基于绝对最小采样（AMS）数据集生成高度准确的 CEST 谱，从而实现 20-30 倍的加速。基于克拉美罗下界度量的谱特定优化非均匀采样（SONUS）方案对于实现这种性能至关重要，同时也揭示了最佳采样方案的更一般特性。这是首次对用于最大限度减少不必要误差的定量多维 NMR 测量的显著加速的 NUS 方案进行系统探索和优化。
