ETH Zurich, Physical Chemistry, Vladimir-Prelog-Weg 2, 8093 Zurich, Switzerland.
ETH Zurich, Physical Chemistry, Vladimir-Prelog-Weg 2, 8093 Zurich, Switzerland.
J Magn Reson. 2018 Aug;293:115-122. doi: 10.1016/j.jmr.2018.06.002. Epub 2018 Jun 15.
Fast magic-angle spinning, coupled with H detection is a powerful method to improve spectral resolution and signal to noise in solid-state NMR spectra. Commercial probes now provide spinning frequencies in excess of 100 kHz. Then, one has sufficient resolution in the H dimension to directly detect protons, which have a gyromagnetic ratio approximately four times larger than C spins. However, the gains in sensitivity can quickly be lost if the rotation angle is not set precisely. The most common method of magic-angle calibration is to optimize the number of rotary echoes, or sideband intensity, observed on a sample of KBr. However, this typically uses relatively low spinning frequencies, where the spinning of fast-MAS probes is often unstable, and detection on the C channel, for which fast-MAS probes are typically not optimized. Therefore, we compare the KBr-based optimization of the magic angle with two alternative approaches: optimization of the splitting observed in C-labeled glycine-ethylester on the carbonyl due to the Cα-C' J-coupling, or optimization of the H-N J-coupling spin echo in the protein sample itself. The latter method has the particular advantage that no separate sample is necessary for the magic-angle optimization.
快速魔角旋转（Fast magic-angle spinning）与 H 检测相结合是提高固态 NMR 谱谱分辨率和信噪比的有力方法。现在，商业探头提供的旋转频率超过 100 kHz。然后，在 H 维度上有足够的分辨率可以直接检测质子，质子的磁旋比大约是 C 自旋的四倍。然而，如果旋转角度设置不精确，灵敏度的提高很快就会丧失。魔角校准最常用的方法是优化在 KBr 样品上观察到的旋转回波（rotary echoes）的数量或边带强度。然而，这通常使用相对较低的旋转频率，在这种情况下，快速魔角探头的旋转往往不稳定，并且在 C 通道上进行检测，而快速魔角探头通常不是针对 C 通道进行优化的。因此，我们将基于 KBr 的魔角优化与两种替代方法进行比较：优化由于 Cα-C' J 偶合而在羰基上观察到的 C 标记甘氨酸乙酯的分裂，或优化蛋白质样品本身中的 H-N J 偶合自旋回波。后一种方法的特别优点是不需要单独的样品来进行魔角优化。
