Reichert D, Bonagamba T J, Schmidt-Rohr K
FB Physik, Martin-Luther Universität Halle-Wittenberg, Halle, 06108, Germany.
J Magn Reson. 2001 Jul;151(1):129-35. doi: 10.1006/jmre.2001.2337.
One- and two-dimensional 13C exchange nuclear magnetic resonance experiments under magic-angle spinning (MAS) can provide detailed information on slow segmental reorientations and chemical exchange in organic solids, including polymers and proteins. However, observations of dynamics on the time scale of seconds or longer are hampered by the competing process of dipolar 13C spin exchange (spin diffusion). In this Communication, we show that fast MAS can significantly slow down the dipolar spin exchange effect for unprotonated carbon sites. The exchange is measured quantitatively using the centerband-only detection of exchange technique, which enables the detection of exchange at any spinning speed, even in the absence of changes of isotropic chemical shifts. For chemically equivalent unprotonated 13C sites, the dipolar spin exchange rate is found to decrease slightly less than proportionally with the sample-rotation frequency, between 8 and 28 kHz. In the same range, the dipolar spin exchange rate for a glassy polymer with an inhomogeneously broadened MAS line decreases by a factor of 10. For methylene groups, no or only a minor slow-down of the exchange rate is found.
在魔角旋转（MAS）条件下的一维和二维¹³C交换核磁共振实验能够提供有关有机固体（包括聚合物和蛋白质）中缓慢链段重排和化学交换的详细信息。然而，由于偶极¹³C自旋交换（自旋扩散）这一竞争过程，对秒级或更长时间尺度上动力学的观测受到了阻碍。在本通讯中，我们表明快速MAS能够显著减缓未质子化碳位点的偶极自旋交换效应。使用仅检测中心带的交换技术对交换进行定量测量，该技术能够在任何旋转速度下检测交换，即使在各向同性化学位移没有变化的情况下也是如此。对于化学等价的未质子化¹³C位点，发现偶极自旋交换速率随样品旋转频率在8至28千赫兹之间略有下降，下降幅度小于成正比关系。在相同范围内，具有不均匀展宽MAS谱线的玻璃态聚合物的偶极自旋交换速率降低了10倍。对于亚甲基，未发现交换速率降低或仅略有降低。
