Zhang Rongchun, Mroue Kamal H, Ramamoorthy Ayyalusamy
Biophysics and Department of Chemistry, University of Michigan, Ann Arbor, MI 48109-1055, USA.
Biophysics and Department of Chemistry, University of Michigan, Ann Arbor, MI 48109-1055, USA.
J Magn Reson. 2016 May;266:59-66. doi: 10.1016/j.jmr.2016.03.006. Epub 2016 Mar 24.
Heteronuclear cross polarization (CP) has been commonly used to enhance the sensitivity of dilute low-γ nuclei in almost all solid-state NMR experiments. However, CP relies on heteronuclear dipolar couplings, and therefore the magnetization transfer efficiency becomes inefficient when the dipolar couplings are weak, as is often the case for mobile components in solids. Here, we demonstrate methods that combine CP with heteronuclear Overhauser effect (referred to as CP-NOE) or with refocused-INEPT (referred to as CP-RINEPT) to overcome the efficiency limitation of CP and enhance the signal-to-noise ratio (S/N) for mobile components. Our experimental results reveal that, compared to the conventional CP, significant S/N ratio enhancement can be achieved for resonances originating from mobile components, whereas the resonance signals associated with rigid groups are not significantly affected due to their long spin-lattice relaxation times. In fact, the S/N enhancement factor is also dependent on the temperature, CP contact time as well as on the system under investigation. Furthermore, we also demonstrate that CP-RINEPT experiment can be successfully employed to independently detect mobile and rigid signals in a single experiment without affecting the data collection time. However, the resolution of CP spectrum obtained from the CP-RINEPT experiment could be slightly compromised by the mandatory use of continuous wave (CW) decoupling during the acquisition of signals from rigid components. In addition, CP-RINEPT experiment can be used for spectral editing utilizing the difference in dynamics of different regions of a molecule and/or different components present in the sample, and could also be useful for the assignment of resonances from mobile components in poorly resolved spectra. Therefore, we believe that the proposed approaches are beneficial for the structural characterization of multiphase and heterogeneous systems, and could be used as a building block in multidimensional solid-state NMR experiments.
在几乎所有的固态核磁共振实验中，异核交叉极化（CP）通常用于提高稀低γ核的灵敏度。然而，CP依赖于异核偶极耦合，因此当偶极耦合较弱时，磁化转移效率会变得低下，固体中的移动成分通常就是这种情况。在此，我们展示了将CP与异核奥弗豪泽效应（称为CP-NOE）或与重聚焦INEPT（称为CP-RINEPT）相结合的方法，以克服CP的效率限制并提高移动成分的信噪比（S/N）。我们的实验结果表明，与传统CP相比，源自移动成分的共振可实现显著的信噪比增强，而与刚性基团相关的共振信号由于其较长的自旋晶格弛豫时间并未受到显著影响。实际上，信噪比增强因子还取决于温度、CP接触时间以及所研究的系统。此外，我们还证明CP-RINEPT实验可成功用于在单个实验中独立检测移动和刚性信号，而不影响数据采集时间。然而，在采集刚性成分的信号期间强制使用连续波（CW）去耦可能会略微损害从CP-RINEPT实验获得的CP谱的分辨率。此外，CP-RINEPT实验可用于利用分子不同区域和/或样品中存在的不同成分的动力学差异进行光谱编辑，对于解析度较差的光谱中移动成分的共振归属也可能有用。因此，我们认为所提出的方法对多相和异质系统的结构表征有益，并且可作为多维固态核磁共振实验的一个组成部分。
