Control of Nuclear Spin Interactions in Solid-state NMR by MAS Sideband Manipulation

核磁共振波谱是确定分子结构的最强大的手段之一，在过去的25年里对蛋白质等生物分子的研究做出了重大贡献。最近，核磁共振也成为研究固相复杂分子体系的一种重要技术。例如膜蛋白、超分子组装体、生物界面、聚合物、分子筛、离子导体、纳米复合材料和催化剂，这些系统将为未来的一系列科学和技术发展奠定基础。关于分子结构的详细信息是从与取向有关的核自旋相互作用中获得的，如化学位移各向异性(CSA)。这些测量可以从粉末固体的宽线核磁共振谱中得到，这些谱显示的奇点直接对应于描述相互作用的张量的主分量。然而，光谱重叠意味着经常需要求助于魔角旋转(MAS)光谱中出现的旋转边带的强度分析，这种方法在CSA的情况下已经成为常规方法。此外，最近的研究表明，对中等数量(约8/10)的自旋边带进行分析，通常会比拟合宽线谱给出更可靠的结果。通常需要相对较低的自旋速度才能获得如此多的边带，因此已经开发了许多二维MAS核磁共振实验，将边带流形从不同的位置分离出来，并进一步提高分辨率。特别是，我们展示了如何记录边带强度与样品以实际MAS速率的某个分数旋转时的预期强度相同的光谱。这个CSA放大实验代表了一种测量旋转边带强度的新方法，也是这里提出的研究计划的起点。在研究过程中，我们将扩大CSA放大的范围，将其他相互作用包括在内，对它们进行缩放，使在传统方法无法实现的情况下测量相应的张量是可行的。我们将在系统中对CSA参数进行测量，这对传统方法具有挑战性。我们将把结果与计算的张量进行比较，以提取结构信息，并在分辨率较差的MAS光谱中进行指认。此外，我们将研究使用MAS边带强度来研究分子动力学的实验如何从CSA放大方法中受益。

Nuclear magnetic resonance (NMR) spectroscopy is one of the most powerful means of determining molecular structure, making significant contributions over the last 25 years to the study of biological molecules, such as proteins. More recently, NMR has also emerged as an important technique for studying complex molecular systems in the solid phase. Examples include membrane proteins, supramolecular assemblies, biological interfaces, polymers, molecular sieves, ionic conductors, nanocomposite materials and catalysts, systems which will underpin a host of scientific and technological developments in the future.Detailed information about molecular structure is obtained from orientation-dependent nuclear spin interactions, such as the chemical shift anisotropy (CSA). Measurements of these can be made from wideline NMR spectra of powdered solids which show singularities corresponding directly to the principal components of the tensor which describes the interaction. However, spectral overlap means that it is often necessary to resort instead to an analysis of the intensities of the rotational sidebands which appear in the magic angle spinning (MAS) spectrum, and this approach has become routine in the case of the CSA. Furthermore, it has been shown recently that analysis of a moderate number (approximately 8 / 10) of spinning sidebands usually gives more reliable results than fitting the wideline spectrum. A relatively low spinning rate is normally required to give this many sidebands, and so many two-dimensional MAS NMR experiments have been developed which separate the sideband manifolds from different sites and further improve resolution. In particular, we have shown how to record spectra in which the sideband intensities are identical to those expected for a sample spinning at some fraction of the actual MAS rate. This CSA amplification experiment represents a new approach to the measurement of spinning sideband intensities and is the starting point for the research programme proposed here.In the course of the research we will broaden the scope of CSA amplification to include other interactions, scaling them to make measurement of the corresponding tensors feasible in situations where conventional methods fail. We will make measurements of CSA parameters in systems which are challenging for conventional methods. We will compare the results with calculated tensors to extract structural information and to make assignments in poorly-resolved MAS spectra. In addition, we will investigate how experiments which use MAS sideband intensities to study molecular dynamics can benefit from the CSA amplification approach.