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

核磁共振波谱是测定分子结构最有力的手段之一，在过去25年中对生物分子（如蛋白质）的研究做出了重大贡献。近年来，核磁共振也成为研究固相复杂分子体系的重要技术。例子包括膜蛋白、超分子组件、生物界面、聚合物、分子筛、离子导体、纳米复合材料和催化剂，这些系统将成为未来一系列科学和技术发展的基础。分子结构的详细信息可以从取向依赖的核自旋相互作用中获得，例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光谱中进行，该光谱显示出与描述相互作用的张量的主成分直接对应的奇点。然而，光谱重叠意味着通常需要转而分析出现在魔角旋转（MAS）光谱中的旋转边带的强度，这种方法在CSA的情况下已成为常规方法。此外，最近的研究表明，分析中等数量的旋转侧带（大约8 / 10）通常比拟合宽线谱给出更可靠的结果。通常需要一个相对较低的旋转速率来获得如此多的边带，因此已经开发了许多二维MAS核磁共振实验，这些实验将边带流形从不同的位置分离出来，进一步提高了分辨率。特别是，我们已经展示了如何记录边带强度与以实际MAS速率的一部分旋转的样品的预期强度相同的光谱。该CSA放大实验代表了一种测量旋转边带强度的新方法，是本文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，包括其他相互作用，缩放它们，以便在常规方法失败的情况下测量相应的张量。我们将测量系统中的CSA参数，这对传统方法来说是具有挑战性的。我们将结果与计算张量进行比较，以提取结构信息并在低分辨率的MAS光谱中进行分配。此外，我们将探讨利用MAS边带强度研究分子动力学的实验如何受益于CSA扩增方法。

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.