High Resolution Solid State Nitrogen-14 NMR

通过开发新的实验和计算方法，我们的目标是使主要自然存在的氮同位素的固体核磁共振谱变得更加容易和更有信息量。14N核磁共振过去非常困难，因为14N原子核的电四极矩产生了非常宽的谱线。我们找到了一种有希望的解决这个问题的方法--它可能为表征一系列生物分子、天然材料和药物开辟了一条新的途径，包括纤维蛋白，这些都是了解淀粉样疾病的关键靶点。在这个项目中，我们将开发高分辨率的14N核磁共振方法，并使用它们来深入了解15N浓缩困难或不可能的系统的结构和动态信息，例如环境样品、天然材料和组织活检。核磁共振是一种分析分子结构和动力学的强大技术，而固态核磁共振在研究从纤维或膜蛋白等非晶态生物固体到非晶态玻璃等一系列材料方面具有独特的地位。尽管其天然丰度很高(99.6%)，但14N同位素相对较少受到关注，因为它是一个自旋1核，具有很大的四极相互作用，这使得核磁共振谱的激发和检测变得困难。因此，到目前为止，大多数固态核磁共振研究都使用了人工(而且非常昂贵)富含15N同位素的材料，可以测量高分辨率的核磁共振光谱。尽管观察14N存在困难，但它的大四极相互作用是当地环境的极好报告，比其他核磁共振相互作用敏感得多，有可能为含氮材料的结构和动力学提供独特的见解。在过去的五年中，~(14)N及其固体核磁共振谱受到越来越多的关注。最近的发展包括间接检测基本的14N核磁共振跃迁，以及检测到部分被禁止的高阶跃迁，称为泛音跃迁，它有更尖锐的线。虽然这些方法的灵敏度和光谱分辨率正在提高，但它们目前还不够健壮和灵敏，无法广泛应用。我们的团队参与了这些努力。我们已经获得了一系列令人兴奋的初步结果来支持我们的研究目标，表明在信号强度、位置分辨率、准确和有效的数据模拟方面可以获得重大改进，并且14N不一定限制我们使用小分子-甚至可以获得关于小蛋白质的数据。为了实现14N核磁共振的潜力，在这些初步结果之后必须在两个关键领域进行重点研究：首先，需要开发有效激发基频和泛音跃迁的方法。为了实现这一目标，我们将使用最先进的计算和数学工具来提高激励效率。这其中的一个关键组成部分将是使用最优控制理论，该理论能够优化实验方法，提供接近理论极限的性能数据，通常比通过更传统的方法获得的性能数据高出一个数量级以上。第二，我们相信在14N核磁共振谱中获得的灵敏度和分辨率可以显著提高。初步泛音数据表明，通过开发新的脉冲序列和优化捕获条件，可以显著缩小共振，并提高效率。

Through the development of novel experimental and computational methods, we aim to make solid state NMR spectroscopy of the major naturally occurring isotope of nitrogen much easier and more informative. 14N NMR used to be very hard due to the electric quadrupole moment of 14N nuclei, which generates very broad spectral lines. We found a promising way around this problem -- it potentially opens a new route to the characterization of a huge range of biomolecules, natural materials and drugs, including fibrillar proteins, which are key targets for the understanding of amyloid diseases. Within this project we will develop high-resolution 14N NMR methods and use them to gain insight into structural and dynamic information for systems where 15N enrichment is difficult or impossible, e.g. environmental samples, natural materials and tissue biopsies. NMR is a powerful technique for the analysis of structure and dynamics of molecules and solid state NMR is uniquely positioned to study a range of materials, from non-crystalline biological solids, such as fibrils or membrane proteins, to amorphous glasses. Despite its high natural abundance (99.6%), 14N isotope has received relatively little attention because it is a spin-1 nucleus with a large quadrupolar interaction that makes excitation and detection of the NMR spectrum difficult. Accordingly, most solid-state NMR studies to date have utilized materials artificially (and very expensively) enriched with the 15N isotope, for which high-resolution NMR spectra can be measured. Despite the difficulty associated with observing 14N, its large quadrupolar interaction is an excellent reporter of the local environment that is far more sensitive than other NMR interactions, with the potential to provide unique insights into structure and dynamics of nitrogen-containing materials. In the last five years, an increasing amount of attention has been given to 14N and its solid state NMR spectroscopy. Recent developments include indirect detection of the fundamental 14N NMR transition, as well as the detection of the partially forbidden higher order transition, known as the overtone transition, which has much sharper lines. Although the sensitivity and spectral resolution of these methods is improving, they are currently not sufficiently robust and sensitive to make them widely applicable.Our group is involved in these efforts. We have obtained a range of exciting preliminary results to support our research targets, demonstrating that major improvements are obtainable for signal intensity, site resolution, accurate and efficient data simulation, and that 14N does not necessarily constrain us to small molecules -- even data on small proteins can be obtained.To realise the potential of 14N NMR, these preliminary result must be followed by focused research in two keys areas:Firstly, there is a need to develop methods for the efficient excitation of both the fundamental and overtone transitions. To achieve this goal we will use state-of-the-art computational and mathematical tools to boost the excitation efficiency. A key component to this will be the use of optimal control theory that has the ability to optimize experimental methods to deliver performance figures close to the theoretical limit, often more than an order of magnitude higher than those obtained through more conventional means.Secondly, we believe that the sensitivity and resolution obtained in the 14N NMR spectra can be significantly enhanced. Preliminary overtone data indicate that significant narrowing of resonances, and gains in efficiency can be achieved through the development of new pulse sequences and the optimisation of acquisition conditions.