High Resolution Solid State Nitrogen-14 NMR

通过发展新的实验和计算方法，我们的目标是使主要的自然存在的氮同位素的固体核磁共振光谱更容易和更丰富的信息。由于14N核的电四极矩，它产生的谱线非常宽，因此过去的14N核磁共振非常困难。我们找到了一种解决这个问题的有希望的方法——它可能为大量生物分子、天然材料和药物的表征开辟了一条新途径，包括纤维蛋白，它是理解淀粉样蛋白疾病的关键靶点。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富集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。核磁共振是分析分子结构和动力学的强大技术，固态核磁共振具有独特的定位，可以研究一系列材料，从非晶体生物固体，如原纤维或膜蛋白，到无定形玻璃。尽管14N同位素的天然丰度很高（99.6%），但由于它是一个自旋为1的原子核，具有很大的四极相互作用，使得核磁共振谱的激发和检测变得困难，因此受到的关注相对较少。因此，迄今为止，大多数固态核磁共振研究都使用人工（而且非常昂贵）富集15N同位素的材料，可以测量高分辨率核磁共振光谱。尽管与观测14N相关的困难，其大的四极相互作用是当地环境的一个很好的报告者，比其他核磁共振相互作用更敏感，有可能为含氮材料的结构和动力学提供独特的见解。近五年来，对14N及其固体核磁共振光谱的研究越来越受到人们的重视。最近的发展包括间接检测基本的14N核磁共振跃迁，以及检测部分禁止的高阶跃迁，称为泛音跃迁，它有更清晰的线条。虽然这些方法的灵敏度和光谱分辨率都在不断提高，但目前它们的鲁棒性和灵敏度还不足以使其广泛应用。我们小组参与了这些努力。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，位点分辨率，准确有效的数据模拟方面可以获得重大改进，并且14N不一定限制我们到小分子-甚至可以获得小蛋白质的数据。为了实现14N核磁共振的潜力，这些初步结果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。其中一个关键部分是使用最优控制理论，该理论有能力优化实验方法，以提供接近理论极限的性能数据，通常比通过更传统的手段获得的数据高出一个数量级。其次，我们认为在14N核磁共振光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，共振显著缩小，通过开发新的脉冲序列和优化采集条件，可以实现效率的提高。

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.