Randomization improves sparse sampling in multidimensional NMR

While a number of strategies have been developed to reduce data collection requirements for multidimensional NMR based on non-Fourier methods of spectrum analysis, there is an increasing awareness that the principal differences in the performance of these methods is attributable to the sampling strategies employed, and not the method of spectrum analysis per se. The ability of maximum entropy reconstruction to utilize essentially arbitrary sampling schemes makes it a useful platform for comparative analysis of sampling strategies. Here we use maximum entropy reconstruction to demonstrate that artifacts characteristic of sparse sampling result from regularity in the sampling pattern, and that they can be substantially reduced by introducing a degree of randomness to an otherwise regular sampling scheme, without requiring additional sampling.     

Optimized angle selection for radial sampled NMR experiments

Sparse sampling offers tremendous potential for overcoming the time limitations imposed by traditional Cartesian sampling of indirectly detected dimensions of multidimensional NMR data. Unfortunately, several otherwise appealing implementations are accompanied by spectral artifacts that have the potential to contaminate the spectrum with false peak intensity. In radial sampling of linked time evolution periods, the artifacts are easily identified and removed from the spectrum if a sufficient set of radial sampling angles is employed. Robust implementation of the radial sampling approach therefore requires optimization of the set of radial sampling angles collected. Here we describe several methods for such optimization. The approaches described take advantage of various aspects of the general simultaneous multidimensional Fourier transform in the analysis of multidimensional NMR data. Radially sampled data are primarily contaminated by ridges extending from authentic peaks. Numerical methods are described that definitively identify artifactual intensity and the optimal set of sampling angles necessary to eliminate it under a variety of scenarios. The algorithms are tested with both simulated and experimentally obtained triple resonance data.     

用于低场磁共振成像的直接射频采样数字接收机

提出一种用于低场磁共振成像谱仪的数字接收机的设计. 它基于直接射频采样技术,本文详细讨论了其设计过程. 实验结果表明,设计的数字接收机具有结构简单、性能优良和实用性较强的特点.     

不同采样模式的固体DQ-SQ实验的压缩感知重建比较

为了提高固体二维双量子-单量子(DQ-SQ)谱的采集速度,根据DQ-SQ谱的自稀疏性,该文使用了一种基于压缩感知技术的重建算法.其优化的能量函数是有限差分约束的l1范数,并使用不同的权重对水平和竖直方向的有限差分项进行约束.该文分别对伪随机采样、全随机采样和e指数采样等采样模式进行了比较,发现伪随机采样表现出最佳的重建结果.进一步研究发现伪随机的极限形式,即t1截尾(t1-cutoff)采样模式效果最佳.     

Phasing arbitrarily sampled multidimensional NMR data

The recent re-introduction of the two-dimensional Fourier transformation (2D-FT) has allows for the transformation of arbitrarily sampled time domain signals. In this respect, radial sampling, where two incremented time dimensions (t(1) and t(2)) are sampled such that t(1)=taucosalpha and t(2)=tausinalpha, is especially appealing because of the relatively small leakage artifacts that occur upon Fourier transformation. Unfortunately radially sampled time domain data results in a fundamental artifact in the frequency domain manifested as a ridge of intensity extending through the peak positions perpendicular to +/- the radial sampling angle. Successful removal of the ridge artifacts using existing algorithms requires absorptive line shapes. Here we present two procedures for retrospective phase correction of arbitrarily sampled data.     

Fast multidimensional NMR: radial sampling of evolution space

Multidimensional NMR spectroscopy can be speeded up by limited radial sampling of the time-domain evolution data. The resulting frequency-domain projections are used to reconstruct the full NMR spectrum. New algorithms are proposed to suppress back-projection artifacts while retaining optimum sensitivity. The method is illustrated by experiments on the 900 MHz HNCO spectrum of a protein, HasA.     

迭代软阈值法压缩感知重建谱峰较宽的二维固体谱

压缩感知技术可以打破传统奈奎斯特采样定理的限制,利用优化算法对欠采数据进行重建,并获得高质量的结果,因此在核磁共振领域得到了广泛的关注．但是当核磁共振谱的谱峰很宽时,基于共轭梯度方法的压缩感知重建却难以得到令人满意的谱图．因此,该文采用凸优化非线性重建算法,使用基于谱图域软阈值的压缩感知算法重建固体二维宽谱（¹H双量子-单量子谱或MQMAS谱）,有效地解决了宽峰在重建时变弱的问题．     

Two-dimensional Fourier transform of arbitrarily sampled NMR data sets

A new procedure for Fourier transform with respect to more than one time variable simultaneously is proposed for NMR data processing. In the case of two-dimensional transform the spectrum is calculated for pairs of frequencies, instead of conventional sequence of one-dimensional transforms. Therefore, it enables one to Fourier transform arbitrarily sampled time domain and thus allows for analysis of high dimensionality spectra acquired in a short time. The proposed method is not limited to radial sampling, it requires only to fulfill the Nyquist theorem considering two or more time domains at the same time. We show the application of new approach to the 3D HNCO spectrum acquired for protein sample with radial and spiral time domain sampling.     

基于低秩矩阵的非均匀采样NMR波谱重建进展

多维核磁共振（Nuclear Magnetic Resonance,NMR）利用多维波谱来分析分子结构,被广泛用于化学、生物学和医学等领域,但信号采样时间随波谱维度和采样点数增加而迅速增长．非均匀采样通过降低间接维采样点数来加速数据采集,并引入合理的重建方法获得完整的NMR波谱．如何快速重建高质量的波谱,是NMR信号处理研究的前沿．本文主要综述近年来基于低秩矩阵的NMR波谱重建方法的发展．首先介绍了低秩矩阵的相关数学基础;然后从一般低秩矩阵和结构化低秩汉克尔矩阵两个角度来论述重建模型,并讨论相关的NMR波谱应用;最后分析了该技术存在的不足,并展望其未来发展的趋势．     

Narrow peaks and high dimensionalities: Exploiting the advantages of random sampling

Level of artifacts in spectra obtained by Multidimensional Fourier Transform has been studied, considering randomly sampled signals of high dimensionality and long evolution times. It has been shown theoretically and experimentally, that this level is dependent on the number of time domain samples, but not on its relation to the number of points required in appropriate conventional experiment. Independence of the evolution time domain size (in the terms of both: dimensionality and evolution time reached), suggests that random sampling should be used rather to design new techniques with large time domain than to accelerate standard experiments. 5D HC(CC-TOCSY)CONH has been presented as the example of such approach. The feature of Multidimensional Fourier Transform, namely the possibility of calculating spectral values at arbitrary chosen frequency points, allowed easy examination of resulting spectrum. We present the example of such approach, referred to as Sparse Multidimensional Fourier Transform.     

Capillary scale NMR flow mapping

Stopped-flow NMR at capillary scale has many advantages over traditional methods of introducing the sample into the probe, particularly when large numbers of samples must be examined. This work describes application of a simple method for direct visualization of a sample inside the flow cell of flow NMR systems to capillary scale analysis. We describe the details of the method and show how it can be used to measure the optimum flow rate for a capillary NMR system and how to determine the optimum sampling efficiency for small samples.     

三维NMR谱

３ＤＮＭＲ谱是２ＤＮＭＲ谱附加一个时间周期扩展而成，３ＤＮＭＲ共振峰的重叠降低，并且灵敏度与常规的２ＤＮＯＥＳＹ实验大体相当，采集３ＤＮＭＲ数据组所需时间太长，因此迄今为止，该方法的实际应用只局限于那些浓度相当高的样品。尽管目前还十分费时，但很可能３ＤＮＭＲ不久即可成为常规的ＮＭＲ技术。     

四维NMR谱

尽管３ＤＮＭＲ谱的分辨率有所提高，但解释较大的三维异核ＮＭＲ谱时，仍存在含糊性。因此，人们希望通过增加维数进一步提高分辨率，在最近几年中出现了４ＤＮＭＲ谱，４ＤＮＭＲ谱常常是说明２ＤＮＭＲ谱的方法，４ＤＮＭＲ实验可简单地看作是由三个２ＤＮＭＲ实验所组成，新的４ＤＮＭＲ技术仍在不断地发展，已有许多将４ＤＮＭＲ谱应用于蛋白质及核酸研究的报道。     

2, 4, 8, 10-四卤代-6-硫-12-H-双苯并[d,g] [1, 3,,3]-二氧磷杂八环的1H和13C NMR研究

通过2D NMR技术对5个2, 4, 8, 10-四卤代-6-硫-12-H-双苯并[d, g] [1, 3, 2]-二氧磷杂八环的¹H和¹³C NMR信号进行了准确的归属,获得了所有芳环碳原子的P-C和部分质子的P-H偶合常数,3/9质子的⁵J_PH属于折线型偶合,偶合常数为0.9～1.0 Hz.     

葛根素-7-磷酸二乙酯的NMR研究

葛根素是从常用中药野葛根中分离提取出来的一种具有很强生物活性的异黄酮碳苷类化合物,设计并合成了一种新的化合物葛根素-7-磷酸二乙酯. 在常温下,这种新的化合物在NMR谱上有两组相关的信号峰,即在溶液中以两种不同的构象存在. 本文利用HSQC和HMBC等多种二维核磁共振技术对这种化合物的NMR谱进行了全归属.     

对氯苯氧基氯苯乙酮异构体的NMR分析

2-氯-4-(4-氯苯氧基)苯乙酮和2-(4-氯苯氧基)-4-氯苯乙酮互为同分异构体,它们的核磁共振谱十分相似,只通过一维核磁共振谱对其结构确定有很大的困难. 本文借助2-(4-氯苯氧基)-4-氯苯乙酮的分子晶体结构信息,利用二维核磁共振方法,对两个化合物的¹H NMR和¹³C NMR进行了指认,为这两种化合物的结构鉴定提供了重要依据.     

高碳糖希夫碱的1H NMR和13C NMR全归属

对7个高碳糖希夫碱进行了¹H和¹³C NMR检测，通过DEPT和¹H-¹H COSY，HMQC，HMBC等2D NMR 技术对其¹H和¹³C NMR数据进行了全归属和较详细的解析，并指出其NMR特征.