Access to experimentally infeasible spectra by pure-shift NMR covariance

Quantitative stray-field T1 relaxometry with the matrix pencil method

The matrix pencil method (MPM) is tested as an approach to quantitatively process multiexponential low-field nuclear magnetic resonance T₁ relaxometry data. The data is obtained by measuring T₁ saturation recovery curves in the highly inhomogeneous magnetic field of a stray-field sensor. 0.9% brine solutions, doped with different concentrations of a Gd³⁺ containing contrast agent, serve as test liquids. Relaxation-times as a function of contrast-agent concentration along with the T₁ relaxation curves for combinations of multiple different test liquids are measured, and the results from processing using MPM as well as inverse Laplace transformation as a benchmark are compared. The relaxation-time resolution limits of both procedures are probed by gradually reducing the difference between the relaxation-times of two liquids measured simultaneously. The sensitivity to quantify the relative contribution of each component to the magnetization build-up curve is explored by changing their volume ratio. Furthermore, the potential to resolve systems with more than two components is tested. For the systems under test, MPM shows superior performance in separating two or three relaxation components, respectively and effectively quantifying the time constants.

Detection and Verification of a Key Intermediate in an Enantioselective Peptide Catalyzed Acylation Reaction

Until now, the intermediate responsible for the acyl transfer of a highly enantioselective tetrapeptide organocatalyst for the kinetic resolution of trans-cycloalkane-1,2-diols has never been directly observed. It was proposed computationally that a π-methylhistidine moiety is acylated as an intermediate step in the catalytic cycle. In this study we set out to investigate whether we can detect and characterize this key intermediate using NMR-spectroscopy and mass spectrometry. Different mass spectrometric experiments using a nano-ElectroSpray Ionization (ESI) source and tandem MS-techniques allowed the identification of tetrapeptide acylium ions using different acylation reagents. The complexes of trans-cyclohexane-1,2-diols with the tetrapeptide were also detected. Additionally, we were able to detect acylated tetrapeptides in solution using NMR-spectroscopy and monitor the acetylation reaction of a trans-cyclohexane-1,2-diol. These findings are important steps towards the understanding of this highly enantioselective organocatalyst.

Covariance NMR: Theoretical concerns, practical considerations, contemporary applications and related techniques

The family of resolution enhancement and spectral reconstruction techniques collectively known as covariance NMR continues to expand, along with the list of applications for these techniques. Recent advances in covariance NMR include the utilization of covariance to reconstruct pure shift NMR spectra, and the growing use of covariance NMR in processing non-uniformly sampled data, especially in solid state NMR and metabolomics. This review describes theoretical and practical considerations for direct and indirect covariance NMR techniques, and summarizes recent additions to the covariance NMR family. The review also outlines some of the applications of covariance NMR, and places covariance NMR in the larger context of methods that use statistical and algebraic approaches to enhance and combine various kinds of spectroscopic data, including tensor-based approaches for multidimensional NMR and heterocovariance spectroscopy.

Identification of metabolites in coriander seeds (Coriandrum Sativum L.) aided by ultrahigh resolution total correlation NMR spectroscopy

NMR is a fast method for obtaining a holistic snapshot of the metabolome and also offers quantitative information without separating the compounds present in a complex mixture. Identification of the metabolites present in a plant extract sample is a crucial step for all plant metabolomics studies. In the present work, we used various two dimensional (2D) NMR methods such as J-resolved NMR, total correlation spectroscopy (TOCSY), and heteronuclear single quantum coherence sensitivity enhanced NMR spectroscopy for the identification of 36 common metabolites present in Coriandrum sativum L. seed extract. The identified metabolites belong to the following classes: organic acids, amino acids, and carbohydrates. ¹ H NMR spectra of such complex mixtures in general display tremendous signal overlap due to the presence of a large number of metabolites with closely resonating multiplet signals. This signal overlapping leads to ambiguity in an assignment, and hence, identification of metabolites becomes tedious or impossible in many cases. Therefore, the utility of pure-shift proton spectrum along the indirect (F₁ ) dimension of the F₁ -PSYCHE-TOCSY spectrum is demonstrated for overcoming ambiguity in assignment of metabolites in crowded spectral regions from Coriandrum sativum L. seed extract sample. Because pure-shift NMR methods yield ultrahigh resolution spectrum (i.e., a singlet peak per chemical site) along one or more dimensions, such spectra provide better identification of metabolites compared with regular 2D TOCSY where signal overlap and peak distortions lead to ambiguity in the assignment. Nine metabolites were unambiguously assigned by pure-shift F₁ -PSYCHE-TOCSY spectrum, which was unresolved in regular TOCSY spectrum.

Multiplicity-edited 1 H-1 H TOCSY experiment

A ¹ H-¹ H total correlation spectroscopy (TOCSY) experiment incorporating ¹³ C multiplicity information is proposed. In addition, broadband ¹ H homodecoupling in the indirect dimension can be implemented using a perfect BIRD module that affords exclusive ¹ H chemical shift evolution with full decoupling of all heteronuclear and homonuclear (including ² J_HH ) coupling constants. As a complement to the normal TOCSY and the recent PSYCHE-TOCSY experiments, this novel multiplicity-edited TOCSY experiment distinguishes between CH/CH₃ (phased up) and CH₂ (phased down) cross-peaks, which facilitates resonance analysis and assignment.

Compressed NMR: Combining compressive sampling and pure shift NMR techniques

Historically, the resolution of multidimensional nuclear magnetic resonance (NMR) has been orders of magnitude lower than the intrinsic resolution that NMR spectrometers are capable of producing. The slowness of Nyquist sampling as well as the existence of signals as multiplets instead of singlets have been two of the main reasons for this underperformance. Fortunately, two compressive techniques have appeared that can overcome these limitations. Compressive sensing, also known as compressed sampling (CS), avoids the first limitation by exploiting the compressibility of typical NMR spectra, thus allowing sampling at sub-Nyquist rates, and pure shift techniques eliminate the second issue "compressing" multiplets into singlets. This paper explores the possibilities and challenges presented by this combination (compressed NMR). First, a description of the CS framework is given, followed by a description of the importance of combining it with the right pure shift experiment. Second, examples of compressed NMR spectra and how they can be combined with covariance methods will be shown.

Non-Uniform and Absolute Minimal Sampling for High-Throughput Multidimensional NMR Applications

Many biomolecular NMR applications can benefit from the faster acquisition of multidimensional NMR data with high resolution and their automated analysis and interpretation. In recent years, a number of non-uniform sampling (NUS) approaches have been introduced for the reconstruction of multidimensional NMR spectra, such as compressed sensing, thereby bypassing traditional Fourier-transform processing. Such approaches are applicable to both biomacromolecules and small molecules and their complex mixtures and can be combined with homonuclear decoupling (pure shift) and covariance processing. For homonuclear 2D TOCSY experiments, absolute minimal sampling (AMS) permits the drastic shortening of measurement times necessary for high-throughput applications for identification and quantification of components in complex biological mixtures in the field of metabolomics. Such TOCSY spectra can be comprehensively represented by graphic theoretical maximal cliques for the identification of entire spin systems and their subsequent query against NMR databases. Integration of these methods in webservers permits the rapid and reliable identification of mixture components. Recent progress is reviewed in this Minireview.

Ultra-high resolution in low field tabletop NMR spectrometers

An approach for resolution enhancement is proposed, for data acquired on low field tabletop NMR spectrometers by employing processing-based (generalized indirect covariance) advancements in pure shift NMR.

Pure shift 1 H NMR: what is next?

Currently, pure shift nuclear magnetic resonance (NMR) is an area of high interest. The aim of this contribution is to describe briefly how this technique has evolved, where it is now and what could be the next challenges in the amazing adventure of the development and application of pure shift NMR experiments. Copyright © 2016 John Wiley & Sons, Ltd.

Measurement of 1H NMR relaxation times in complex organic chemical systems: application of PSYCHE

In complex organic molecules, relaxation times measured from the PSYCHE homonuclear broadband decoupling methods provide a wealth of information on intramolecular dynamics and intermolecular interactions.