Optimization of Diffusion-Filtered NMR Experiments for Selective Suppression of Residual Nondeuterated Solvent and Water Signals from 1H NMR Spectra of Organic Compounds

Improving the analysis of phase-separated bio-fuel samples with slice-selective total correlation NMR spectroscopy

Separated samples are a particular challenge for NMR experiments. The boundary is severely detrimental to high-resolution spectra and normal NMR experiments simply add the two spectra of the two layers together. Pyrolysis bio-oils represent an increasingly important alternative fuel resource yet readily separate, whether due to naturally high water content or due to blending, a common practice for producing a more viable fuel. Slice-selective NMR, where the NMR spectrum of only a thin slice of the total sample is acquired, is extended here and improved, with slice-selective two-dimensional correlation experiments used to resolve the distinct chemical spectra of the various components of the phase-separated blended fuel mixtures. Analysis of how the components of any blended biofuel samples partition between the two layers is an important step towards understanding the separation process and may provide insight into mitigating the problem.

Gradient-based pulse sequences for benchtop NMR spectroscopy

Benchtop NMR spectroscopy has been on the rise for the last decade, by bringing high-resolution NMR in environments that are not easily compatible with high-field NMR. Benchtop spectrometers are accessible, low cost and show an impressive performance in terms of sensitivity with respect to the relatively low associated magnetic field (40-100 MHz). However, their application is limited by the strong and ubiquitous peak overlaps arising from the complex mixtures which are often targeted, often characterized by a great diversity of concentrations and by strong signals from non-deuterated solvents. Such limitations can be addressed by pulse sequences making clever use of magnetic field gradient pulses, capable of performing efficient coherence selection or encoding chemical shift or diffusion information. Gradients pulses are well-known ingredients of high-field pulse sequence recipes, but were only recently made available on benchtop spectrometers, thanks to the introduction of gradient coils in 2015. This article reviews the recent methodological advances making use of gradient pulses on benchtop spectrometers and the applications stemming from these developments. Particular focus is made on solvent suppression schemes, diffusion-encoded, and spatially-encoded experiments, while discussing both methodological advances and subsequent applications. We eventually discuss the exciting development and application perspectives that result from such advances.

Measuring Protein Concentration by Diffusion-Filtered Quantitative Nuclear Magnetic Resonance Spectroscopy

The concentration of macromolecules in solution is a crucial property in many areas of research, including the development and commercialization of biological therapeutics. For proteins in particular, none of the reported methods for measuring concentration detect a molecular property that is known a priori; rather, they rely on ligand binding, degradation and derivitization, or an intrinsic property that must be determined experimentally. The purpose of this report is to describe (1) a diffusion-filtered qNMR experiment (DF-qNMR) for quantitating macromolecules in complex matrices and (2) an overall method for measuring absolute protein concentration based on this DF-qNMR experiment. This method combines protein denaturation with the diffusion filter to produce clean spectra of the protein with well-resolved resonances, regardless of the matrix complexity. The concentration is then obtained by comparing the peak area of the valine/isoleucine/leucine methyl groups to an external, certified, small-molecule quantitation standard. The method, which is referred to as VILMHA (valine isoleucine leucine methyl hydrogen analysis), was tested on three proteins of various sizes. In all cases, the measured concentration was within 1.8% of the labeled value for the undiluted standard reference material evaluated. In addition, the RSD's were less than 1.25% in all cases and less than 1% in most cases. The accuracy, precision, and ease of use make this method superior to existing absolute protein concentration methods. Furthermore, VILMHA is ideally suited to serve as the basis for converting the relative protein concentration methods into absolute methods or establishing molecular-specific parameters. Finally, DF-qNMR has the potential to quantitate other types of macromolecules (e.g., such as polymers, surfactants, etc.) in the presence of small-molecule contaminants.

Cyclodextrin-based poly(pseudo)rotaxanes for transdermal delivery of carvedilol

This work aimed to design supramolecular gels combining Soluplus or Solutol and alfa- and hydroxypropyl-β-cyclodextrin (α-CD, HPβ-CD) for carvedilol (CAR) transdermal delivery. Poly(pseudo)rotaxane formation (appearance, SEM, ¹H NMR), drug solubilization, rheological properties and in vitro release were investigated. CAR-CD complexes were prepared in situ or by spray drying. For Solutol, poly(pseudo)rotaxanes were formed immediately after mixing with α-CD and did not influence CAR solubility. Differently, Soluplus poly(pseudo)rotaxanes took 24-48 h to be formed and CAR solubility decreased compared to Soluplus micelles. Soluplus 20% + α-CD (5-10%) showed higher G' and G'' but also faster CAR release than Solutol poly(pseudo)rotaxanes, which is explained by the different location of PEG chains in the two amphiphilic polymers. Faster drug release was achieved incorporating HPβ-CD or CAR-HPβ-CD spray-dried complexes. The results evidenced the versatility of the formulations in terms of rheological behavior and drug release patterns, which can be adjusted for CAR transdermal delivery.

Tandem ATRP/Diels–Alder synthesis of polyHEMA-based hydrogels

An efficient controlled process for the preparation of HEMA-based hydrogels by means of the Diels–Alder reaction.

Application of diffusion-edited NMR spectroscopy for selective suppression of water signal in the determination of monomer composition in alginates

Alginate is a linear copolymer of 1-4 linked β-D-mannuronic acid (M) and 1-4 linked α-L-guluronic acid (G). The physical properties of these polysaccharides such as gel properties and viscosity are largely correlated to the monomer composition (M/G ratio), the sequence of the polymer and the molecular weight. Determination of the M/G ratio is therefore important and NMR spectroscopy is among the most common methods used to accurately obtain this ratio. Instead of using time consuming, possibly sample altering, acid hydrolysis to reduce the viscosity of the alginate sample prior to analysis, samples of low concentrations can be used. However, this results in a water peak in the NMR spectrum that is several orders of magnitude larger than the alginate signals and water suppression is required. In this article, a diffusion-edited NMR experiment that suppresses the water peak while retaining the signals of interest has been used to enable correct M/G ratio determination. This approach exploits the difference in translational diffusion between the larger alginate molecules and the smaller water molecules. Using this method, the monomer composition of 20 different alginate powders was determined. The diffusion parameters were optimized to allow measurement for samples covering a large range of M/G ratios and viscosities. Thus, such method should be useful for analyzing large numbers of unknown alginate samples using, for example, automation procedures.

A NMR reverse diffusion filter for the simplification of spectra of complex mixtures and the study of drug receptor interactions

A reverse diffusion filter NMR experiment (Drev) is proposed for the study of small molecules in binding with macromolecules. The filtering efficiency of Drev to eliminate the signals of the macromolecule is shown to be superior to conventional transverse relaxation filters at least for macromolecules containing a significant fraction of flexible residues. The Drev filter was also a useful complement for ligand-based NMR screening in combination with saturation transfer difference experiments.

NMR spectroscopy as a tool to close the gap on metabolite characterization under MIST

Withdrawals from the market due to unforeseen adverse events have triggered changes in the way therapeutics are discovered and developed. This has resulted in an emphasis on truly understanding the efficacy and toxicity profile of new chemical entities (NCE) and the contributions of their metabolites to on-target pharmacology and off-target receptor-mediated toxicology. Members of the pharmaceutical industry, scientific community and regulatory agencies have held dialogues with respect to metabolites in safety testing (MIST); and both the US FDA and International Conference on Harmonisation have issued guidances with respect to when and how to characterize metabolites for human safety testing. This review provides a brief overview of NMR spectroscopy as applied to the structure elucidation and quantification of drug metabolites within the drug discovery and development process. It covers advances in this technique, including cryogenic cooling of detection circuitry for enhanced sensitivity, hyphenated LC-NMR techniques, improved dynamic range through new solvent-suppression pulse sequences and quantitation. These applications add to the already diverse NMR toolkit and further anchor NMR as a technique that is directly applicable to meeting the requirements of MIST guidelines.