High-resolution magic-angle spinning (HR-MAS) nuclear magnetic resonance (NMR) is an essential tool to characterize a variety of semisolid systems, including biological tissues, with virtually no sample preparation. The "non-destructive" nature of NMR is typically compromised, however, by the extreme centrifugal forces experienced under conventional HR-MAS frequencies of several kilohertz. These features limit the usefulness of current HR-MAS approaches for fragile samples. Here, we introduce a full protocol for acquiring high-quality HR-MAS NMR spectra of biological tissues at low spinning rates (down to a few hundred hertz). The protocol first consists of a carefully designed sample preparation, which yields spectra without significant spinning sidebands at low spinning frequency for several types of sample holders, including the standard disposable inserts classically used in HR-MAS NMR-based metabolomics. Suppression of broad spectral features is then achieved using a modified version of the recently introduced PROJECT experiment with added water suppression and rotor synchronization, which deposits limited power in the sample and which can be suitably rotor-synchronized at low spinning rates. The performance of the slow HR-MAS NMR procedure is demonstrated on conventional (liver tissue) and very delicate (fish eggs) samples, for which the slow-spinning conditions are shown to preserve the structural integrity and to minimize intercompartmental leaks of metabolites. Taken together, these results expand the applicability and reliability of HR-MAS NMR spectroscopy. These results have been obtained at 400 and 600 MHz and suggest that high-quality slow HR-MAS spectra can be expected at higher magnetic fields using the described protocol.
We demonstrate the acquisition of ultrafast 2D NMR spectra of semi-solid samples, with a high-resolution magic-angle-spinning setup. Using a recent double-quantum NMR pulse sequence in optimised synchronisation conditions, high-quality 2D spectra can be recorded for a sample under magic-angle spinning. An illustration is given with a semi-solid sample of banana pulp.
a b s t r a c tThe microalga Botryococcus braunii has been studied for its hydrocarbon biofuel production potential. The range of the lipids it synthesizes cannot be readily profiled, because their main chemical class standards (long chain alkenes and botryococcenes) are at present unavailable. The aim of this study was to develop a direct, specific GCMS analysis method for the rapid chemotyping of these lipids. The SIM (Selected Ion Monitoring)-GCMS program we developed discriminates classes and subclasses of the targeted molecules through their main chemical motifs identified with fragment ions of specific m/z selected by unsupervised chemometrics. Our validation results indicate that the SIM mode enhanced their detection by significantly increasing the signalto-noise ratio ca. 20-fold. The profiling results from TL extracts of diverse B. braunii strains show that under the culture conditions tested, these strains could be distinguished by their chemical signatures, which also signaled different culture conditions involving various physiological states. We were able to make a rapid estimate of the biofuel potential of these strains, focusing on alkenes and botryococcenes. The direct injection SIM-GCMS method was successfully applied here for rapid chemotaxonomy, displaying good sensitivity and specificity for new high molecular weight hydrocarbons from B. braunii.
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