Improving the accuracy of 1H–19F internuclear distance measurement using 2D 1H–19F HOESY

With the rise in fluorinated pharmaceuticals, it is becoming increasingly important to develop new ¹⁹F NMR-based methods to assist in their analysis. Crucially, obtaining information regarding the conformational dynamics of a molecule in solution can aid the design of strongly binding therapeutics. Herein, we report the development of a 2D ¹H–¹⁹F Heteronuclear Overhauser Spectroscopy (HOESY) experiment to measure ¹H–¹⁹F internuclear distances, with accuracies of ~5% when compared with ¹H–¹⁹F internuclear distances calculated by quantum chemical methods. We demonstrate that correcting for cross-relaxation of ¹H, using the diagonal peaks from the 2D ¹H–¹H Nuclear Overhauser Enhancement Spectroscopy (NOESY), is critical in obtaining accurate values for ¹H–¹⁹F internuclear distances. Finally, we show that by using the proposed method to measure ¹H–¹⁹F internuclear distances, we are able to determine the relative stereochemistry of two fluorinated pharmaceuticals.     

Targeted multidimensional gas chromatography using a heart-cutting device and cryogenic focusing for the determination of benzophenone derivatives in foodstuffs

Photoinitiators are used to promote the polymerization process during the curing of varnishes or inks on cartonboard packaging. Depending on storage conditions and shelf life, these substances are able to migrate through the packaging layer into the foodstuff. This type of contamination phenomenon is therefore becoming a critical issue in terms of food safety. In order to tackle this problem, a fast and selective method was developed for the determination of benzophenone and three methylbenzophenone isomers in cereal-based foodstuffs and their cardboard packaging. Food samples or packages were efficiently extracted by pressurized liquid extraction using acetonitrile, and the extracts were directly injected onto the analytical system. The analysis was performed by multidimensional gas chromatography–mass spectrometry using a heart-cutting approach to reduce the background noise from complex matrices. The strategy employed two distinct cuts each containing its proper deuterated internal standard leading to accurate quantification. By integrating a cryofocusing effect, an enhancement in signal/noise ratio was achieved by a factor >10, which markedly decreased the sensitivity threshold. Moreover, baseline separation of the critical isomers allowed their unequivocal determination. The method was fully validated on cereal-based foodstuffs based upon an analysis of variance, and excellent performances were obtained at the decision limit making this method well suited for official food controls.     

Structural Mimics of Acetylene Hydratase: Tungsten Complexes Capable of Intramolecular Nucleophilic Attack on Acetylene

Bioinspired complexes employing the ligands 6-tert-butylpyridazine-3-thione (SPn) and pyridine-2-thione (SPy) were synthesized and fully characterized to mimic the tungstoenzyme acetylene hydratase (AH). The complexes [W(CO)(C₂H₂)(CHCH-SPy)(SPy)] ( 4 ) and [W(CO)(C₂H₂)(CHCH-SPn)(SPn)] ( 5 ) were formed by intramolecular nucleophilic attack of the nitrogen donors of the ligand on the coordinated C₂H₂ molecule. Labelling experiments using C₂D₂ with the SPy system revealed the insertion reaction proceeding via a bis-acetylene intermediate. The starting complex [W(CO)(C₂H₂)(SPy)₂] ( 6 ) for these studies was accessed by the new acetylene precursor mixture [W(CO)(C₂H₂)_n(MeCN)_3−nBr₂] (n=1 and 2; 7 ). All complexes represent rare examples in the field of W−C₂H₂ chemistry with 4 and 5 being the first of their kind. In the ongoing debate on the enzymatic mechanism, the findings support activation of acetylene by the tungsten center.