Tautomerization and Isomerization in Quantitative NMR: A Case Study with 4-Deoxynivalenol (DON)

Biotransformation of Deoxynivalenol to the Novel Metabolite Deoxynivalenol-8,15-hemiketal-7-glucoside by the Bacillus subtilis Glycosyltransferase YjiC

The mycotoxin deoxynivalenol (DON) is a chronic problem in cereals in temperate areas worldwide. Above regulatory levels, DON contamination can result in significant economic loss both to the primary producer and the feed industry in terms of increased costs. Here we report the enzymatic biotransformation of DON to a novel stable metabolite by a soil-borne strain of Bacillus subtilis. Proteomic analysis of activity-enriched protein fractions from this B. subtilis strain identified the glycosyltransferase YjiC as the enzyme responsible for the observed DON biotransformation. Liquid chromatography high-resolution tandem mass spectrometry and NMR spectroscopic analysis demonstrated that YjiC glycosylates DON at the 7-hydroxyl position, producing the novel metabolite DON-8,15-hemiketal-7-glucoside (HKDON7G). In toxicity experiments, duckweed exposed to 20 μM HKDON7G showed no phytotoxicity when compared to DON. Stability testing of HKDON7G demonstrated that it is significantly more resistant to enzymatic and microbial hydrolysis compared to DON-3-glucoside. This study is the first to report a chemical modification to the 7-hydroxyl position of DON and presents a novel mechanism for the detoxification of DON-contaminated food and feed.

Global Perspectives on Mycotoxin Reference Materials (Part I): Insights from Multi-Supplier Comparison Study Including Aflatoxin B1, Deoxynivalenol and Zearalenone

We conducted a comprehensive examination of liquid mycotoxin reference standards. A total of 30 different standards were tested, each containing 10 samples of three distinct substances: Aflatoxin B1, Deoxynivalenol, and Zearalenone. The standards were sourced from 10 different global market leading manufacturers. To facilitate comparison, all the standard sets were adjusted to the same concentration level. The standards were analyzed using the techniques LC-MS/MS, HPLC-DAD, and LC-HRMS to assess their quality attributes. Regarding the validation of the reference values, it was observed that 30% of the suppliers provided reference standards that were either below the lower acceptance limit or above the higher acceptance limit, confirmed by both the LC-MS/MS and HPLC-DAD methods. Furthermore, a total of 12 impurities were found in the DON standards, 10 in the AFB1 standards, and 8 in the ZON standards, distributed across all the suppliers. Therefore, this study suggests relevant adjustments to the ISO 17034 standard, proposing that the purity of a raw material should be uniformly based on q-NMR analysis, as most manufacturers state the purity of their certificates is determined using HPLC-UV or LC-MS/MS. Liquid standards with a shelf life of ≤1 year should not exceed an uncertainty of 3%. Standards that have a longer shelf life should not have more than 5% uncertainty. This study also emphasizes the importance of stability. The standards should undergo continuous long-term monitoring; otherwise, products may exhibit a target value of only 80%, as seen in one instance. It is also recommended to include proof of HPLC and LC-MS/MS analyses on the certificate of each released batch of a final product.

O-versus S-Metal Coordination of the Thiocarboxylate Group: An NMR Study of the Two Tautomeric Forms of the Ga(III)-Photoxenobactin E Complex

Photoxenobactin E (1) is a natural product with an unusual thiocarboxylic acid terminus recently isolated from an entomopathogenic bacterium. The biosynthetic gene cluster associated with photoxenobactin E, and other reported derivatives, is very similar to that of piscibactin, the siderophore responsible for the iron uptake among bacteria of the Vibrionaceae family, including potential human pathogens. Here, the reisolation of 1 from the fish pathogen Vibrio anguillarum RV22 cultured under iron deprivation, its ability to chelate Ga(III), and the full NMR spectroscopic characterization of the Ga(III)-photoxenobactin E complex are presented. Our results show that Ga(III)-photoxenobactin E in solution exists in a thiol-thione tautomeric equilibrium, where Ga(III) is coordinated through the sulfur (thiol form) or oxygen (thione form) atoms of the thiocarboxylate group. This report represents the first NMR study of the chemical exchange between the thiol and thione forms associated with thiocarboxylate-Ga(III) coordination, including the kinetics of the interconversion process associated with this tautomeric exchange. These findings show significant implications for ligand design as they illustrate the potential of the thiocarboxylate group as a versatile donor for hard metal ions such as Ga(III).

Overcoming NMR line broadening of nitrogen containing compounds: A simple solution

This study presents a straightforward solution to the challenge of elucidating the structures of nitrogen containing compounds undergoing isomerization. When spectral line broadening occurs related to isomerization, be it prototropic tautomerism or bond rotations, this poses a significant obstacle to structural elucidation. By adding acids, we demonstrate a simple approach to overcome this issue and effectively sharpen NMR signals for acid stable prototropic tautomers as well as the conformational isomers containing a morpholine or piperazine ring.