NMR investigation on the non-enzymatic breakdown of the glucosinolate sinigrin and its acetylated derivative by copper ions.

Copper is an essential trace element present in many organisms, and the maintenance of its homeostasis is involved in the prevention of diseases, including cancer. Glucosinolates are sulfur-containing glycosides mainly found in cruciferous plants, and are hydrolyzed by the myrosinase enzyme to bioactive compounds such as isothiocyanates that contribute to plant defense against herbivores and exhibit various health-promoting effects in humans. The interaction between Cu and glucosinolates, however, is largely uncharacterized. In this 1H nuclear magnetic resonance (NMR)-based study, we demonstrated that the glucosinolate sinigrin or its O-acetylated derivative can readily react with Cu(I) ions in solution without the presence of the plant myrosinase enzyme, although Cu(II) ions were found to be unreactive. In aqueous solution, the Cu(I) ions interact with the vinyl group of sinigrin as shown by 1H NMR spectroscopy. The glycosidic CS bond of sinigrin is eventually cleaved, resulting in the formation of 2-butenenitrile. The reaction can also occur in the presence of Cu(II) and ascorbic acid generating Cu(I) in situ. When the reaction is conducted without triflate in organic solvent, Cu(I) ions were also demonstrated to be able to desulfate O-acetylated sinigrin without cleavage of the glycosidic CS bond. Our findings indicate that Cu(I)-mediated non-enzymatic cleavage plays a key role in the fate of dietary glucosinolates and suggest that these compounds and their hydrolysis products may help sequester excess copper. This study will be developed for other glucosinolates with additional spectroscopic, chromatographic, and analytical methods for further understanding.