Inter- and intramolecular isotopic correlations in some cyanogenic glycosides and glucosinolates and their practical importance

Article abstract of DOI:10.1016/0031-9422(96)00290-7

Sinalbin is a complex organic salt from white mustard. The δ ¹³C- values of the aromatic parts of the anion glucosinalbin (p- hydroxyphenylacetic acid) and of the cation sinapin (sinapic acid) are identical (-32.2‰) and both aromatic compounds are depleted by 6.4‰ with respect to the glucose moiety (-25.8‰) bound in glucosinalbin. The δ ¹³C value of the choline part of the molecule can be correlated to its metabolic origin from glucose. However, there is an unexpected dramatic ¹³C- enrichment in the first C atom of glucosinalbin, originally the C-2 of tyrosine. This enrichment of approximately 11% relative to the mean value of the aglycone is found in the same position of four other glucosinolates and cyanogenic glycosides derived from phenylalanine or tyrosine. An isotope effect on the phenylalanine-ammonia-lyase reaction is discussed as the most probable cause for this finding. In contrast, sinigrin, the glucosinolate from black mustard, shows a relative ¹³C-depletion of 6.5‰ in the corresponding first C-atom of the aglycone allyl mustard oil. The proposed reason for this depletion is an isotope effect on an aldol reaction during the biosynthetic introduction of this C-atom into the aglycone skeleton. Synthetic allyl mustard oil is depleted by 14% in the same position relative to the δ ¹³C-value of the whole molecule, probably owing to an isotope effect on the first step of its chemical synthesis, the binding of allyl amine to CS₂. This difference can be used to detect adulterations of mustard.