Hydroxyl and Methoxyl Derivatives of Benzylglucosinolate in Lepidium densiflorum with Hydrolysis to Isothiocyanates and non-Isothiocyanate Products: Substitution Governs Product Type and Mass Spectral Fragmentation

A system of benzylic glucosinolates was found and characterized in common pepperweed, Lepidium densiflorum Schrad. The major glucosinolate was the novel 4-hydroxy-3,5-dimethoxybenzylglucosinolate (3,5-dimethoxysinalbin),present at high levels in seeds, leaves, and roots. Medium-level glucosinolates were 3,4-dimethoxybenzylglucosinolate and 3,4,5trimethoxybenzylglucosinolate. Minor glucosinolates included benzylglucosinolate, 3-hydroxy- and 3-methoxybenzylglucosinolate, 4-hydroxybenzylglucosinolate (sinalbin), the novel 4-hydroxy-3-methoxybenzylglucosinolate (3-methoxysinalbin), and indole-type glucosinolates. A biosynthetic connection is suggested. NMR, UV, and ion trap MS/MS spectral data are reported, showing contrasting MS fragmentation of p-hydroxyls and p-methoxyls. Additional investigations by GC-MS focused on glucosinolate hydrolysis products. Whereas glucosinolates generally yielded isothiocyanates, the dominating 3,5-dimethoxysinalbin with a free p-hydroxyl group produced the corresponding alcohol and syringaldehyde (4-hydroxy-3,5-dimethoxybenzaldehyde). After thermal deactivation of the endogenous myrosinase enzyme, massive accumulation of the corresponding nitrile was detected. This case study points out how non-isothiocyanate glucosinolate hydrolysis products are prevalent in nature and of interest in both plant pathogen interactions and human health.