Acylated glucosinolates with diverse acyl groups investigated by high resolution mass spectrometry and infrared multiphoton dissociation

With the aim of developing a procedure for detecting and identifying intact acylated glucosinolates (a-GLSs) found in trace quantities in natural plant samples, extracts of Barbarea vulgaris seeds were analyzed by reversed-phase liquid chromatography coupled with electrospray ionization and Fourier-transform ion cyclotron resonance mass spectrometry (RPLC-ESI FTICR MS). After a preliminary optimization of fragmentation conditions, based on a non-acylated parent glucosinolate (glucobarbarin) and three previously identified a-GLSs (the 6'-isoferuloyl esters of glucobarbarin, gluconasturtiin and glucobrassicin), infrared multiphoton dissociation (IRMPD) was employed for a tandem MS-based elucidation of the molecular structures of novel a-GLSs. As a result, three acylated derivatives of glucobarbarin, esterified at the thioglucose moiety with a coumaric acid isomer, sinapic acid or an isomer and a dimethoxycinnamic acid isomer, were identified. In addition, a further acylated glucosinolate was tentatively identified as the isoferuloyl ester of an unidentified hydroxylic derivative of glucobarbarin. This is the first demonstration of diversity in the acyl moieties of thioglucose-acylated glucosinolates, which may reflect the substrate specificity of the endogenous acyl transferase. As expected, 6'-isoferuloyl-glucobarbarin was detected as the main acylated GLS in extracts of B. vulgaris seeds. A quantitative estimate suggested that non-isoferuloyl substituted glucobarbarins correspond to ca. 0.026% of the level of 6'-isoferuloyl glucobarbarin. The formation of an uncommon distonic radical anion, most likely generated in the gas phase upon methyl radical (CH3·) loss from the isoferuloyl anion, is demonstrated.