| Abstract: | Quantitative analysis of each of the S-alk(en)yl-L-cysteine sulphoxides was achieved by the following procedure. A methanol: chloroform: water extract of onion tissue was purified by electrophoresis. Alk(en)yl-L-cysteine sulphoxides were separated by thin-layer chromatography (t.l.c.) on commercial silica-gel plates and were reacted with ninhydrin, the spot intensity being measured by an integrating densitometer. The synthetic analogue (±)-S-1-butyl-L-cysteine sulphoxide was used as an internal standard in each extract. In white bulb onion (cv. Southport White Globe), (±)-S-1-propyl-L-cysteine sulphoxide is the predominant flavour precursor at a concentration of 2.9 ± 0.4 mg g?1 fresh weight. ±-S-1-methyl-L-cysteine sulphoxide and (±) trans-S-1-propenyl-L-cysteine sulphoxide are present at the lower concentrations of 0.9 ± 0.25 mg g?1 fresh weight and 0.6 ± 0.15 mg g?1 fresh weight respectively. Stable n-butyl, trifluoroacetyl derivatives of standards and of the alk(en)yl-L-cysteine sulphoxides in extracts, were formed. Subsequent g.c.-m.s. enabled parent ions and fragmentation patterns to be recorded and allowed the conclusive identification of the endogenous alk(en)yl-L-cysteine sulphoxides. |
