REVERSED-PHASE HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHY OF S-ALK(EN)YL-L-CYSTEINE DERIVATIVES IN ALLIUM-SATIVUM INCLUDING THE DETERMINATION OF (+)-S-ALLYL-L-CYSTEINE SULFOXIDE, GAMMA-L-GLUTAMYL-S-ALLYL-L-CYSTEINE AND GAMMA-L-GLUTAMYL-S-(TRANS-1-PROPENYL)-L-CYSTEINE

The separation of six S-alk(en)yl-L-CySteine sulphoxides and gamma-L-glutamyl-S-alk(en)yl-L-cysteines as genuine constituents of Allium sativum L. is reported. After automated precolumn derivatization with o-phthaldialdehyde-tert.-butanethiol the reaction products, sulphur-substituted isoindole derivatives, were analysed by reversed-phase high-performance liquid chromatography (RP-HPLC) followed by UV detection at 337 and 260 nm or fluorescence detection (excitation wavelength 230 nm, emission wavelength 420 nm). The method described allowed the qualitative and quantitative determination of the characteristic genuine polar garlic components in a single run. The accuracy and precision of the assay method, including external calibration, were evaluated. To validate the system the two main gamma-glutamyl peptides, gamma-L-glutamyl-S-allyl-L-cysteine and gamma-L-glutamyl-S-(trans-1-propenyl)-L-cysteine, were determined using two different chromatographic procedures: they were determined as isoindole derivatives with UV detection as described above and by RP-HPLC with UV detection at 210 nm without previous derivatization. The method can be applied to the standardization of garlic and garlic preparations. Several garlic bulb samples were investigated and the total amount of the three main compounds was found to vary by a factor of about 2.5.