Chemical assignment of structural isomers of sulfur-containing metabolites in garlic by liquid chromatography-fourier transform ion cyclotron resonance-mass spectrometry

Abstract

Background: The chemical assignment of metabolites is crucial to understanding the relation between food composition and biological activity. Objective: This study was designed to detect and chemically assign sulfur-containing metabolites by using LC-Fourier transform ion cyclotron resonance-mass spectrometry (FTICR-MS) in Allium plants. Methods: Ultrahigh resolution (> 250,000 full width at half-maximum) and mass accuracy (< 1 mDa) by FTICR-MS allowed us to distinguish ions containing sulfur isotopes (32S and 34S). Results: Putative 69 S-containing monoisotopic ions (S-ions) were extracted from the metabolome data of onion (Allium cepa), green onion (Allium fistulosum), and garlic (Allium sativum) on the basis of theoretical mass differences between 32S-ions and their 34S-substituted counterparts and on the natural abundance of 34S. Eight S-ions were chemically assigned by using the reference data according to the guidelines of the Metabolomics Standards Initiative. Three ions detected in garlic were assigned as derived from the isomers γ-glutamyl-S-1-propenylcysteine and γ-glutamyl-S-2-propenylcysteine and as S-2-propenylmercaptoglutathione on the basis of differences in key product ions identified in reference tandem MS spectra. Conclusion: The ability to discriminate between such geometric isomers will be extremely useful for the chemical assignment of unknown metabolites in MS-based metabolomics.