Characterisation and determination of phytochelatins in plant extracts by electrospray tandem mass spectrometry

A method based on pneumatically assisted electrospray ionisation tandem mass spectrometry (ESI MS-MS) was developed for the identification, sequencing and determination of phytochelatin (PC) peptides in plant tissue and plant cell cytosols. The ionization and fragmentation conditions were optimized using a series of (GluCys)2Gly (PC2), (GluCys)3Gly (PC3), and (GluCys)4Gly (PC4) standards prepared from glutathione by enzymatically (γ-glutamylcysteine dipeptyl transpeptidase) assisted biosynthesis in the presence of Cd2+. Phytochelatins were found to ionize readily to produce a characteristic mono-protonated ion. The collision-induced dissociation (CID) of this ion followed by mass spectrometry (MS-MS mode) allowed the determination of the amino acid sequence of each of the PCs. Calibration curves were linear up to a concentration of 2 μg ml-1 in the MS and MS-MS modes with the detection limits at the low ng ml-1 level. The method was applied to the determination of phytochelatin peptides biosynthesized by a number of plant cell cultures exposed to the Cd stress. The results agreed with those obtained by an independent procedure based on reversed-phase HPLC with post-column derivatization of the -SH groups with 5,5'-dithiobis-2-nitrobenzoic acid and spectrophotometric detection.