Differentiation of some positional and diastereomeric isomers of Boc-carbo-β3 dipeptides containing galactose, xylose and mannose sugars by electrospray ionization tandem mass spectrometry (ESI MS/MS)☆

Abstract Electrospray ionization (ESI) tandem mass spectrometry has been used to distinguish the positional and diastereomeric isomers of Boc-C-linked carbo-β 3 dipeptides ( 1 – 38 ) synthesized from glycine (Gly), β-h-glycine (β-hGly), β-h-alanine (β-hAla) and C-linked carbo-β 3 -amino acid (Caa) that contain galactose, xylose and mannose sugars as side chains with “ R ” and “ S ” configurations at the amine center. The major fragmentation of [ M  + H] + of the dipeptides ( 1 – 38 ) yields mainly two ions: (i) [ M  + H-C(CH 3 ) 3  + H] + (‘ a ’) and (ii) [ M  + H-Boc + H] + (‘ b ’) corresponding to losses of 2-methyl-prop-1-ene and -Boc moiety from [ M  + H] + ions, respectively. The diastereomeric dipeptide isomers with Caa ( R ) and ( S ) configurations at the N-terminus can easily be distinguished by the difference in the abundance of ion ‘ a ’ and ‘ b ’. The isomeric peptides with Caa ( R ) at the N-terminus gives prominent [ M  + H-C(CH 3 ) 3  + H] + (‘ a ’) where as it is insignificant or totally absent for peptides which have Caa ( S ) at the N-terminus. This is presumably due to the different steric crowdings around the Boc-group in the different diastereomers. The positional isomers of dipeptides can also be differentiated by the difference in the abundance of ion ‘ a ’ and ‘ b ’ in the CID of [ M  + H] + ions. The CID of [ M  + H-Boc + H] + ions of the isomeric peptides also show y 1 + ions at different m / z values. All these results suggest that the CID of [ M  + H] + ions is highly useful for distinguishing the Boc-NH-Caa-β 3 dipeptide isomers with Caa of “ S ” configuration from the isomers with Caa of “ R ” configuration and the positional isomers.