Differentiation of some positional and diastereomeric isomers of Boc-carbo-β 3 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.