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)

Electrospray ionization (ESI) tandem mass spectrometry has been used to distinguish the positional and diastereomeric isomers of Boc-C-linked carbo-β3 dipeptides (1–38) synthesized from glycine (Gly), β-h-glycine (β-hGly), β-h-alanine (β-hAla) and C-linked carbo-β3-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(CH3)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(CH3)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 y1+ 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-β3 dipeptide isomers with Caa of “S” configuration from the isomers with Caa of “R” configuration and the positional isomers.