Differentiation of some positional and diastereomeric isomers of Boc-carbo-beta(3) dipeptides containing galactose, xylose and mannose sugars by electrospray ionization tandem mass spectrometry (ESI MS/MS)

摘要

Electrospray ionization (ESI) tandem mass spectrometry has been used to distinguish the positional and diastereomeric isomers of Boc-C-linked carbo-beta(3) dipeptides (1-38) synthesized from glycine (Gly), beta-h-glycine (beta-hGly), beta-h-alanine (beta-hAla) and C-linked carbo-beta(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(CH3)(3) + H](+) (W) 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-terminus can easily be distinguished by the difference in the abundance of ion 'a' and W. The isomeric peptides with Caa (R) at the at the N N-terminus gives prominent [M+ H-C(CH3)(3) + H](+) (W) 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 diastereorners. The positional isomers of dipeptides can also be differentiated by the difference in the abundance of ion 'a' and W 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-beta(3) dipeptide isomers with Caa of "S" configuration from the isomers with Caa of "R" configuration and the positional isomers. (c) 2005 Elsevier B.V. All rights reserved.