Does low-energy collision-induced dissociation of lithiated and sodiated carbohydrates always occur at anomeric carbon of the reducing end?

Rationale: Collision-induced dissociation (CID) tandem mass spectrometry is one of the major methods in the structural determination of carbohydrates. Previous experimental studies and theoretical investigation of lithiated and sodiated underivatized carbohydrates seem to indicate that dehydration reactions and cross-ring dissociation of low-energy CID mainly occur at the anomeric carbon of the reducing end. However, these studies only investigated a few carbohydrates. Methods: ESI-MS/MS spectra of [M + Li](+) and [M + Na](+) ions of several (18)O1-labeled monosaccharides and disaccharides at O1 of the reducing end were studied using a linear ion trap mass spectrometer. Results: Dissociations from the losses of both labeled and unlabeled neutral fragments were observed. The branching ratios of dissociations from the losses of unlabeled neutrals for dehydration reactions are larger than that for cross-ring dissociation; lithiated carbohydrates are larger than sodiated carbohydrates, and 1-4 linkages of disaccharides are larger than the other linkages. For some lithiated carbohydrates, dehydration reactions from the losses of unlabeled neutrals have larger branching ratios than that from the losses of labeled neutrals. The fragments from the losses of unlabeled neutrals investigated using MS3 showed that the losses of unlabeled H2O mainly occur at the reducing monomer for sodiated carbohydrates, but the losses of unlabeled C2H4O2 for lithiated carbohydrates can occur at both reducing and nonreducing monomers. The ratio of B-1 and Y-1 ions to C-1 and Z(1) ions of disaccharides is related to the cis or trans configuration of the O1 and O-2 atoms in the nonreducing monomer. The results are explained by the properties of transition states of dissociation channels. Conclusions: Our data shows that dehydration reactions and cross ring dissociation do not always occur at the anomeric carbon atom of the reducing monomer.