The role of mobile protons in negative ion CID of oligosaccharides.

Carbohydrates of all classes consist of glycoform mixtures built on common core units. Determination of compositions and structures of such mixtures relies heavily on tandem mass spectrometric data. Analysis of native glycans is often necessary for samples available in very low quantities and for sulfated glycan classes. Negative tandem mass spectrometry (MS) provides useful product ion profiles for neutral oligosaccharides and is preferred for acidic classes. In previous work from this laboratory, site-specific influences of sialylation on product ion profiles in the negative mode were elucidated. The present results show how the interplay of two other acidic groups, uronic acids and sulfates, determines product ion patterns for chondroitin sulfate oligosaccharides. Unsulfated chondroitin oligosaccharides dissociate to form C-type ions almost exclusively. Chondroitin sulfate oligosaccharides produce abundant B- and Y-type ions from glycosidic bond cleavage with C- and Z-types in low abundances. These observations are explained in terms of competing proton transfer reactions that occur during the collisional heating process. Mechanisms for product ion formation are proposed based on tandem mass spectra and the abundances of product ions as a function of collision energy.