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Fragmenting Bio-Sugars Yields Sweet Structural Information

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Researchers from the University of Michigan have shown that the application of electron capture dissociation (ECD) techniques to glycoconjugates can deliver more saccharide-linkage information than other techniques.

The study was published in the latest issue of Analytical Chemistry and could help further our understanding of proteomics.

Glycoconjugates, oligosaccharide-containing biomolecules, are a group of important biological compounds that contain carbohydrates linked to other biological species and play an extensive role in nature.

The multiple functions of these compounds, such as influencing protein folding, immune response and cellular interactions such as cell-cell recognition and cell-matrix interactions, are largely due to the increased degree of complexity that the oligosaccharide groups impart on the biomolecules.

The structure and linkage of these oligosaccharide groups can also have an impact on the activity of biological drugs. For instance, Mitsuo Satoh and co-workers at the Tokyo Research Laboratories have shown that the structure of asparagine-linked oligosaccharides attached to human immunoglobulin G1 (IgG1) antibodies can affect their pharmacokinetics and cytoxicity.

Mass spectrometry (MS) has been shown to be an important tool for oligosaccharide characterisation, with tandem mass spectrometry (MS/MS) being used extensively for oligosaccharide structural analysis.

During a MS experiment, oligosaccharides undergo two main types of fragmentation, either glycosidic cleavage, which breaks the bonds between the saccharide units and provides saccharide sequence and branching information; or cross-ring cleavages, which can provide important saccharide linkage information.

ECD techniques introduce low energy electrons to trapped gas phase ions that cause more cross-ring cleavage than other techniques such as collision-induced dissociation (CID) and infrared multiphoton dissociation (IRMPD).

While ECD produces significantly different types of fragment ions to other techniques, the technique can suffer from low fragmentation efficiency, especially with compounds without basic functionalities as they do not undergo multiple ionisations easily.

The researchers managed to overcome this problem by utilising alkali earth and transition metals as charge carriers during the initial electrospray ionisation step to cause at least two charges to form on the oligosaccharides.

The researchers showed that for certain metal-adducts of oligosaccharides, particularly maltoheptaose, cross-ring cleavage is the dominant fragmentation pathway in ECD. In contrast, IRMPD techniques yielded fragments that were dominated by glycosidic cleavages.

While the researchers have managed to extend the utility of the ECD technique, their results "do not indicate that there is a clearly optimal metal which will maximise cross-ring fragmentation for both branched and linear oligosaccharides in ECD."