Oligosaccharide fragmentation mechanism

Yamagah, T Suzuki, H. Tachibana, K. Semiquantitative analysis of isomeric oligosaccharides by negative-ion mode UV-MALDl TOF postsource decay mass spectrometry and their fragmentation mechanism study at Af-acetyl hexosamine moiety. 

The term monomeric mechanism will be used for the mechanism depicted in the left-hand part of Scheme 2 (sequence a). In this case, the monosaccharide residues are transferred consecutively from the corresponding glycosyl donors (Z-A or Z -B) onto a membrane-bound glycosyl acceptor. The acceptor is generally a monosaccharide residue, which may be a fragment of an oligosaccharide chain linked to a hydrophobic molecule embedded in a cell membrane. In many instances, the acceptor that is used for assembly of the polymeric chain (Y) is not identical to the final acceptor (X) of the chain, and further transfer of the chain from Y to X, or liberation of the polysaccharide molecule in the case of exocellular polysaccharides, is a necessary step in the biosynthesis. 

In bacterial lipopolysaccharides, O-specific chains composed of repeating, or modified repeating, units are linked to a unique oligosaccharide sequence of the core region which is connected to a lipid A fragment serving as a hydrophobic anchor embedded in the bacterial outer-membrane. Biosynthesis of O-specific chains was found to occur independently on formation of other structural fragments of the lipopolysaccharide molecule. Both block and monomeric mechanisms were demonstrated for the biosynthesis of these polymers. 

The ionization source is the mechanical device that allows ionization to occur. Matrix-assisted laser desorption/ionization (MALDI) and electrospray ionization (ESI) are now the most common ionization sources for biomolecular MS, because both of them are soft ionization techniques that is, the sample ionization process generates few or no fragments even for large biomolecules, such as proteins and oligosaccharides, so that the intact molecular ions can be easily observed. 

Some examples of studies with MALDI and IMS include the use of MALDI to generate sodiated parent ions of a number of oligosaccharides without fragmentation and cationized forms of bradykinin. Cross sections of the ions were obtained and compared with predictions made by molecular mechanics or molecular dynamics calculations. Comparison of MALDI/IM/TOF-MS with MALDI/IM/MS analysis yielded results similar to that by nanoelectrospray MS. Dipeptides and biogenic amines were analyzed with a system that consisted of an atmospheric pressure MALDI source, an IMS, and an orthogonal TOF-MS, and sensitivity was improved when a localized CD source was added. ... 

IRLD) in which the primary mechanism is alkali ion attachment has been particularly useful for the analysis of neutral polymers. Figures 6.13 and 6.14 show the IRLD mass spectra of an oligosaccharide and an oligomeric mixture of polypropylene glycol, respectively. In the former, delayed extraction of the ions produces the opportunity for considerable in-source fragmentation that can be used for structural analysis. 

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