Characterization of isomeric carbohydrates using metal coordination, enzymatic digestion, and tandem mass spectrometry.

摘要

The amino-sugars, glucosamine, galactosamine, and mannosamine (and their acetylated counterparts) are building blocks of carbohydrates, which are important biologically for many reasons. For example, sulfated N-acetyl-galactosamine accounts for approximately 50% of the carbohydrate content in articular cartilage. Because of the biological interest in these compounds, mass spectrometric methods were developed to identify the aforementioned amino-sugars. Specifically, stereoisomeric N-acetylhexosamine and hexosamine monosaccharides were differentiated using mass spectrometry by first derivatizing the carbohydrates with [Co(Cl)₂(DAP)₂]Cl, then subjecting the complexes to collision-induced dissociation (CID) in a quadrupole ion trap mass spectrometer.; The monosaccharides were differentiated based on the presence or absence of unique product ions observed in CID spectra. Because the mass spectra are very reproducible, the relative abundances of the distinguishing product ions were used to quantify mixtures of the diastereomers. Both the hexosamines and the N-acetylhexosamines were quantified in this fashion, and the technique was used to identify the stereochemistry of N-acetylhexosamine monosaccharides found in three biologically relevant lipooligosaccharides.; The multi-component quantification protocol was also used to determine the disaccharide composition in chondroitin (cartilage) polysaccharides. To achieve this, the polysaccharides were first enzymatically digested to produce a mixture of isomeric, sulfated disaccharides. The disaccharides were quantified using the relative abundances of product ions found in the CID spectra of the enzymatic digestion mixture. In additional experiments, the quantification protocol was used in combination with selective and nonselective enzymatic digestions, in such a manner that sequence information in the chondroitin polysaccharide was deduced. This information has not been obtained by any other method.; In addition to the analytical methods described above, mechanistic studies were undertaken to develop a better understanding of why the metal-carbohydrate complexes underwent stereoselective dissociations. In all of the complexes studied herein, the coordination of the carbohydrate to the metal plays a key role in the dissociation pathways observed. The effects of the 2 auxiliary ligand and coordination number on the stereoselectivity of the dissociations were also studied, and the coordination number was found to have a much more substantial effect on the dissociation than the size of the auxiliary ligand.