Methods for routine sequencing of carbohydrates lag behind those for sequencing nucleic acids and proteins. The lack of such methods makes fully characterizing carbohydrates difficult. Sequencing a carbohydrate is a challenge because many of the sugar building blocks are isomers of one another, making it hard to distinguish between them, and the stereochemistry of glycosidic bonds connecting the sugars must be characterized to fully analyze the carbohydrate’s structure. Reseachers
led by Isabelle Compagnon of the University of Lyon report a hybrid method that allows them to sequence carbohydrates by combining infrared laser spectroscopy with mass spectrometry (Nat. Commun. 2017, DOI: 10.1038/s41467-017-01179-y). They break carbohydrates into monosaccharide fragments in the mass spectrometer with gas collisions. Those fragments are then further dissociated with IR photons from a tunable light source to record the IR spectra of the fragments. The researchers first used the method to analyze disaccharides. They showed that the IR fingerprints of a particular type of monosaccharide fragment—the so-called C fragment—can be used to identify the monosaccharide content and can contain stereochemical information about the glycosidic bond between the sugars. The team then applied the sequencing approach to longer oligosaccharides. “The key insight that this study provides is that the chirality of the glycosidic bond oxygen is not altered even when the glycosidic bond is cleaved,” says Nicolas C. Polfer of the University of Florida, who also combines mass spectrometry with IR spectroscopy. “This observation has important ramifications for carbohydrate sequencing, as the glycosidic bond is generally the most labile bond in larger saccharides.” Nicola Pohl, a chemistry professor at Indiana University, Bloomington, who also develops analytical methods for carbohydrates, wonders how well the method will translate to a wide range of monosaccharides. “Will this IR technique produce enough distinct signatures to be diagnostic for all possible sugars?” she asks.