Evidence of gas-phase pyranose-to-furanose isomerization in protonated peptidoglycans.

Peptidoglycans are diverse co- and post-translational modifications of key importance in myriad biological processes. Mass spectrometry is employed to infer their biomolecular sequences and stereochemisties, but little is known about the critical gas-phase dissociation processes involved. Here, using tandem mass spectrometry (MS/MS and MSn), isotopic labelling and high-level simulations, we identify and characterize a facile isomerization reaction that produces furanose N-acetylated ions. This reaction occurs for both O- and N-linked peptidoglycans irrespective of glycosidic linkage stereochemistry (α/β). Dissociation of the glycosidic and other bonds thus occur from the furanose isomer critically altering the reaction feasibility and product ion structures.