We combine conformer-selective, cryogenic infrared spectroscopy, quantum mechanical computations, and O-18 substitution at the reducing end to determine the structural preferences of protonated glucosamine in the gas phase. Cryogenic infrared- infrared (IR-IR) double resonance spectroscopy of helium-tagged, protonated glucosamine provides vibrational fingerprints of individual conformers, and O-18 isotopic labeling facilitates the match with computed structures and provides a selective probe of the anomeric hydroxyl. This is key for using vibrational spectroscopy for glycan analysis and determining the generality of anomeric memory during glycosidic bond cleavage.