  O-acetylations are functional modifications which can be found on different
hydroxyl groups of glycans and which contribute to the fine tuning of their
biological activity. Localizing the acetyl modifications is notoriously
challenging in glycoanalysis, in particular because of their mobility: loss or
migration of the acetyl group may occur through the analytical workflow.
Whereas migration conditions in the condensed phase have been rationalized,
little is known about the suitability of Mass Spectrometry to retain and
resolve the structure of O-acetylated glycan isomers. Here we use the resolving
power of infrared ion spectroscopy in combination with ab initio calculation to
assess the structure of O-acetylated monosaccharide ions in the gaseous
environment of a mass analyzer. N-acetyl glucosamines were synthetized with an
O-acetyl group in positions 3 or 6, respectively. The protonated ions produced
by electrospray ionization were observed by mass spectrometry and their
vibrational fingerprints were recorded in the 3 $\mu$m range by IRMPD
spectroscopy (InfraRed Multiple Photon Dissociation). Experimentally, the
isomers show distinctive IR fingerprints. Additionally, ab initio calculations
confirm the position of the O-acetylation and resolve their gas phase
conformation. These findings demonstrate that the position of O-acetyl groups
is retained through the transfer from solution to the gas phase, and can be
identified by IRMPD spectroscopy.
