15:50 - 16:20 Young Investigators

## Sugars in the Gas Phase – From Structure to Reaction Mechanisms

Kevin Pagel

Institut für Chemie und Biochemie, Freie Universität Berlin, Takustraße 3, 14195, Berlin, GER

Department of Molecular Physics, FHI der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195, Berlin, GER

Currently, the vast majority of glycans are characterized using mass spectrometry-based techniques (MS). Measuring the molecular weight of a sugar, however, immediately poses a fundamental problem: entire classes of monosaccharide building blocks exhibit an identical atomic composition and, consequently, an identical mass. Therefore, glycan MS data can be highly ambiguous and often it is not possible to clearly assign a particular structure.

Recently, we assessed the potential of cold-ion IR spectroscopy for oligosaccharide analysis. 1 Gas-phase IR spectra of a series of synthetically derived mono-, di- and trisaccharide standards were recorded. For each of these oligosaccharides, unique and highly diagnostic absorption patterns with a variety of well-resolved bands was obtained. This unprecedented resolution reveals remarkable differences in the overall IR signatures and allows a simple, fingerprint-based discrimination between isomers, even if they merely differ in the stereochemistry of a single $\ce{OH}$ group.

Encouraged by these results, we very recently employed cold-ion IR spectroscopy to study reactive glycosyl cation intermediates that occur during glycosidic bond formation. These intermediates often determine the stereochemical outcome of the reaction, however, due to their transient and short-lived nature their exact structure remained elusive to date. Our results show that glycosyl cations with participating C2 protective groups form a covalent bond with the anomeric carbon that leads to C1-bridged acetoxonium-type structures. The resulting bicyclic structure strongly distorts the ring, which leads to a unique conformation for each individual monosaccharide. This novel level of mechanistic understanding fundamentally impacts glycosynthesis and allows to tailor building blocks and reaction conditions.

1. E. Mucha, A.I. González Flórez, M. Marianski, D.A. Thomas, W. Hoffman, W.B. Struwe, H.S. Hahm, S. Gewinner, W. Schöllkopf, P.H. Seeberger, G. von Helden, K. Pagel, Angew. Chem. Int. Ed. 2017, 56, 11248–11251.