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Elusive oxocarbenium ions revealed and characterized by IR ion spectroscopy

Date of news: 20 April 2018

Elusive oxocarbenium ions revealed and characterized by IR ion spectroscopy

Glycosyl cations, including the elusive oxocarbenium ion, have been experimentally formed and structurally characterized for the first time by combining tandem mass spectrometry and IR ion spectroscopy using the free electron laser FELIX and quantum chemical calculations.

The stereo-selective synthesis of glycosidic bonds is a huge challenge in oligosaccharide synthesis. In these glycosylation reactions, carbohydrate building blocks are covalently linked in order to form larger oligosaccharides. The transient intermediates, the glycosyl cations are key in the mechanism and outcome of glycosylation reactions. However, due to their high reactivity and short life times, these glycosyl cations are very hard to characterize spectroscopically. Only recently, the structure of a glucose oxocarbenium ion has been shown under very harsh conditions by NMR. These conditions are, however, not compatible with all commonly used protecting groups and a more general method to determine the oxocarbenium structure has been published in a JACS communication by Rijs and Boltje.


The figure illustrates our approach: first the glycosyl cation is formed by activating the precursor carbohydrate inside the mass spectrometer, subsequently it IR ion spectrum is recorded and compared to quantum chemical calculations to determine its exact structure.

The use of tandem mass spectrometry coupled with IR ion spectroscopy and quantum chemical calculations is a novel, mild and versatile approach to generate and structurally characterize elusive glycosyl cations under isolated conditions. The robustness and generality of the approach has been examined by for example testing different glycans; i.e. manno-, gluco and galactosides, and the effect of neighboring group participation on the resulting conformation of the glycosyl cation. When neighboring group participation was fully eliminated, the elusive oxocarbenium was formed. Since the IR spectra show a plethora of diagnostic features highlighting the conformational preferences of the glycosyl cation. These experiments are a first step in elucidation the glycosylation reaction mechanism and predicting its stereoselectivity.

Article (currently online as JACS ASAP)
Elferink, M. Severijnen, J. Martens, R. Mensink, G. Berden, J. Oomens, F. Rutjes, A.M. Rijs, and T. Boltje, “Direct Experimental Characterization of Glycosyl Cations by Infrared Ion Spectroscopy”, J. Am, Chem. Soc. (2018), DOI: 10.1021/jacs.8b01236