Direct Experimental Characterization of Glycosyl Cations by Infrared Ion Spectroscopy

Abstract

Glycosyl cations are crucial intermediates formed during enzymatic and chemical glycosylation. The intrinsic high reactivity and short lifetime of these reaction intermediates make them very challenging to characterize using spectroscopic techniques. Herein, we report the use of collision induced dissociation tandem mass spectrometry to generate glycosyl cations in the gas phase followed by infrared ion spectroscopy using the FELIX infrared free electron laser. The experimentally observed IR spectra were compared to DFT calculated spectra enabling the detailed structural elucidation of elusive glycosyl oxocarbenium and dioxolenium ions.

Scheme 1. Reactive Intermediates in Glycosylation Reactions and Their Respective Reaction Pathways

LG = Leaving group, E = Electrophile, A = Anion, P = Protecting group.

Scheme 2. Structures of the Mannosyl Donors Used in This Study

Figure 1

Comparison of the calculated spectra (filled) of 2-O-acetyl- (1a) or 3-O-acetyl- (1b) participation with the measured IR ion spectrum of mannosyl cation derived from 1 (black line in both panels).

Figure 2

Comparison of the calculated spectra (filled) of the ³E (2a) or ⁴H₃ (2b) oxocarbenium ion conformer with the measured IR ion spectrum of the m/z = 219 CID fragment of 2 (black line in both panels). Energies are relative to the 0.0 kJ/mol structure as reported in Figure S6.

Figure 3

Comparison of calculated spectra (filled) with the measured IR ion spectra (black line) of mannuronic acids 3a, 4a, 4b (A–C) and a mixture of isomers 4a and 4b (D).