Conformational preferences of protonated N-acetylated hexosamines probed by InfraRed Multiple Photon Dissociation (IRMPD) spectroscopy and ab initio calculations

Abstract

The gas phase IR spectra of three protonated N-acetyl hexosamines and their methyl-blocked α and β anomers are measured in the 3μm spectral range using IRMPD spectroscopy. Using molecular dynamics exploration of the conformational space followed by MP2 energy calculations and DFT simulation of the IR spectra, we propose energetic and spectroscopic arguments to elucidate (i) the site of protonation, (ii) the conformational preference of the carbohydrate ring, and (iii) the contributions of the α and β anomers to the experimental spectra. We report that protonation occurs on the carbonyl group. All isomers of N-acetyl hexosamines are observed in a 4C1 chair conformation regardless of the methylation at the reducing end. Subtle structural variations of this common conformation are described. Finally, anomer-diagnostic spectroscopic features are proposed for GlcNAc and ManNAc.