Selective reactivity of glycosyl cation stereoisomers: the role of intramolecular hydrogen bonding.

The impact of the stereochemistry of the glycosyl cation species upon its dynamic properties is examined together with their vibrational spectra in order to gain insight into the effects of configurational isomerism on conformer dynamics and proton mobility. Ab initio molecular dynamics (AIMD) simulations and infrared multiple photon dissociation (IRMPD) spectroscopy explore the conformational and reactive dynamics of two pairs of glycosyl cation isomers: (1) protonated α- and ß- anomers of methyl-D-galactopyranoside and (2) the oxocarbenium ions of the D-aldohexose C2 epimers galactose and talose. Analysis of these simulations together with experimental spectroscopy, interpreted by anharmonic calculations, points to the key role played by the intramolecular hydrogen bonds which are present in a unique pattern and extent in each isomer. We find that the reactivity of galactoside stereoisomers toward acid-catalyzed nucleophilic substitution, as gauged by the ability to form free oxocarbenium ions, differs markedly in a way that agrees with experimental measurements in the condensed phase. Other properties such as conformer stability and vibrational transitions were also found to reflect the characteristic hydrogen bonding interactions present in each isomer. In both systems, the stereochemistry is shown to determine the strength of intramolecular hydrogen bonding as well as between which substituents proton transfer is possible. We expect that the critical impact of non-covalent interactions on stereoisomer selectivity may be a widely found phenomenon whose effects should be further investigated.

Gas phase dynamics, conformational transitions and spectroscopy of charged saccharides: the oxocarbenium ion, protonated anhydrogalactose and protonated methyl galactopyranoside.

Protonated intermediates are postulated to be involved in the rate determining step of many sugar reactions. This paper presents a study of protonated sugar species, isolated in the gas phase, using a combination of infrared multiple photon dissociation (IRMPD) spectroscopy, classical ab initio molecular dynamics (AIMD) and quantum mechanical vibrational self-consistent field (VSCF) calculations. It provides a likely identification of the reactive intermediate oxocarbenium ion structure in a d-galactosyl system as well as the saccharide pyrolysis product anhydrogalactose (that suggests oxocarbenium ion stabilization), along with the spectrum of the protonated parent species: methyl d-galactopyranoside-H+. Its vibrational fingerprint indicates intramolecular proton sharing. Classical AIMD simulations for galactosyl oxocarbenium ions, conducted in the temperature range ∼300-350 K (using B3LYP potentials on-the-fly) reveal efficient transitions on the picosecond timescale. Multiple conformers are likely to exist under the experimental conditions and along with static VSCF calculations, they have facilitated the identification of the individual structural motifs of the galactosyl oxocarbenium ion and protonated anhydrogalactose ion conformers that contribute to the observed experimental spectra. These results demonstrate the power of experimental IRMPD spectroscopy combined with dynamics simulations and with computational spectroscopy at the anharmonic level to unravel conformer structures of protonated saccharides, and to provide information on their lifetimes.