Protonated sugars: Vibrational spectroscopy and conformational structure of protonated O-methyl α-D-galactopyranoside

Bridging protons provide a common structural motif in biological assemblies such as proton wires and proton-bound dimers. Here we present a proof-of-principle computational and vibrational spectroscopic investigation of an intra-molecular proton-bound dimer, O-methyl -D-galactopyranoside (MeGal-H ⁺), generated in the gas phase through photo-ionisation of its complex with phenol in a molecular beam. Its vibrational spectrum corresponds well with a classical molecular dynamics simulation conducted on-the-fly and also with the lowest-energy structures predicted by DFT and ab initio calculations. They reveal proton-bound structures that bridge neighbouring pairs of oxygen atoms, preferentially O6 and O4, linked together within the carbohydrate scaffold. Motivated by the possibility of an entry into the microscopic mechanism of its acid (or enzyme)-catalysed hydrolysis, we also report the corresponding predictions for its singly hydrated complex.