Computational Studies of Protonated β-d-Galactose and Its Hydrated Complex: Structures, Interactions, Proton Transfer Dynamics, and Spectroscopy

We present an exploration of proton transfer dynamics in a monosaccharide, based upon ab initio molecular dynamic (AIMD) simulations, conducted “on-the-fly”, in β-d-galactose-H⁺ (βGal-H⁺) and its singly hydrated complex, βGal-H⁺-H₂O. Prior structural calculations identify O6 as the preferred protonation site for O-methyl α-d-galactopyranoside, but the β-anomeric configuration favors the inversion of the pyranose ring from the ⁴C₁ chair configuration, to ¹C₄, and the formation of proton bridges to the (axial) O1 and O3 sites. In the hydrated complex, however, the proton bonds to the water molecule inserted between the O6 and Ow sites, and the ring retains its original ⁴C₁ conformation, supported by a circular network of co-operatively linked hydrogen bonds. Two distinct proton transfer processes, operating over a time scale of 10 ps, have been identified in βGal-H⁺ at 500 K. One of them leads to chemical reaction and the formation of an oxacarbenium ion (accompanied by the loss of an H₂O molecule). In the hydrated complex, βGal-H⁺-H₂O, this reaction is suppressed, and the proton transfer, which involves multiple jumps between the sugar and the H₂O, creates an H₃O⁺ ion, relevant, perhaps, to the reactivity of protonated sugars both in the gas and condensed phases. Anticipating future spectroscopic investigations, the vibrational spectra of βGal-H⁺ and βGal-H⁺-H₂O have also been computed through AIMD simulations conducted at average temperatures of 300 and 40 K and also through vibrational self-consistent field (VSCF) calculations at 0 K.