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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<sup>+</sup> (βGal-H<sup>+</sup>) and its singly hydrated complex,
βGal-H<sup>+</sup>-H<sub>2</sub>O. 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 <sup>4</sup>C<sub>1</sub> chair configuration, to <sup>1</sup>C<sub>4</sub>, 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 <sup>4</sup>C<sub>1</sub> 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<sup>+</sup> at 500 K. One of them leads to chemical
reaction and the formation of an oxacarbenium ion (accompanied by
the loss of an H<sub>2</sub>O molecule). In the hydrated complex,
βGal-H<sup>+</sup>-H<sub>2</sub>O, this reaction is suppressed,
and the proton transfer, which involves multiple jumps between the
sugar and the H<sub>2</sub>O, creates an H<sub>3</sub>O<sup>+</sup> 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<sup>+</sup> and βGal-H<sup>+</sup>-H<sub>2</sub>O 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