2 research outputs found
Monosaccharide-Water Complexes: Vibrational Spectroscopy and Anharmonic Potentials
Ab initio vibrational self-consistent field (VSCF) calculations
are used to predict the vibrational spectra of an extended series
of monosaccharide·D<sub>2</sub>O complexes, including glucose,
galactose, mannose, xylose, and fucose in their α and β
anomeric forms, and compared with recently published experimental
data for their (phenyl-tagged) complexes. Anharmonic VSCF-PT2 frequencies
are calculated directly, using ab initio hybrid HF/MP2 potentials,
to assess their accuracy in reproducing the vibrational anharmonicities
and provide a more rigorous basis for vibrational and structural assignments.
The average discrepancies between the calculated and experimental
frequencies are ∼1.0–1.5%, and the first-principles
spectroscopic calculations, free of any empirical scaling, yield results
of high accuracy. They encourage confidence in their future application
to the assignment of other carbohydrate systems, both free and complexed,
and an improved understanding of their intra- and intermolecular carbohydrate
interactions
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