Molecular dynamics and kinetics of monosaccharides in solution. A broadband ultrasonic relaxation study

Between 100 kHz and 2 GHz ultrasonic absorption spectra have been measured for aqueous solutions of D-galactose, D-mannose, D-glucose, D-arabinose, D-ribose, D-lyxose, and D-xylose, as well as of the methylated derivatives methyl-β-D-xylopyranoside, methyl-β-D-glucopyranoside, and methyl-β-D-arabinopyranoside at 25 °C. A 1 molar solution of the latter carbohydrate did not show absorption in excess of the asymptotic high frequency contribution. The other solutions revealed relaxation characteristics which are described by up to three Debye spectral terms per spectrum. The relaxation times τ_α...τ∈ of these terms indicate the existence of five relaxation regions for the carbohydrate solutions under investigation (500 ≤ τ_α ≤ 1500 ns; 40 ≤ τ_β ≤ 150 ns; 3 ≤ τ_γ ≤ 12 ns; 0.5 ≤ τ_δ ≤ 2.1 ns; 0.1 ≤ τ∈ ≤ 0.8 ns; 0.5 ≤ c ≤ 3.2 mol/L; 25 °C). These regions have been attributed to ring isomerization processes such as chair conformational changes and pseudorotations, to rotational isomerization of exocyclic groups, and to a carbohydrate association mechanism. Additional broadband dielectric relaxation measurements of some solutions showed that the reorientational motions of the hydration water molecules are much faster (relaxation time ≤ 0.03 ns) than the aforementioned molecular processes.