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 me thyl-beta-D-xylopyranoside, methyl-beta-D-glucopyranoside, and methyl-beta- D-arabinopyranoside at 25 degrees C. A 1 molar solution of the latter carbo hydrate 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 relaxa tion times tau(alpha)...tau(epsilon) of these terms indicate the existence of five relaxation regions for the carbohydrate solutions under investigati on (500 less than or equal to tau(alpha) less than or equal to 1500 ns; 40 less than or equal to tau(beta) less than or equal to 150 ns; 3 less than o r equal to tau(gamma) less than or equal to 12 ns; 0.5 less than or equal t o tau(delta) less than or equal to 2.1 ns; 0.1 less than or equal to tau(ep silon) less than or equal to 0.8 ns; 0.5 less than or equal to c less than or equal to 3.2 mol/L; 25 degrees C). These regions have been attributed to ring isomerization processes such as chair conformational changes and pseu dorotations, to rotational isomerization of exocyclic groups, and to a carb ohydrate association mechanism. Additional broadband dielectric relaxation measurements of some solutions showed that the reorientational motions of t he hydration water molecules are much faster (relaxation time less than or equal to 0.03 ns) than the aforementioned molecular processes.