Molecular Dynamics and Kinetics of Monosaccharides in Solution. A Broadband Ultrasonic Relaxation Study J. Stenger, | M. Cowman, † F. Eggers, ‡ E. M. Eyring, § U. Kaatze,* ,| and S. Petrucci ⊥ Drittes Physikalisches Institut, Georg-August-UniVersita ¨ t, Bu ¨ rgerstrasse 42-44, D-37073 Go ¨ ttingen, Germany, Department of Chemical Engineering, Chemistry, and Material Science, Polytechnic UniVersity, Brooklyn, New York 11201, Max-Planck-Institut fu ¨ r Biophysikalische Chemie, Am Fassberg, D-37077 Go ¨ ttingen, Germany, Department of Chemistry, UniVersity of Utah, Salt Lake City, Utah 84112, and Chemistry Department, Polytechnic UniVersity, Route 110, Farmingdale, New York 11735 ReceiVed: NoVember 12, 1999; In Final Form: February 18, 2000 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--D-xylopyranoside, methyl--D-glucopyranoside, and methyl--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 R ...τ ǫ of these terms indicate the existence of five relaxation regions for the carbohydrate solutions under investigation (500 e τ R e 1500 ns; 40 e τ  e 150 ns; 3 e τ γ e 12 ns; 0.5 e τ δ e 2.1 ns; 0.1 e τ ǫ e 0.8 ns; 0.5 e c e 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 e 0.03 ns) than the aforementioned molecular processes. 1 Introduction Constituting one of the four major classes of biomolecules, carbohydrates play multiple roles in living nature. They serve as the main resource of energy in biological cells and form significant components of nucleic acids; they are linked to many proteins and lipids and contribute essentially to the structure and function of cell walls of bacteria and plants. Monosaccha- rides, though following the simple empirical formula (CH 2 O) n , display a great variety of structures and a fascinating confor- mational flexibility. The properties of monosaccharides in aqueous solutions are, therefore, of considerable significance not only in biochemistry. The interactions of these outstanding molecules with the unique hydrogen bond network of water are also a topic of current wide interest in liquid-state physics. Aiming at an experimental characterization of the molecular dynamics of monosaccharides in water we performed a sys- tematic comparative study of their acoustical relaxation spectra. For this purpose, ultrasonic absorption measurements have been performed in the frequency range from 100 kHz to 2 GHz, corresponding with a relaxation time domain from about 1 µs to 100 ps. Previous ultrasonic studies of aqueous solutions of D-glucose and of some other carbohydrates 1-4 have indicated that interesting processes such as ring conformational intercon- versions or exocyclic group rotations can be investigated using ultrasonic spectrometry at those frequencies. These processes are hardly accessible by other methods. 2 Experimental Section A. Carbohydrate Solutions. A survey of the monosaccha- rides considered in this paper is given in Figure 1. To accentuate the chemical differences between the compounds, only the dominating conformation of each carbohydrate in aqueous solution is shown. For simplicity and clearness chemical equilibria between the displayed structure and other molecular forms and confirmations are first neglected here. D(+)Glucose is included for reasons of comparison though, the spectra for solutions of this saccharide are taken from a previous paper. 4 D(+)glucose and methyl -D-glucopyranoside had been pur- chased from Fluka (Neu-Ulm, Germany), the other compounds from Sigma (Steinheim, Germany). With the exception of methyl--D-arabinopyranoside (>97%), D(+)mannose (> 98%), and D(+)galactose (>98%), the purity of the chemicals was higher than 99%. After being dried for at least 12 h at 60 °C under reduced pressure, the monosaccharides have been used as delivered by the manufacturer. Solutions have been prepared in volumetric flasks by weighing the carbohydrate and adding doubly distilled and deionized water up to the fiduciary mark. The water had been also sterilized by UV irradiation. To allow the anomer equilibrium to be established the first measurements have been started not earlier than 15 h after sample preparation, respectively. Between measurements the solutions were stored at 4 °C. Nevertheless, no solution was used longer than 10 days after the time of preparation. A survey of the solutions is given in Table 1 where some parameters of the liquids are also presented. The density F of the samples has been measured using * Corresponding author. † Polytechnic University, Brooklyn. ‡ Max-Planck-Institut. § University of Utah. ⊥ Polytechnic University, Farmingdale. | Drittes Physikalisches Institut. 4782 J. Phys. Chem. B 2000, 104, 4782-4790 10.1021/jp9940194 CCC: $19.00 © 2000 American Chemical Society Published on Web 04/21/2000