Ultrasonic Relaxation and Fast Chemical Kinetics of Some Carbohydrate Aqueous Solutions

Abstract

Molecular relaxation properties of the monosaccharides (a) D-glucose, (b) methyl -D-glucopyranoside, (c) methyl R-D-mannopyranoside, (d) D-xylose, (e) D-arabinose, (f) methyl -D-xylopyranoside, (g) methyl -Darabinopyranoside, (h) methyl R-L-(6-deoxy)mannopyranoside, and (i) 1,6-anhydro--D-glucopyranoside, all in aqueous solution, have been studied using broad band ultrasonic spectrometry in the frequency range 0.2-2000 MHz.Ultrasonic excess absorption with relaxation characteristics near 80 MHz was found for glucose and the methyl glucosides of D-glucose and D-mannose, but no relaxation process was detected for the other monosaccharides in the same frequency range. From structural aspects it is deduced that the most likely process causing the observed relaxation is the rotation of the exocyclic -CH 2 OH group, placing rotational isomerization on the nanosecond time scale. Relaxation parameters for D-glucose and methyl -D-glucopyranoside solutions were further investigated as a function of concentration and temperature, in order to confirm the assignment of the relaxation process, and to determine some of its thermodynamic and kinetic parameters.

Carbohydrate solution simulations: Producing a force field with experimentally consistent primary alcohol rotational frequencies and populations

“…Rotation takes place on a nanosecond time scale, in agreement with recent experimental estimates. 20,21 The rotameric distribution of ␤-D-glucose for a 10 ns simulation (Fig. 3) is gg:gt:tg Ϸ 69:31:0, indicating that hydroxymethyl equilibrium populations have been achieved.…”

“…Exchange between the different conformations of the hydroxymethyl groups has been estimated to occur on a time scale of between 10 Ϫ10 and 10 Ϫ3 s, 2,6 with recent studies placing rotational isomerization more specifically on the nanosecond time scale. 20,21 As hydroxymethyl transitions are fairly infrequent on the time scale of molecular dynamics simulations, it is difficult to judge from a nanosecond MD simulation whether the primary alcohol rotameric populations are correctly represented by a given force field. Difficulties with achieving a representative sample for ␣(1-6) linked oligosaccharides using a CHARMM parameter set have been recently reported.…”

Conformation, dynamics, solvation and relative stabilities of selected β-hexopyranoses in water: a molecular dynamics study with the gromos 45A4 force field

“…For galactose, one set of studies 93,100,102,104 suggests that the three rotamers are significantly populated, while the two most recent determinations 95,96 support a predominance of the t and g À rotamers (with a higher population for t, g + being nearly absent). Ultrasonic-relaxation spectroscopy 36,87,106 and NMR 89,107,108 measurements suggest that the hydroxymethyl group rotation takes place on the ns timescale.…”

Molecular Dynamics and Conformational Kinetics of Mono‐ and Disaccharides in Aqueous Solution

“…The d-term is clearly missing, for example, in the spectra of solutions of the aldopentoses d-arabinose, d-xylose, d-lyxose, and d-ribose without an exocyclic ÀCH 2 OH group. It is also absent in the ultrasonic spectra of 1,6-anhydro-b-d-glucopyranoside and a-l-(6-deoxy)mannopyranoside, [27] the latter showing that substitution of the hydroxymethyl group in position 6 of dmannose by a methyl group removes the d-relaxation term.…”

“…[8,[10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] Acoustical spectrometry, available in the frequency range from about 10 4 to 10 10 Hz, and dielectric spectrometry between roughly 10 5 and 10 11 Hz have proven valuable methods to experimentally study the conformational changes and molecular motions of aqueous carbohydrate solutions. [27][28][29][30][31][32][33] As an example, both, the sonic excess attenuation-per-wavelength spectrum and the negative imaginary part of the dielectric spectrum of a 1 mol L À1 solution of d-fructose in water are displayed in Figure 1 and show that various relaxation phenomena may be observed within the frequency range of measurements. These relaxation phenomena include chair-chair ring inversions, pseudorotations, as well as exocyclic side group Acoustical attenuation spectra between 10 kHz and 2 GHz, complex dielectric spectra between 300 kHz and 40 GHz, and time-resolved non-equilibrium measurements are reported for aqueous solutions of various mono-and disaccharides with and without 2:1 valent salts.…”