Chair-chair conformational flexibility, pseudorotation, and exocyclic group isomerization of monosaccharides in water

Abstract

Acoustical absorption spectra between 10 kHz and 2 GHz are reported for various monosaccharides in water. With the exception of solutions of methyl-beta-D-arabinopyranoside (0.5 mol/l) the spectra reveal absorption with relaxation characteristics in excess to the asymptotic high frequency absorption term. Up to three relaxation terms per spectrum emerge within the measuring frequency range. Regression analysis of the measured spectra in terms of a suitable analytical spectral function yields five relaxation regimes with relaxation times on the order of 1 mus, 100 ns, 10 ns, 1 ns, or 100 ps, respectively. These relaxation regimes are assigned to the chair-chair ring inversion, two modes of pseudorotation, an exocyclic side group isomerization and a molecular association mechanism. Particular emphasis is given to the ring inversion which is additionally verified by time resolved measurements of nonequilibrium tautomer systems, utilizing the coupling of the inversion to the carbohydrate mutarotation. Further evidence is derived from measurements of solutions of D-fructose in mixtures of ethanol and water. (C) 2002 American Institute of Physics.