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-␤-D-arabinopyranoside ͑0.5 mol/l͒ 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 s, 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.

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

“…The finding of the relaxation time t a of fructose solutions to be almost independent of saccharide concentration ( Figure 10, ref. [31]) suggests the low-frequency a-term to be due to an unimolecular reaction of the type SÐS*.…”

“…Additionally, DV % DV s follows for all solutions exhibiting an a-relaxation term. The slope of the A a versus c relation in Figure 12 yields DV = (7.1 AE 0.4) mL mol À1 , whereas the relaxation amplitude of the 1 mol L À1 solutions of aldopentoses d-arabinose, dlyxose, and d-ribose (Table 1) corresponds with a distinctly smaller reaction volume (0.9 mL mol À1 DV 1.4 mL mol À1 [31] ).…”

“…These terms have been assigned to the regimes a, b, g, d, e, with relaxation times on the order of 1 ms, 100 ns, 10 ns, 1 ns, and 100 ps, [31] respectively, each regime reflecting a particular molecular mechanism, as will be outlined in details in the Discussion section.…”

Acoustical Spectroscopy of Carbohydrate Aqueous Solutions: Saccharides; Alkyl Glycosides; Cyclodextrins. Part II. Association and Complexation

“…An unambiguous example for the existence of only one relaxation (with relaxation time τ 1 ) is solutions of 1-methyl-β-D-arabinopyranoside with Ca(ClO 4 ) 2 added (Cowman et al, 1999). Like 1,6-anhydro-β-glucopyranoside, the monosaccharide solution without salt does not reveal acoustical excess attenuation (Polacek et al, 2002). Hence the Debye-type relaxation term in the carbohydrate-salt solution clearly reflects the complex formation.…”

Acoustical Spectroscopy of Carbohydrate Aqueous Solutions: Saccharides; Alkyl Glycosides; Cyclodextrins. Part I. Conformer Variations

“…The five relaxation times relevant to the monosaccharide aqueous solutions are of the order of 1 µs (τ α ), 100 ns (τ β ), 10 ns (τ γ ), 1 ns (τ δ ), and 100 ps (τ ε ), respectively (Polacek et al, 2002). The latter relaxation term has been tentatively discussed in terms of association (Stenger et al, 2000;Polacek et al, 2002). Terms "α" to "δ" are being assigned to conformational variations as will be briefly set below.…”