Microsecond to subnanosecond molecular relaxation dynamics of the interaction of Ca²⁺ with some carbohydrates in aqueous solution

Ultrasonic absorption spectra in the frequency range 1-500 MHz are reported for solutions of methyl-β-D-arabinopyranoside and of 1,6-anhydro-β-D-glucopyranoside, both with and without added Ca²⁺ and Ba²⁺ ions. These carbohydrates have been chosen because neither monosaccharide shows any relaxation process, over the same frequency range, in salt-free aqueous solutions. Spectra for both carbohydrates in aqueous solution with added Ca(ClO₄)₂, CaCl₂, or Ba(ClO₄)₂ can be represented by an asymptotic high-frequency contribution and a Debye relaxation term centered between 50 and 200 MHz, corresponding to a process with a relaxation time of approximately 3-4 ns. Spectra for solutions of Ca(ClO₄)₂ + 1,6-anhydro-β-D-glucopyranoside also exhibit a second relaxation process at lower frequencies, with a relaxation time of approximately 30 ns. These results clearly indicate cation-carbohydrate complex formation. The observed relaxation terms are interpreted as a modified Eigen-Winkler-type multistep process, associated with formation of monodentate cation-carbohydrate complexes and, where possible, subsequent rearrangement to tridentate complexes.