Microsecond to Subnanosecond Molecular Relaxation Dynamics of the Interaction of Ca²⁺ with Some Carbohydrates in Aqueous Solution

Abstract

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 Ca 2+ and Ba 2+ 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(ClO 4 ) 2, CaCl 2, or Ba(ClO 4 ) 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(ClO 4 ) 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.