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-beta-D-arabinopyranoside and of 1,6-anhydro-beta-D -glucopyranoside, both with and without added Ca2+ and Ba2+ ions. These car bohydrates have been chosen because neither monosaccharide shows any relaxa tion 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, c orresponding to a process with a relaxation time of approximately 3-4 ns. S pectra for solutions of Ca(ClO4)(2) + 1,6-anhydro-beta-D-glucopyranoside al so exhibit a second relaxation process at lower frequencies, with a relaxat ion time of approximately 30 ns, These results clearly indicate cation-carb ohydrate complex formation. The observed relaxation terms are interpreted a s a modified Eigen-Winkler-type multistep process, associated with formatio n of monodentate cation-carbohydrate complexes and, where possible, subsequ ent rearrangement to tridentate complexes.