Dynamic study of interaction between beta-cyclodextrin and aspirin by the ultrasonic relaxation method.

A single ultrasonic relaxational phenomenon was observed in aqueous solutions containing both beta-cyclodextrin (beta-CD) as host and nonionized or ionized acetylsalicylic acid (aspirin) as guest. The observed relaxation was responsible for a dynamic complexation reaction between beta-CD and aspirin molecules, concomitant with a volume change during the reaction. The kinetic and equilibrium constants for the complexation in the acid (nonionized) form of the aspirin system were derived from the guest concentration dependence of the relaxation frequency. The equilibrium constant for the carboxylate (ionized) form of aspirin was determined from the concentration dependence of a maximum absorption per wavelength, and the rate constants were calculated by using the determined equilibrium constant and the observed relaxation frequencies, which remained nearly almost constant over the concentration range studied. The results showed that the effect of charge on the aspirin molecule was reflected only in the dissociation process from the beta-CD cavity, while no remarkable change was seen in the association process whose rate was diffusion controlled. The results could be explained on the basis of the difference of the hydrophobic moieties in the two guests that were included in the host cavity. The results of the standard volume change for the complexation reaction were closely related to the number of expelled water molecules originally located in the beta-CD cavity and the volume of the aspirin molecule incorporated into the beta-CD cavity.

Dynamic interaction between alkylammonium ions and beta-cyclodextrin by means of ultrasonic relaxation.

Ultrasonic absorption measurements in the frequency range from 0.8 to 220 MHz were carried out in aqueous solutions of pentylammonium chloride (PEACL) and hexylammonium chloride (HEACL) with beta-cyclodextrin (beta-CD) at pH approximately 7.2 and 25 degrees C. A single relaxational absorption was attributed to a perturbation of a chemical relaxation associated with the formation of a complex between beta-CD and the alkylammonium chlorides. The rate and equilibrium constants for the complexation reaction were determined from the concentration dependence of the relaxation frequency. Increasing the chain length of the alkylammonium ion led to an increase in the stability of the complex and slowed the exit rate of the ion from the beta-CD cavity. The standard volume change of the reaction was obtained from a maximum absorption per wavelength and was attributed to water molecules being expelled from the cavity with concomitant alkylammonium ion insertion.

Ultrasonic relaxation due to inclusion complex of amino acid by beta-cyclodextrin in aqueous solution.

Ultrasonic absorption coefficients in the frequency range of 0.8-95 MHz were measured in aqueous solutions of L-methionine or L-norleucine (guest) in the presence and absence of beta-cyclodextrin (beta-CD, host) at 25 degrees C. A single relaxational absorption was observed only in the solutions containing the guest and host. The ultrasonic relaxation was attributed to a perturbation of a chemical equilibrium associated with an interaction between beta-CD and the amino acid to form the host-guest complex. The kinetic and thermodynamic parameters in the system of L-norleucine with beta-CD were determined from the concentration dependence of the relaxation frequency and the maximum absorption per wavelength. Because of the concentration independence of the relaxation frequency in L-methionine system with beta-CD, the equilibrium constant and the standard volume change of the complexation reaction were estimated first from the concentration dependence of maximum absorption per wavelength, and subsequently the rate constants were calculated with the help of the estimated equilibrium constant and the observed relaxation frequency. The results obtained in this study were compared with those for systems of beta-CD with other amino acids or alcohols having comparable hydrophobicity.