Kinetic study for molecular recognition of amino acid by cyclodextrin in aqueous solution

A system of P-cyclodextrin and L-isoleucine in aqueous solution was studied by the ultrasonic relaxational method to obtain kinetic parameters for the complexation reaction at 25 degreesC. Ultrasonic absorption coefficients w ere measured as a function of frequency in the range 0.8-95 MHz and solute concentration at three different pH's. The frequency dependence of the abso rption observed as a function of frequency was analyzed by a Debye-type equ ation with a single relaxation time at neutral pH. The cause of the relaxat ion was attributed to a perturbation of a chemical equilibrium due to forma tion of inclusion complex consisting of P-cyclodextrin and L-isoleucine. Th e forward and backward rate constants, the equilibrium constant, and the st andard volume change of the reaction were determined from the ultrasonic pa rameters. Comparing these obtained results with those at low pH, it was fou nd that the complexation reaction at low pH is the same as that at neutral pH range. However, at the high pH, another reaction is generated, in additi on to the complexation reaction presumably due to a proton transfer reactio n. Further, the results for the complexation reaction were compared with th ose reported previously for several nonelectrolytes, and the structure of a mino acid was considered to influence the complexation reaction process con siderably, especially the process for the departure of guest from the host cavity.