Kinetics of inclusion reactions of β-cyclodextrin with several dihydroxycholate ions studied by NMR spectroscopy

The H-1 NMR spectra of some aqueous dihydroxycholate-beta-cyclodextrin systems show separate 18-CH3 signals for complexed and free dihydroxycholate ions. The observed variations in line shapes with concentration and temperature were investigated, and kinetic data were derived for both the formation and decomplexation processes. Analysis of the results indicates that, within the concentration range studied, the dominant exchange mechanism involves a unimolecular decomplexation step (dihydroxycholate, beta-cyclodextrin) --> dihydroxycholate + beta-cyclodextrin. For the chenodeoxycholate system the enthalpies of activation are 43 +/- 5 and 45 +/- 2 kJ/mol, the entropies of activation are -5 +/- 10 and -66 +/- 7 J/(K mol) for the formation and decomplexation reactions, respectively, and its decomplexation rate constant is 34 +/- 1 s(-1) at 300 K. Other dihydroxycholate-cyclodextrin systems show similar activation parameters but slightly lower reaction rates at 300 K. Dehydration plays a major role in the formation process, while the decomplexation rates appear to be controlled by the conformation of the dihydroxycholates.