Stoichiometric and microenvironmental effects on hydrolysis of propantheline and oxyphenonium bromides in cyclodextrin solutions

Abstract

The effects of α‐, β‐, and γ‐cyclodextrins (CDs) on the basic hydrolysis of propantheline bromide (PB) and oxyphenonium bromide (OB) are analyzed in terms of the stoichiometry and microenvironments of their complexes. The rate constant of each species is evaluated with binding constant data for the 1:1, 1:2, and 2:1 complexes. The dielectric constant of the binding site of PB is estimated from the ultraviolet maximum wavelength in reference with the ethanol–water and dioxane–water systems. The energy‐optimized structures of some complexes of PB with β‐ and γ‐CD are obtained by molecular mechanics. Because the ester linkage of PB in the 1:1 complex with α‐CD and in the 2:1 complex with γ‐CD is located near hydroxyls of the CD rim, these complexes catalyze the hydrolysis of PB. In contrast, the hydrolysis is inhibited by the formation of the 1:1 and 1:2 complexes of β‐CD and the 1:1 complex of γ‐CD because the ester linkage of PB is rather deeply incorporated into the CD cavities for these complexes. All the CDs inhibit the hydrolysis of OB. The rate constant of the 1:1 complex of OB and CD is in the decreasing order α‐CD > γ‐CD > β‐CD. This order is consistent with that of the local dielectric constants of the binding sites. © 2001 Wiley‐Liss, Inc. and the American Pharmaceutical Association J Pharm Sci 90:740–748, 2001