Inclusion complexation of barbiturates with .BETA.-cyclodextrin in aqueous solution. I. Spectroscopic study on the mode of interaction.

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Abstract
Inclusion complexation of various barbituric and thiobarbituric acid derivatives with .beta.-cyclodextrin in aqueous solution was studied by UV, circular dichroism (CD), and NMR. The induced CD observed for barbituric acids were substantially different from those for thiobarbituric acids. Chemical shift changes in 1H- and 13C-NMR spectra suggested that not only the 5-substituent but also the heterocyclic moiety of barbiturate participated in inclusion formation. The stability constant of the inclusion complex was determined by the UV method at various pH and temperatures from which the thermodynamic parameters were determined. Free barbituric acids were more favorable to form the inclusion complex than the ionized form. The relationship between the number of carbon atoms in 5-substituents and the formation constant was sigmoidal but barbituric acids bearing a cyclic substituent at 5-C deviate from the correlation curves. Isoequilibrium relationships between .DELTA.H [enthalpy change] and .DELTA.S [entropy change] values were observed having a compensation temperature of 372.degree. K.