A study of analytical applications of the catalytic properties of cyclodextrins

Tuanying Chen, Purdue University

Abstract

This thesis describes the evaluation of the catalytic properties of modified cyclodextrins for analytical applications. The $\beta$-dimethyl cyclodextrin was modified by adding one and two imidazolyl groups at carbon 3 positions. The modifications produced enhancements of catalytic activity for the hydrolysis of p-nitrophenyl acetate at neutral pH by factors of 1000 or more relative to the unmodified cyclodextrins. The catalytic properties of the monosubstituted cyclodextrin were evaluated for the quantification of p-nitrophenyl acetate in the concentration range of 10 to 90 $\mu$mol/L. Results obtained by equilibrium, initial-rate and error-compensating predictive kinetic methods were compared. The equilibrium and predictive-kinetic options yielded virtually identical results with linear changes with concentration throughout the range studied and several-fold larger than the initial-rate option and dependencies on temperature, pH and catalyst concentration 5- to 10-fold smaller than the initial-rate option. A new approach for the quantitative resolution of mixtures of structural isomers has been described. The method is based on the observation that rate constants for the cyclodextrin-catalyzed hydrolysis of selected structural isomers are significantly different. By using curve-fitting methods it is possible to use these differences in rate constants to resolve kinetic responses for mixtures into the responses for the individual components. The new approach is evaluated for the o-, m- and p-isomers of nitrophenyl acetate. At pH 10, with $\beta$-cyclodextrin as catalyst, ratios of rate constants for the three isomers differ by ratios of 1 to 6.7 to 1.6 in the order mentioned above. Results are reported for both two- and three-component mixtures. For two-component mixtures of the o- and p-isomers which have rate constants differing by only 1.6-fold, linear least-squares slope and intercept of determined vs. prepared concentrations for the ortho isomer were 1.00 $\pm$ 0.02 and 2 $\pm$ 2.2 $\mu$mol/L for three runs on each of five samples in the concentration range from 22 to 176 $\mu$mol/L.

Degree

Ph.D.

Advisors

Pardue, Purdue University.

Subject Area

Analytical chemistry|Pharmacology

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