KINETICS AND THERMODYNAMICS OF INCLUSION OF PARA-NITROPHENOLATE WITH ALPHA-CYCLODEXTRIN MEASURED WITH PULSE VOLTAMMETRY

The rate constants and formal equilibrium constant are determined for inclusion of p-nitrophenolate anion into alpha-cyclodextrin at pH = 13.5, where alpha-cyclodextrinm is 82% dissociated to the basic (anionic) form. The conditional dissociation constant is 2.99 x 10(-3) M (at 20-degrees-C), about an order of magnitude larger than the value reported for pH 11, where alpha-cyclodextrin is completely in the acid form. This observation illustrates the importance of Coulombic interactions in substrate-receptor interactions. The values of rate constants, determined from a nonlinear least-squares analysis of the shape of square-wave voltammograms, are k(f) = 3.5 x 10(5) S-1 and k(b) = 1.2 x 10(8) S-1 M-1 at 20-degrees-C. This type of analysis is typically precluded by distortion of wave shape due to adsorption of cyclodextrin on the mercury electrode. This artifact was eliminated by optimization of voltammetric parameters and by competitive adsorption of tetramethylammonium ion. Values of diffusion coefficients of p-nitrophenolate in the complexed and uncomplexed forms are 3.28 x 10(-6) and 1.05 x 10(-5) cm-2 s-1, respectively.