Thermodynamics of the hydrolysis and cyclization reactions of α-, β-, and γ-cyclodextrin

Abstract

A thermodynamic investigation of the hydrolysis and cyclization reactions of cyclomaltohexa-, hepta-, and octa-ose (α-, β-, and γ-cyclodextrins) has been performed using microcalorimetry and high-performance liquid-chromatography. The calorimetric measurements lead to standard molar enthalpy changes Δ r H m° ( T = 298.15 K, KH 2PO 4 buffer ( m = 0.10 mol kg −1), pH = 4.58 to 5.15) for the following reactions: ▪ Equilibrium constants were determined for the following generalized cyclization reactions ( T = 329.6 K, 0.005 mol kg −1 K 2HPO 4 buffer adjusted to pH = 5.55 with H 3PO 4) catalyzed by cyclomaltodextrin glucanotransferase: ▪ Here, G l is d-glucose and the G n 's ( n is a positive integer) are linear maltodextrins; u, v, and w are, respectively, integers ≥7, ≥8, and ≥9. Values of the equilibrium constants, standard molar Gibbs energy change Δ r G m°, standard molar enthalpy change Δ r H m°, standard molar entropy change Δ r S m°, and standard molar heat-capacity change Δ r C p,m° are tabulated for the above reactions at T = 298.15 K. The values of Δ r G m° and Δ r S m° for the first three above-mentioned reactions rely upon an estimated value of Δ r S m° for the hydrolysis reaction of maltose to d-glucose. The thermodynamics of the disproportionation reaction ▪ is also discussed. Values of the quantities Δ r H m°/ N, Δ r G m°/ N, Δ r S m°/ N, and Δ r C p,m°/ N for the three above-mentioned hydrolysis reactions where N is the number of (1 → 4)-α- d-glucosidic bonds broken in each of these reactions, have been calculated and compared with thermodynamic quantities for the similar hydrolysis reaction of a linear oligosaccharide. A thermodynamic investigation of the hydrolysis and cyclization reactions of α-, β-, and γ-cyclodextrin has been performed using microcalorimetry and high-performance liquid-chromatography.