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

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 ΔrHm° (T = 298.15 K, KH2PO4 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 K2HPO4 buffer adjusted to pH = 5.55 with H3PO4) catalyzed by cyclomaltodextrin glucanotransferase: Here, Gl is d-glucose and the Gnʹs (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 ΔrGm°, standard molar enthalpy change ΔrHm°, standard molar entropy change ΔrSm°, and standard molar heat-capacity change ΔrCp,m° are tabulated for the above reactions at T = 298.15 K. The values of ΔrGm° and ΔrSm° for the first three above-mentioned reactions rely upon an estimated value of ΔrSm° for the hydrolysis reaction of maltose to d-glucose. The thermodynamics of the disproportionation reaction is also discussed. Values of the quantities ΔrHm°/N, ΔrGm°/N, ΔrSm°/N, and ΔrCp,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.