Thermodynamics of the hydrolysis and cyclization reactions of alpha-, beta-, and gamma-cyclodextrin.

ABSTRACT

A thermodynamic investigation of the hydrolysis and cyclization reactions of cyclomaltohexa-, hepta-, and octa-ose (alpha-, beta-, and gamma-cyclodextrins) has been performed using microcalorimetry and high-performance liquid-chromatography. The calorimetric measurements lead to standard molar enthalpy changes delta rHm0 (T = 298.15 K, KH2PO4 buffer (m = 0.10 mol kg-1), pH = 4.58 to 5.15) for the following reactions alpha-cyclodextrin(aq) + 6H2O(l) = 6 D-glucose(aq), beta-cyclodextrin(aq) + 7H2O(l) = 7 D-glucose(aq), gamma-cyclodextrin(aq) + 8H2O(l) = 8 D-glucose(aq). 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 Gu(aq) = alpha-cyclodextrin(aq) + G(u-6)(aq), Gv(aq) = beta-cyclodextrin(aq) + G(v-7)(aq), Gw(aq) = gamma-cyclodextrin(aq) + G(w-8)(aq). Here, G1 is D-glucose and the Gn's (n is a positive integer) are linear maltodextrins; u, v, and w are, respectively, integers > or = 7, > or = 8, and > or = 9. Values of the equilibrium constants, standard molar Gibbs energy change delta rGm0, standard molar enthalpy change delta rHm0, standard molar entropy change delta rSm0, and standard molar heat-capacity change delta rCp,m0 are tabulated for the above reactions at T = 298.15 K. The values of delta rGm0 and delta rSm0 for the first three above-mentioned reactions rely upon an estimated value of delta rSm0 for the hydrolysis reaction of maltose to D-glucose. The thermodynamics of the disproportionation reaction Gm(aq) + Gn(aq) = Gm-1(aq) + Gn+1(aq) is also discussed. Values of the quantities delta rHm0/N, delta rGm0/N, delta rSm0/N, and delta rCp,m0/N for the three above-mentioned hydrolysis reactions where N is the number of (1-->4)-alpha-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.