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

Article abstract of DOI:10.1016/S0008-6215(97)00073-6

A thermodynamic investigation of the hydrolysis and cyclization reactions of cyclomaltohexa-, hepta-, and octa-ose (α-, β-, and γ-cyclodextrins) has been performed using micro calorimetry and high-performance liquid-chromatography. The calorimetric measurements lead to standard molar enthalpy changes Δ(r)H(m)°(T= 298.15 K, KH₂PO₄ buffer (m = 0.10 mol kg^-1), pH = 4.58 to 5.15) for the following reactions: α-cyclodextrin(aq) + 6H₂0(l) = 6 D-glucose(aq), β-cyclodextrin(aq) + 7 H₂0(l) = 7 D-glucose(aq), γ-cyclodextrin(aq)+ 8H₂0(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: G(u)(aq) = α-cyclodextrin(aq)+ G(u-6)(aq), G(v)(aq) = β-cyclodextrin(aq) + G(v-7)(aq), G(w)(aq) = γ-cyclodextrin(aq) + G(w-8)(aq). Here, G₁ 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)C(p.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 a 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 G(m)(aq) + G(n)(aq) = G(m-1)(aq) + G(n + 1)(aq) 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 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 thermodynamics quantities for the similar 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.