Tewari Y B, Goldberg R N, Sato M
Biotechnology Division, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA.
Carbohydr Res. 1997 Jun 11;301(1-2):11-22. doi: 10.1016/s0008-6215(97)00073-6.
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.
利用微量热法和高效液相色谱法，对环麦芽六糖、七糖和八糖（α-、β-和γ-环糊精）的水解和环化反应进行了热力学研究。量热测量得出了以下反应在标准摩尔焓变ΔrHm0（T = 298.15 K，KH2PO4缓冲液（m = 0.10 mol kg-1），pH = 4.58至5.15）：α-环糊精（aq）+ 6H2O（l）= 6 D-葡萄糖（aq），β-环糊精（aq）+ 7H2O（l）= 7 D-葡萄糖（aq），γ-环糊精（aq）+ 8H2O（l）= 8 D-葡萄糖（aq）。测定了环麦芽糊精葡糖基转移酶催化的以下广义环化反应（T = 329.6 K，用H3PO4调节至pH = 5.55的0.005 mol kg-1 K2HPO4缓冲液）的平衡常数：Gu（aq）=α-环糊精（aq）+ G（u - 6）（aq），Gv（aq）=β-环糊精（aq）+ G（v - 7）（aq），Gw（aq）=γ-环糊精（aq）+ G（w - 8）（aq）。这里，G1是D-葡萄糖，Gn（n为正整数）是线性麦芽糊精；u、v和w分别为≥7、≥8和≥9的整数。列出了上述反应在T = 298.15 K时的平衡常数、标准摩尔吉布斯能变ΔrGm0、标准摩尔焓变ΔrHm0、标准摩尔熵变ΔrSm0和标准摩尔热容变ΔrCp,m0的值。上述前三个反应的ΔrGm0和ΔrSm0值依赖于麦芽糖水解为D-葡萄糖的反应的ΔrSm0估计值。还讨论了歧化反应Gm（aq）+ Gn（aq）= Gm - 1（aq）+ Gn + 1（aq）的热力学。计算了上述三个水解反应中ΔrHm0/N、ΔrGm0/N、ΔrSm0/N和ΔrCp,m0/N的值，其中N是每个反应中断裂的（1→4）-α-D-糖苷键的数量，并与线性寡糖类似水解反应的热力学量进行了比较。
