Thermodynamics of the Hydrolysis Reactions of 1,4-beta-D-xylobiose, 1,4-beta-D-xylotriose, D-cellobiose, and D-Maltose

Microcalorimetry and high-performance liquid chromatography have been used to conduct a thermodynamic investigation of the following reactions:(1)1, 4 - β - d -xylobiose (aq) + H₂ O (l) = 2 d -xylose (aq),(2)1, 4 - β - d -xylotriose + 2 H₂ O (l) = 3 d -xylose (aq),(3)d -maltose (aq) + H₂ O (l) = 2 α - d -glucose (aq),(4)d -cellobiose (aq) + H₂ O (l) = 2 α - d -glucose (aq) .The results of the equilibrium measurements were K = (1.46 ± 0.15) · 10³ for reaction (1) and K = (551 ± 34) for reaction (3). Although it was not possible to measure directly a value for the equilibrium constant for reaction (4), it was possible to obtain the value K = 657 for this reaction via a thermochemical pathway calculation. The results of the calorimetric measurements were standard enthalpies of reaction Δ_rH^{{ring operator}} = (0.12 ± 0.26) kJ · mol^-1 for reaction (1) and Δ_rH^{{ring operator}} = -(0.06 ± 0.18) kJ · mol^-1 for reaction (2). It is noted that values of Δ_rH^{{ring operator}} for reactions (1) and (2) are equal to each other within their respective experimental errors. This fact is consistent with earlier observations that, for reactions involving the making/breaking of N saccharide linkages, the assignment of characteristic values of Δ_rH^{{ring operator}}/N or Δ_rG^{{ring operator}}/N or Δ_rS^{{ring operator}}/N for a specified linkage, is accurate in predicting the values of Δ_rH^{{ring operator}}, Δ_rG^{{ring operator}}, and Δ_rS^{{ring operator}} for reactions involving saccharides that contain multiples or combinations of such linkages. Also, the values of the standard entropy changes Δ_rS^{{ring operator}} for the hydrolysis reactions (3) and (4) fall into the range of values {(32 to 48) J · K^-1 · mol^-1)} previously noted for the hydrolysis of six-carbon disaccharides. In order to tie the results of this study into the thermochemical literature, a reaction catalog of related property values was created. Selected property values from this reaction catalog were then used to calculate "best" values of the standard Gibbs free energy of formation Δ_fG^{{ring operator}}, the standard enthalpy of formation Δ_fH^{{ring operator}}, the standard molar entropy S^{{ring operator}}_m, and the standard molar heat capacity C_p,m^{{ring operator}}, for the substances of interest to this investigation.