Thermodynamics of the hydrolysis reactions of 1,4-β-d-xylobiose, 1,4-β-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:equation(1)1,4-β-d-xylobiose(aq)+H2O(l)=2d-xylose(aq),1,4-β-d-xylobiose(aq)+H2O(l)=2d-xylose(aq),equation(2)1,4-β-d-xylotriose+2H2O(l)=3d-xylose(aq),1,4-β-d-xylotriose+2H2O(l)=3d-xylose(aq),equation(3)d-maltose(aq)+H2O(l)=2α-d-glucose(aq),d-maltose(aq)+H2O(l)=2α-d-glucose(aq),equation(4)d-cellobiose(aq)+H2O(l)=2α-d-glucose(aq).d-cellobiose(aq)+H2O(l)=2α-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 ΔrH∘ = (0.12 ± 0.26) kJ · mol−1 for reaction (1) and ΔrH∘ = −(0.06 ± 0.18) kJ · mol−1 for reaction (2). It is noted that values of ΔrH∘ 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 ΔrH∘/N or ΔrG∘/N or ΔrS∘/N for a specified linkage, is accurate in predicting the values of ΔrH∘, ΔrG∘, and ΔrS∘ for reactions involving saccharides that contain multiples or combinations of such linkages. Also, the values of the standard entropy changes ΔrS∘ 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 ΔfG∘, the standard enthalpy of formation ΔfH∘, the standard molar entropy S∘m, and the standard molar heat capacity Cp,m∘, for the substances of interest to this investigation.