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Abstract
The thermodynamics of the hydrolysis of lactose to glucose and galactose have been investigated using both high pressure liquid chromatography and heat-conduction microcalorimetry. The reaction was carried out over the temperature range 282-316 K and in 0.1 M sodium acetate buffer at a pH of 5.65 using the enzyme beta-galactosidase to catalyze the reaction. For the process lactose(aq) + H2O(liq) = glucose(aq) + galactose(aq), delta G0 = -8.72 +/- 0.20 kJ.mol-1, K0 = 34 +/- 3, delta H0 = 0.44 +/- 0.11 kJ.mol-1, delta S0 = 30.7 +/- 0.8 J.mol-1.K-1, and delta Cop = 9 +/- 20 J.mol-1.K-1 at 298.15 K. The standard state is the hypothetical ideal solution of unit molality. Thermochemical cycle calculations using enthalpies of combustion and solution, entropies, solubilities, activity coefficients, and apparent molar heat capacities have also been performed. These calculations indicate large discrepancies which are attributable primarily to errors in literature data on the enthalpies of combustion and/or third law entropies of the crystalline forms of the substrates.
Highlights
The thermodynamics of the hydrolysis of lactose to glucose and galactose have been investigated using both high pressure liquid chromatography and heatconduction microcalorimetry
As analternative method of treating the data, the concentrations of galactose and glucose were calculated from the known initial concentrations of the substrateb)
Note that equilibrium constants determined from experiments starting with only lactose in solutioynielded results in agreement with those obtained from experiments iwnhich the initial reaction mixture contained approximately equal amounts of glucose and galactose
Summary
The primary aim of this investigation is to determine thermodynamic parameters (equilibrium constants, Gibbs energy, enthalpy, and heat capacity changes) for the hydrolysis of lactose to glucose and galactose. Because the glucose and galactose have the same retention times, it was necessary to use an average response factor for these substancesand toassume that their concentrations in solution were equal when calculating equilibrium constants for the hydrolysis reaction. As analternative method of treating the data, the concentrations of galactose and glucose were calculated from the known initial concentrations of the substrateb) This method yielded equilibrium constants that were 5% higher than the equilibrium constants determined using response factors found using freshly prepared standard solutions. Note that equilibrium constants determined from experiments starting with only lactose in solutioynielded results in agreement with those obtained from experiments iwnhich the initial reaction mixture contained approximately equal amounts of glucose and galactose.
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