Thermodynamics of the conversion of fumarate to L-(-)-malate.

The thermodynamics of the conversion of aqueous fumarate to L-(-)-malate has been investigated using both heat conduction microcalorimetry and a gas chromatographic method for determining equilibrium constants. The reaction was carried out in aqueous Tris-HCl buffer over the pH range 6.3-8.0, the temperature range 25-47 degrees C, and at ionic strengths varying from 0.0005 to 0.62 mol kg-1. Measured enthalpies and equilibrium ratios have been adjusted to zero ionic strength and corrected for ionization effects to obtain the following standard state values for the conversion of aqueous fumarate 2- to malate 2- at 25 degrees C: K = 4.20 +/- 0.05, delta G degrees = -3557 +/- 30 J mol-1, delta H degrees = -15670 +/- 150 J mol-1, and delta C degrees p = -36 +/- J mol-1 K-1. Equations are given which allow one to calculate the combined effects of pH and temperature on equilibrium constants and enthalpies of this reaction.