The Free Energy, Heat, and Entropy of Forma- Tion of Lmalic Acid by Henry Borsook

In the previous communication (l), it was shown that the free energy of the bivalent fumarate ion at 25” is -144,630 calories. In the present communication the measurement of the equilibrium at 25” between fumaric acid and malic acid in the presence of fumarase is reported, and from the values obtained computations are made of the free energy and heat of formation of the bivalent I-malate ion and of the free energy and entropy of solid I-malic acid. It has been known for many years that a reversible hydrolysis of fumaric acid to I-malic acid occurs in the presence of minced muscle or of extracts made from it. The values obtained for the equilibrium constant have varied with different workers. Recently Lehmann (2) has described a potentiometric measurement of this equilibrium constant. In order to obtain the value of AH for this reaction and thereby the means of computing the equilibrium constants at various temperatures, the authors carried out a number of such potential measurements at 25”. In his employment of this potentiometric method the assumption was made by Lehmann that the only reactions taking place, even when the enzyme preparation contained fumarase, were: fumaric acid + Hz0 -+ I-malic acid; and succinic acid + methylene blue -+ fumaric acid + leucomethylene blue. The observations described below, of the authors, show that this is not generally true, and that the value of the equilibrium constant obtained from potentiometric measurements may not be accepted unreservedly. The measurements of Lehmann were made on either initially equi-