Simultaneous determination of delta G, delta H and delta S by an automatic microcalorimetric titration technique. Application to protein ligand binding.

A methodological study has been made with a syringe titration unit attached to an LKB batch microcalorimeter. The precision and accuracy of the instrument assembly have been evaluated by neutralization reactions and by dilution of sucrose solutions. As an example, heat quantities on the order of 10 mJ accompanying the addition of 10 microliter titrant solution could be determined with an accuracy of better than 1%. A stepwise titration procedure was used to characterize the binding of indole-3-propionic acid to alpha-chymotrypsin. The following thermodynamic data were obtained (25 degrees C, acetate buffer, pH 5.80): delta G0=-18.46 +/- 0.17 kJ X mol-1, delta H0=-15.26 +/- 0.20 kJ X mol-1, delta S0 = 10.85 +/- 1.21 JK-1 X mol-1.