Microcalorimetric method to determine competitive binding. Action of a psychotropic drug (dipotassium chlorazepate) on L-tryptophan . human serum albumin complex.

A mathematical treatment and an original microcalorimetric method are developed to verify an eventual competitive binding between any two substances for the same macromolecule. To apply this method, a competitive binding of L-tryptophan and one benzodiazepin (dipotassium chlorazepate) for human serum albumin is perfectly demonstrated. The association constants and the enthalpy variations are equal to 14 000 +/- 2000 M-1 and --6.6 +/- 0.2 kcal/mol for human serum albumin . tryptophan complex and 13 000 +/- 1000 M-1 and --10.0 +/- 0.2 kcal/mol for human serum albumin . chlorazepate complex. In all cases the stoichiometry is equal to one. The binding of tryptophan to human serum albumin is partially stereospecific; the association constant and the enthalpy variation for D-tryptophan complex are equal, respectively, to 1000 +/- 200 M-1 and --2.6 +/- 0.3 kcal/mol.