Thermodynamics of the binding of phenothiazines to human plasma, human serum albumin and alpha 1-acid glycoprotein: a calorimetric study.

A flow microcalorimetric study has been carried out to investigate the interactions between phenothiazine derivatives and human plasma, human serum albumin (HSA) and alpha 1-acid glycoprotein (AGP) at pH 7.4 and 37 degrees C. The direct analyses of enthalpic titration curves allowed the determination of the binding enthalpy change (delta H), the apparent binding constant (K), and the number of the binding sites (n), as well as the evaluation of the apparent free energy (delta G), and entropy (delta S) changes. The overall binding of phenothiazines was exothermic with negative delta H, which was compensated for by changes in delta S. The values of delta G were relatively insensitive to variation in the molecular details of the binding reaction. HSA possessed two classes of binding sites for phenothiazines. The first (n1 = 1), with high affinity (K1 = 10(5)-10(6) M-1) was characterized by small negative delta H and positive delta S values due to hydrophobic interaction. The second class of sites had a low affinity (K2 = 10(3)-10(4) M-1) and high capacity (n2 = 3-8) and contributed to the negative delta H and delta S values. The binding and thermodynamic parameters were influenced by the aliphatic side chain moieties on the phenothiazine nucleus. On the other hand, the drugs were bound to AGP at a single common binding site with a binding affinity of the order of 10(4)M-1, characterized by negative delta H and delta S values, which partially reflected the effect of a van der Waals' interaction.(ABSTRACT TRUNCATED AT 250 WORDS)