Partitioning thermodynamics of selected endocrine disruptors between water and synthetic membrane vesicles: Effects of membrane compositions

The thermodynamics of partitioning of selected endocrine disruptors between water and synthetic membrane vesicles were investigated. For most of the chemicals investigated, partitioning is dominated by the enthalpy change for unsaturated lipid membrane vesicles and by the entropy contribution for saturated lipid membrane vesicles. The contribution of the entropy terms in determining the free-energy change becomes more important compared with the enthalpy terms with increased branching of p-substituted phenols. These results suggest that the thermal energy required for and the entropy gain associated with the creation of the cavity in the lipid bilayer is of critical importance in differentiating the process from 1-octanol/water partitioning. In addition, partitioning thermodynamics are significantly influenced by cholesterol content in the lipid membranes. Results of the present study and those in the literature suggest that partitioning processes significantly depend on the physical state of the lipid membranes and log K_ow-based quantitative structure-activity relationships should be carefully applied for predicting bioconcentration by considering lipid compositions.