Thermodynamics of porphyin binding to human serum albumin using affinity capillary electrophoresis

The interaction thermodynamics of heptacarboxylporphyrin (HCP) and protopor hyrin (PP) with human serum albumin (HSA) was studied by affinity capillary electrophoresis (ACE) over the temperature range of 25 - 50 degrees C, whe re HCP PP bound to HSA via 1:1 molecular association. The binding equilibri um constants (pH 7.4, phosphate buffer) for the binding of HCP with HSA wer e found to decrease wi ih an increase in temperature, whereas the binding c onstants of the PP/HSA system appeared to be independent of temperature cha nges over the range studied. The van't Hoff relationship (25 - 50 degrees C ) was found to be linear for the interaction of either HCP or PP with HSA. However, the interaction thermodynamics for both of these porphyrins with H SA were found to be quite different. in particular the interaction of HCP ( a hydrophilic porphyrin) with HSA appeared to be based on an enthalpy-drive n process, whereas the binding between PP (a hydrophobic porphyrin) and HSA driven by a favorable change in entropy. The ability of using ACE to evalu ate the interaction thermodynamics of serum proteins (e.g, HSA) with ligand s (e.g., porphyrins and related compounds) should aid in the development of new and more effective photosensitizers in the photodynamic therapy of can cer.