Effects of pH and ionic strength on the thermodynamics of human serum albumin-photosensitizer binding

Fluorescence spectroscopy was used to measure the effects of pH and ionic strength on thermodynamic parameters governing the interaction of human serum albumin with zinc phthalocyanine tetrasulfonic acid. Fluorescence emission of zinc phthalocyanine increases at 686 nm with increasing concentrations of the protein. The non-linear correlation between protein concentration and emission of the photosensitizer was fitted using Chipman's analysis to calculate the binding affinities. The standard enthalpy and entropy changes were estimated from van’t Hoff analysis of data that were acquired from temperature ramping studies. Results show that reaction is primarily driven by solution dynamics and that the change in enthalpy for the system becomes increasingly unfavorable with increasing pH and ionic strength. The effect of ionic strength on the entropy change for binding is shown to be significantly greater than the effects of pH. The interplay between entropy and enthalpy changes is demonstrated.

► The pH dependence of entropy and enthalpy changes was determined for zinc phthalocyanine tetrasulfonic acid, ZnPcS4 binding to human serum albumin, HSA.
► The ionic strength dependence of entropy and enthalpy changes was determined for ZnPcS4 acid binding to HSA.
► The primary driving force governing the interaction between ZnPcS4 and HSA over the range of pH and ionic strength was solution dynamics.
► The interplay between entropy and enthalpy changes was demonstrated.