Energetics of albumin-disuccinylmaslinic acid binding determined by fluorescence spectroscopy

• The interaction between SMA and BSA was examined with spectroscopic techniques.
• The study was performed at different pHs and NaCl concentrations.
• Electrostatic forces dominate the binding process at pH higher than the i.e.p of BSA.
• Below the i.e.p., hydrogen bonds and van der Waals forces control the process.
• Enthalpy–entropy compensation occurs in the association processes.

Interaction of disuccinylmaslinic acid (SMA) with bovine serum albumin (BSA) has been investigated by fluorescence spectroscopic methods under different experimental conditions. From the temperature dependence of the binding process an extensive analysis of thermodynamic parameters has been made in connection with the drug structure. SMA binds to BSA mainly through electrostatic interactions at physiological pH (7.4) and low ionic strength. An increased electrolyte concentration provoked hydrogen bonds and van der Waals forces to control the complex formation. When pH was higher than the isoelectric point of albumin (i.e.p. 4.9) the attachment of the drug was favored by both negative enthalpy and positive entropy changes. These results suggest a dominance of electrostatic forces in the association process. Conversely, at pH values lower than i.e.p. the unfavorable negative entropy changes prompt the involvement of hydrogen bonds in the binding. A noteworthy enthalpy–entropy compensation phenomenon has been detected. The binding processes controlled mainly by hydrogen bonds and van der Waals interactions (pH 2.8, 4.2, and 7.4 at high NaCl concentration) fall in the same compensation line. The observation of this entropy-enthalpy compensation suggests that water reorganization plays an important role in the binding of SMA to BSA. Heat-capacity change (ΔCp) has been deduced from temperature dependence of enthalpy. Minimal values of ΔCp were found when electrostatic forces controlled the SMA–BSA association. The formation of a drug–albumin complex has been corroborated by electrophoretic mobility measurements.