Interaction of human serum albumin with dendritic polyglycerol sulfate: Rationalizing the thermodynamics of binding.

We study the thermodynamics of the interaction between human serum albumin (HSA) and dendritic polyglycerol sulfate (dPGS) of different sizes (generations) by isothermal titration calorimetry (ITC) and computer simulations. The analysis by ITC revealed the formation of a 1:1 complex for the dPGS-G2 of second generation. The secondary structure of HSA remained unchanged in the presence of dPGS-G2, as shown by circular dichroism. For higher generations, several HSA are bound to one polymer (dPGS-G4: 2; dPGS-G5.5: 4). The Gibbs free energy ΔG b was determined at different temperatures and salt concentrations. The binding constant K b exhibited a logarithmic dependence on the salt concentration thus indicating a marked contribution of counterion-release entropy to ΔG b. The number of released counterions (∼4) was found to be independent of temperature. In addition, the temperature dependence of ΔG b was small, whereas the enthalpy ΔH ITC was found to vary strongly with temperature. The corresponding heat capacity change ΔC p,ITC for different generations was of similar values [8 kJ/(mol K)]. The nonlinear van't Hoff analysis of ΔG b revealed a significant heat capacity change ΔC p,vH of similar magnitude [6 kJ/(mol K)] accompanied by a strong enthalpy-entropy compensation. ΔG b obtained by molecular dynamics simulation with implicit water and explicit ions coincided with experimental results. The agreement indicates that the enthalpy-entropy compensation assigned to hydration effects is practically total and the binding affinity is fully governed by electrostatic interactions.