DSC Study of the Thermal Stability of S-Protein and S-Peptide/S-Protein Complexes

The thermal denaturation of S-protein is investigated at pH 7.0 by means of DSC measurements. The process is reversible and can be assimilated to a two-state transition. The low values of denaturation temperature and enthalpy, between 38.5 and 40.0 °C and 165 and 180 kJ mol - 1, respectively, demonstrate that the loss of S-peptide strongly decreases the structural stability. The interaction between S-peptide and S-protein is thermodynamically characterized, at pH 7.0, by studying the thermal stability of S-peptide/S-protein complexes at different molar ratios. A two-dimensional nonlinear regression analysis of the excess heat capacity surface as a function of temperature and S-peptide concentration enables us to determine the thermodynamic parameters of binding equilibrium. The values obtained are K b (38.6 °C) = (1.10 ± 0.15) × 10 6 M - 1, Δ b H (38.6 °C) = (−185 ± 10) kJ mol - 1, and Δ b C p = (−3.5 ± 0.5) kJ K - 1 mol - 1 . These figures result in satisfactory agreement with literature values.