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 degrees 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 degrees C) = (1.10 +/- 0.15) x 10(6) M(-1). Delta(b)H(38.6 degrees C) = (-185 +/- 10) kJ mol(-1), and Delta(b)C(p) = (-3.5 +/- 0.5) kJ K-1 mol(-1). These figures result in satisfactory agreement with literature values.