The thermal denaturation of S-protein is investigated at pH 7.0 by mea
ns of DSC measurements. The process is reversible and can be assimilat
ed to a two-state transition, The low values of denaturation temperatu
re and enthalpy, between 38.5 and 40.0 degrees C and 165 and 180 kJ mo
l(-1), respectively, demonstrate that the loss of S-peptide strongly d
ecreases the structural stability. The interaction between S-peptide a
nd S-protein is thermodynamically characterized, at pH 7.0, by studyin
g the thermal stability of S-peptide/S-protein complexes at different
molar ratios. A two-dimensional nonlinear regression analysis of the e
xcess 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 f
igures result in satisfactory agreement with literature values.