The thermal denaturation of human oxyhaemoglobins A, A2, C and S.

1. The time-courses of thermal denaturation of human oxyhaemoglobins A, A(2), C and S at 45 degrees C were studied by following the increase in protein fluorescence. Haemoglobins S and C were less stable than haemoglobin A, whereas haemoglobin A(2) was considerably more stable. 2. The time-courses of denaturation did not follow first-order kinetics and could be fitted most simply to a co-operative scheme in which the partial denaturation of the alpha chain preceded that of the beta chain. 3. The denaturation of these haemoglobins was studied as a function of temperature by using optical rotatory dispersion. Haemoglobin A(2) was again more stable than the others. The addition of small quantities of haemoglobin A(2) had a disproportionate effect on the stability of haemoglobin C. 4. The thermodynamic parameters of the denaturation process were calculated.