1. The time-courses of thermal denaturation of human oxyhaemoglobins A, A2, C and S at 45°C were studied by following the increase in protein fluorescence. Haemoglobins S and C were less stable than haemoglobin A, whereas haemoglobin A2 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 α chain preceded that of the β chain. 3. The denaturation of these haemoglobins was studied as a function of temperature by using optical rotatory dispersion. Haemoglobin A2 was again more stable than the others. The addition of small quantities of haemoglobin A2 had a disproportionate effect on the stability of haemoglobin C. 4. The thermodynamic parameters of the denaturation process were calculated.

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