Spin contribution to the thermodynamics of ligand binding to methemoglobins and metmyoglobins. I. Azide ion binding to horse-heart myoglobin.

The binding of azide ion to horse-heart myoglobins has been studied at pH 6.98 and at various temperatures. The results show that the azide complex is not completely loe spin and that the spin population is strongly dependent on temperature. We have shown that with some assumption it is possible to calculate the approximate fraction of high and low spin present at any temperature from the absorbance measurements. Corrections to the spin contribution at pH 6.98 to these values has been calculated.