The ferric form of the three human embryonic hemoglobins and their reactions with azide ions.

The ferric forms of the three human embryonic hemoglobins exhibit pH titration's with pKa values in the range 8.25-8.6, which correlate with the reduced proteins affinity for oxygen. Azide ions bind to each ferric protein in a process consisting of two separate binding steps. The equilibrium constants for this process are in the 10-30 microM range and can be assigned to the individual chain within the proteins as reactivity appears independent of the nature of the partner chain. The kinetics of the azide binding reactions occur on the second time scale at mM ligand concentrations and also indicate the presence of two distinct processes that have been assigned to each of the two chains within the proteins. Non of these processes indicate any evidence of heme-heme interactions within the ferric form of the proteins. These findings are discussed in comparison with previously reported properties of adult ferric hemoglobin.