The properties and reactions of haemoglobin F(1) and their bearing on the dissociation equilibrium of haemoglobin.

1. Hb-F(I), with the previously proposed structure alpha(A) (2)gamma(F)gamma(AcF), would be an exception to the general hypothesis that haemoglobins are in mobile equilibrium with their sub-units at all pH values. However, studies presented in this paper suggest that this is not so. 2. Gel-filtration and sedimentation analyses show that Hb-F(I) dissociates at acid pH and gives only the usual types of hybrids in recombination experiments. When self-hybridized, Hb-F(I) is the main haemoglobin species re-formed, although small but increasing amounts of Hb-F(II) appear on prolonged exposure to acid. 3. Exchange experiments with isotopically labelled Hb-F(II) and unlabelled Hb-F(I) show no exchange of sub-units at neutral pH or after brief exposure to acid pH. Under equilibrium conditions at acid pH non-alpha(A)-chains do not exchange, although alpha(A)-chains equilibrate completely between the two species. 4. These results indicate that Hb-F(I) does not contain gamma(F)-chains and its possible structure is discussed on this basis. Since the dissociation properties of Hb-F(I) are not markedly different from those of Hb-A or Hb-F(II) it is concluded that Hb-F(I), like other haemoglobins, is an equilibrium system.