Triple-point behavior of human haemoglobin.

The recent crystallographic observation of Brzozowski et al. demonstrates the existence of half-oxygenated haemoglobin crystals. At first sight this seems to imply the discovery of a unique molecule, defined by exactly two bound oxygens. However, the formation of crystals with a specific degree of ligation is predicted to occur when two distinct crystalline phases coexist, namely at the triple point. It seems likely that the crystals studied by Brzozowski et al. were selected from a mixture of T and R crystalline forms obtained in conditions where the triple point existed. In general, the equilibria between haemoglobin solid and liquid phases are governed by the chemical potential of a control ligand and the ligand-binding properties of each phase. We report here that a unique situation arises when there are two different crystalline phases, each with characteristic oxygen-binding properties. One can then predict the existence of a triple point, defined by a specific oxygen partial pressure, where both solid phases coexist. At this point the degree of oxygen saturation in each solid phase is uniquely specified. These considerations could explain the specifically ligated crystals found by Brzozowski et al.