Structures of human oxy- and deoxyhaemoglobin at different levels of humidity: variability in the T state.

High-salt crystals of human oxy- and deoxyhaemoglobin have been studied at different levels of environmental humidity and solvent content. The structure of the oxy form remains relatively unchanged at all levels. The deoxy form, however, undergoes a water-mediated transformation when the relative humidity around the crystals is reduced below 93%. The space group is maintained during the transformation, but the unit-cell volume nearly doubles, with two tetrameric molecules in the asymmetric unit of the low-humidity form compared with one in the native crystals. Interestingly, the haem geometry in the low-humidity form is closer to that in the oxy form than to that in the native deoxy form. The quaternary structure of one of the tetramers moves slightly towards that in the oxy form, while that in the other is more different from the oxy form than that in the high-salt native deoxy form. Thus, it would appear that, as in the case of the liganded form, the deoxy form of haemoglobin can also access an ensemble of related T states.