Met-, oxy- and carboxyhaemoglobin of horse crystallize with the space group
C
2
, and two molecules in the unit cell which must therefore lie on twofold axes and must be identical in orientation. As has been shown by Perutz, the crystals can be obtained in at least six expanded or shrunk stages, with varying water contents. Furthermore, the water can be replaced by salt solution. The projection on the face is centrosymmetric, so that
F(h0l)
is real and a Fourier projection of the structure can be constructed if the signs of (
F
) can be determined. A comparison of the various shrinkage stages has made it possible to reduce the alternative ways of assigning the signs to a relatively small number, though it has not yet been found possible to obtain a unique solution. The present note reports the progress which has so far been made. The haemoglobins from different mammalian sources (horse, ox, sheep, man) appear to be very similar in their structure. Their similarity is shown by their having the same molecular weight and volume, by their optical properties, by their X-ray Patterson diagrams, and by the way they pack into the crystal structures. The cell dimensions and space groups of some dozen different crystalline types have been determined by Perutz and his collaborators. Four of these forms, including that mentioned above, have especially simple features of packing, and from this it is possible to deduce the overall dimension of the molecule. The protein molecule itself has a volume of 83000 Å
3
, a density of 0.43 electron per Å
3
, and approximate dimensions 55 × 55 × 65 Å. It appears to be surrounded by a layer of bound water, into which salt does not penetrate, and which raises its volume to 115000 Å
3
.
Oxidation of human haemoglobin by copper. Mechanism and suggested role of the thiol group of residue β-93