The structure of haemoglobin - VI. Fourier projections on the 010 plane

In the preceding papers the molecular Fourier transform was measured as a continuous function of c$^{\ast}$ along nine lines of constant h in the plane of k=0. Fourier inversion of this transform gives an electron density map of a single row of molecules in projection on the 010 plane. This map shows molecules of irregular outline and complex internal structure. Because of lack of resolution and the great depth of material projected neither polypeptide chains nor haem groups can as yet be recognized. Along the c$^{\ast}$ direction the molecular dimensions can be measured with accuracy; along the a direction this is not yet possible, owing to overlap of neighbouring molecules in projection. Indications of the length of the molecules in that direction are obtained from the Fourier projection of a lattice slightly shrunk along a. At low resolution the molecular dimensions can be estimated from a Fourier projection derived by inversion of the 'salt-water' transform described in part III, which corresponds to the diffraction by a row of 'ghost' molecules having the same shape as haemoglobin, but a uniform density throughout. The tilted spheroid of 71 $\times $ 54 angstrom, proposed by us in part II, is a good approximation to the outline of the molecule shown on this projection. Its only unexpected feature is a hollow at the central dyad where the thickness of the molecule is apparently reduced to 32 angstrom. It is not yet clear what this hollow represents.