The crystal structure of myoglobin III. Sperm-whale myoglobin

Myoglobin of Physeter catodon (sperm whale) forms monoclinic crystals with space group P2$_{1}$ (type A) in ammonium sulphate, and orthorhombic crystals with space group P2$_{1}$2$_{1}$2$_{1}$ (type B) in phosphate. Almost identical crystal forms are obtained from related species of whale. Two-dimensional Patterson projections have been computed from some principal zones of reflexions in each type of crystal. They are used to derive the chain directions of the polypeptide chains in the molecules relative to the crystal axes, and to suggest plausible methods of packing in the two forms. The data are best fitted by a molecule having dimensions about 25 $\times $ 34 $\times $ 42 angstrom, the latter dimension being parallel to the chain direction. Measurements of electron-spin resonance (Bennett & Ingram 1956) and of optical dichroism have been used to determine the orientation of the planes of the haem group and the angle between them and the chains. It is concluded that the angle is 40 to 50 degrees in the two forms, in good agreement with the value found for type F crystals in an earlier paper, namely, 41 degrees. Thus in all three crystal types A, B and F the data are consistent with the molecule having the same set of dimensions, associated with a common chain direction and a common relation between chain direction and haem group.