THE STRUCTURE OF GLOBULAR PROTEINS I I 



with the myoglobin molecule, but that they lodge in interstices in the lattice. This 

 means that if the same molecule is crystallized in a new form, with a different packing 

 arrangement, it reacts quite differently with heavy-atom ligands. Thus sperm whale 

 myoglobin crystallizes in a monoclinic form from ammonium sulphate, and in an 

 orthorhombic form from phosphate. In the former, p-chloromercuribenzene 

 sulphonate enters the lattice at one place on the molecule and indeed proved to be 

 one of our most useful heavy atoms. In the latter, the behaviour is entirely different ; 

 the group becomes attached at four different sites on the molecule and is really of 

 little use for analytical purposes. 



