9] CRYSTALLOGRAPHY OF MYOGLOBIN 131 



Fig. 4, which shows the points of attachment (in projection) of the principal 

 ligands which were successfully put on. A number of interesting chemical 

 questions arise; for example, AUI4- occupies the same site as Hgl42", sug- 

 gesting that the central atom, and even the charge, may play only a sub- 

 sidiary role in determining the mode of attachment. Again, Ag+ and AuCl4~ 

 are attached to the same site, in spite of their charges being apparently 



Fig. 3. DifFerence-Fourier projection of /7-chloro-mercuri-benzene sulphonate derivative 

 of Type A myoglobin along y, showing positions of mercury atoms. 



opposite. However, it is found that the gold atom becomes attached only 

 if the crystals are left for many months in their mother liquor before X-ray 

 pictures are taken, suggesting that some secondary chemical process is a 

 necessary preliminary to combination. 



There are other examples in which we understand the chemistry rather 

 better. Thus both /j-iodo-nitrosobenzene and H03S.C6H4.Hg.S.C6H4.NC 



o> 



□ PCMBS 



A Au, Ag 



o Hgi; 



• I.CgH4.N0 



m PCMS-S.CgH^.NC 



- I 



■ Hg di-ammine 



Fig. 4. Heavy-atom complexes of myoglobin type A, showing the positions of heavy 

 atoms in the unit cell in y projection. 



(PCMBS — S.C6H4.NC) are specific haem group reagents, and although it 

 was not possible to use them for determining the signs of the protein re- 

 flexions, we were able to use the protein signs established by other methods 

 to work backwards and locate the heavy atoms in these reagents with rela- 

 tion to the crystal axes. In each complex the heavy atom is separated from 

 the iron atom of the haem group by the same number of intermediate atoms, 

 and we do in fact find, as Fig. 4 shows, that they are in almost the same 



