9] CRYSTALLOGRAPHY OF MYOGLOBIN 129 



position on the protein molecule, and the signs of the protein reflexions so 

 derived agree perfectly with those obtained from the mercuri-iodide sub- 

 stitution. Similar two-dimensional projections of other protein crystals have 

 been prepared, as has already been mentioned, but in no case can the pro- 

 jection be interpreted in chemical terms. Nevertheless these projections re- 

 present an important stage in the development of the subject, since they 



1 I I I I I I I I I I I I I I I I I I I I I I I I I 



Fig. 2. Fourier projection of Type A myoglobin crystals along y, including all terms 

 of spacing greater than 6-6 Â. 



are the first definitely correct pictures of protein structures to be obtained. 

 It has, however, become clear that isomorphous replacement, powerful 

 method of analysis though it is, will only be exploited usefully if it can be 

 extended to three dimensions in order to circumvent the difficulty of overlap 

 in projections. We shall consider how this might be done after a brief dis- 

 cussion of the chemical problems involved in attaching heavy-atom deriva- 

 tives to myoglobin. 



THE ATTACHMENT OF HEAVY GROUPS 

 TO MYOGLOBIN 



In haemoglobin the convenient presence of free sulphydryl groups makes 

 possible the use of any of the standard — SH group reagents, such as p- 

 chloro-mercuribenzoate or methyl-mercury, for attaching mercury atoms to 

 the molecule. Myoglobin contains no free sulphydryl groups in any species 

 which we have so far examined, and accordingly less straightforward methods 

 must be adopted. The isomorphous replacement method cannot easily be 

 applied, however, unless the heavy atom is attached specifically to a single 

 site in the molecule, or at the most to a very few sites. In myoglobin the 

 Ips 



