292 X-ray Analyses of Proteins and Nucleic Acids / 1 5 : 6 



expected for helices. As noted above, the straight chain portions are 

 right-handed a-helices. In order to obtain Figures 11a and b, it was 

 necessary to use four different substitutions of heavy atoms to check the 

 results and obtain suitable starting points for phase guessing. The 

 model in Figures 11a and b shows one continuous polypeptide chain as 

 demanded by chemical evidence. 



Similar studies of hemoglobin (molecular weight about 65,000) have 

 shown that it consists of four subunits, each with a heme group. These 

 studies at a resolution of 5.5 A showed that each subunit of hemoglobin 

 is a continuous polypeptide chain folded around itself in a form very 

 similar to that of the myoglobin molecule. There are two identical 

 pairs of subunits in each molecule. These are shown in black and white 

 in the model illustrated in Figure lie, which summarizes the X-ray 

 diffraction studies of Perutz and his co-workers. 



The myoglobin and hemoglobin studies involved three new ideas not 

 used in the 1920's in X-ray determination of inorganic crystalline 

 structure. The first was the technique of the substitution of heavy 

 atoms into the unit cell (or isomorphous replacement, as it is called). 

 The second involved the, use of high-speed computers to adjust the 

 phases until the electron density postulated and the diffraction spots 

 observed were consistent with one another. The third novel technique 

 was the use of electron spin resonance to locate the iron atoms. Every- 

 thing indicates that with the development of higher-speed computers, 

 with better programming for electronic computers, and with the prep- 

 aration of increasing numbers of crystalline proteins, more and more 

 protein structures will be studied by these methods. 



6. Nucleic Acid Structure 



Although it was known that DNA was included in the chromosomes, it 

 was formerly believed that the nucleic acid was only significant for 

 structural reasons. The current views completely interchange the 

 protein and DNA role in the chromosome, regarding the protein as a 

 protective agent for the DNA. A major factor in increasing the signifi- 

 cance assigned to DNA was the determination of a satisfactory steric 

 model by Crick and Watson. Their interpretation made use of the 

 theory referred to in the last section for X-ray diffraction by helical 

 structures. 



Crick and Watson showed from the X-ray diffraction patterns that 

 DNA consists of two antiparallel helices. The spiral is very large, 

 having a diameter of 18 A and a spacing between turns of 34 A. Thus, 

 it is a wide chain with lots of room for other molecules to fit in, if they 



