15:5/ X-ray Analyses of Proteins and Nucleic Acids 289 



be fitted by a model involving a fiber made of three a-helices twisted 

 around each other in a helical fashion. By far, the most direct demon- 

 stration of the a-helix is in the globular protein myoglobin, discussed 

 subsequently. 



In addition to their a-helix, Pauling and Corey made pleated sheet 

 models of proteins similar to the /S model of Astbury but did not restrict 

 the peptide bonds to one plane. Both 

 this and the a-helix have retained the 

 ideas of the a structure being com- 

 pressed and the /S stretched out, and 

 also of hydrogen bonds being respon- 

 sible for holding the shape of the 

 protein fibers. They are superior to 



Astbury's earlier models in fitting 



known bond angles and in their agree- , 



ment with the experimental results of 



X-ray diffraction studies. 



Many attempts have been made 



to apply the general methods des- . 



cribed in the previous section to 



crystals of globular proteins. Perhaps 



the most studied crystal has been that 



of the blood protein, hemoglobin. 



However, the structure of the similar 



but simpler protein, myoglobin, was 



worked out to a resolution of about 



6 A before much progress was made 



with hemoglobin. Myoglobin is a 



red pigment similar to hemoglobin — 



but occurring in muscle rather than — 



blood. It is believed to function by 



buffering the oxygen concentration Figure 10. Diffraction pattern of a 



within the muscle. Myoglobin has helix - Notice the clear area in the 



a molecular weight of about 16,000, center of the pattern - 



very low for a typical protein. This 



corresponds to 153 amino acid residues, that is, about 1,200 atoms other 



than hydrogen in each myoglobin molecule. It means that to locate 



all of these atoms in the molecule, one would need to measure the 



intensity and to guess the phases of perhaps 20,000 diffraction spots. 



The results of analyzing and adjusting the phase for about 400 

 diffraction spots for myoglobin crystals, substituted with heavy atoms, 

 showed there were two myoglobin molecules per unit cell of the crystal 

 and located the polypeptide chains and the iron-containing heme group 



