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



within the myoglobin molecule. This procedure was then repeated to 

 include 9,600 diffraction spots which showed the electron density of the 

 myoglobin molecule with a resolution of about 2 A. This is not quite 

 sufficient to indicate the separate atoms. This resolution is sufficient to 

 confirm that the major part of the myoglobin molecule consists of right- 

 handed a-helices. To fit the single polypeptide chain of myoglobin 



COOH 



(b) 



Figure II. Kendrew's model of myoglobin, (a) General 

 shape of the polypeptide chain. The gray area is the heme 

 group. The round dark atom represents a heavy atom attached 

 for isomorphous replacements. The tilt of the heme group 

 is incorrect, (b) Course of the polypeptide chain as deter- 

 mined by a three-dimensional fourier synthesis with 2 A resolu- 

 tion. After J. C. Kendrew, et al., "Structure of Myoglobin," 

 Nature 185: 422 (1960). 



into one globular molecule, it must be bent and twisted at various 

 corners. Where this occurs, the a-helical form is lost usually for 3 or 4 

 amino acid residues. There is also one group of about 13-18 amino 

 acid residues not in the form of an a-helix. 



Figure 11a shows a photograph of Kendrew's model of myoglobin, 

 built to represent the structure which would give the 400 diffraction 

 spots used. In addition, electron spin resonance measurements were 

 used to locate the iron atoms in the heme group (see Chapter 28). 

 However, the latter data were misinterpreted so that the heme group 

 was tilted at the wrong angle. Figure lib, for comparison, shows the 

 form of the polypeptide chain revealed by the 9,600 diffraction spot 

 study. Although not illustrated in the figure, all of the chains are 

 shown by the latter study to be hollow, cylindrical tubes of the form 



