PROTEIN STRUCTURE 



would seem to be highly improbable" (5, p. 430). The identi- 

 fication of an interhelical packing peak does not define rigorous 

 interhelical parallelism. The experimental scattering curves 

 simply indicate that there are certainly considerable regions of 

 interchain order in the molecule. 



X-ray crystal structure studies of hemoglobin have led to 

 this same general conclusion. The vector structure for hemo- 

 globin suggests a molecular structure which may be represented 



Fig. 7. Horse methemoglobin. Idealized representation of mol- 

 ecule in a plane projection. The circles correspond to the end-on 

 view of rodlike regions of high electron density in close-packed array. 

 W. L. Bragg, E. R. Howells, and M. F. Perutz, Acta Cryst., 5, 136 (1952). 



in cross section as shown in Figure 7. In this idealized model 

 the circles represent the end-on projection of peptide chains 

 stacked in close-packed array with each chain coiled in some 

 regular configuration. The molecular length parallel to the a 

 axis and normal to this section is approximately 62 A. The 

 molecule is bisected by a twofold symmetry axis normal to a. 

 An individual single coiled chain could not therefore be longer 

 than «/2, that is 31 A. In considering agreement between this 

 model and the experimental data both Bragg, Howells, and 

 Perutz (16) and Crick (30) have shown that the model is over- 

 simplified. 



409 



