BARBARA W. LOW AND JOHN T. EDSALL 



represent it as a close-packed array of cylindrical rodlike regions 

 of high electron density, in each of which a length of peptide 

 chain is coiled or folded into a regular chain configuration. 



References to individual experimental studies would not in most cases 

 provide the logical basis for the general conclusions outlined here. The 

 reader is referred to the critical accounts of Low (66) and Kendrew (55) men- 

 tioned earlier and to other general reviews by Kendrew (56) and Kendrew 

 and Perutz (57). There he will find both extensive and documented exposition 

 of these and other conclusions which have been treated rather sketchily here. 



A most striking piece of evidence for the existence of regions 

 of regular chain configurations in hemoglobin is, of course, the 

 1.5 A spacing observed by Perutz. Polarized infrared radiation 

 studies of some crystalline proteins (40,43) confirm the hy- 

 pothesis of oriented regions of ordered chain configuration. 



The x-ray scattering distribution from a large number of 

 globular proteins, studied as amorphous powders, can be com- 

 pared with the interference intensity function calculated from 

 the parameters of any well-defined chain configuration. Arndt 

 and Riley (5,90,91) have recently developed this method 

 extensively : they have included /3-carbon atom positions in the 

 calculated functions. Their studies show that the left-handed 

 (ai) helix is the most plausible predominant chain configuration 

 in globular proteins, and that the helical regions are associated 

 together in some sort of relatively compact array. Both these 

 conclusions are qualitative. To quote their conclusions, "It is, 

 of course, possible that alternative configurations exist over 

 limited segments of the chain or as separate chains in small 

 amounts. . . .The closely related ao-helix configuration is a 

 special case; this could occur as an important proportion of the 

 whole molecule, although it is unlikely to be the major constit- 

 uent" (5, p. 437) and elsewhere they comment that "on the 

 basis of the protein data alone, it is impossible to assert that 

 the Tfi, TTo, and 4i3 (0^2) helices are markedly inconsistent with 

 observation." (The 4i3 helix (17) is for other reasons highly 

 improbable.) "Equally, the admixture of a certain proportion 

 of a2 would not be an impossibility, although a 100% a2 structure 



408 



