BARBARA W. LOW AND JOHN T. EDSALL 



stretched-out state keratin gives a (3 pattern with a strong 

 meridional reflection at 3.33 A and equatorial reflections at 

 9.7 and 4.65 A. Collagen gives the third distinct pattern with 

 a strong meridional reflection at 2.8 A and equatorial reflection 

 at 11.6 and 4.6 A. Collagen fibers show only limited short- 

 range (-^15%) extensibility. No new difl"raction pattern is 

 obtained from the stretched fiber ; the spacings of the principal 

 reflections simply show a proportional increase (27). The axial 

 spacing of j8-keratin and of silk fibroin are similar. In silk 

 fibroin Meyer and Mark (75) related the x-ray diff'raction dia- 

 gram to stretched-out polypeptide chains lying parallel to the 

 fiber axis. Although their detailed model has been superseded, 

 this general description characterizes the /3-configuration. 

 The a-jS transformation thus corresponds (8) approximately 

 to the twofold extension of chains folded or coiled along the 

 fiber axis direction. 



The early model structures for both the a and /5 con- 

 figurations were designed to satisfy the geometry of the x-ray 

 diffraction patterns, the mechanism of a-/3 transformation, and 

 the physical requirements of density and side-chain packing. 

 Detailed and precise dimensions for the bond lengths and bond 

 angles of the intrachain residue were not then available, and 

 adequate chemical stabilization of the configuration was not 

 considered in detail. During the years succeeding Astbury's 

 classification, many investigators put forward new models for 

 the a structure limited by additional chemical requirements, 

 including the importance of hydrogen bonding between CO and 

 NH of neighboring residues. These studies have been discussed 

 in some detail by Low (66) and by Kendrew (55) in their recent 

 accounts of work on protein structure and polypeptide chain 

 configuration. It is stimulating to read some of the original 

 papers by Astbury and his co-workers (7,8). The prime physical 

 requirements established by these investigators for the a and ^ 

 chain configurations are still valid. 



In attempting to describe an appropriate structure for 

 collagen Astbury and his co-workers suggested that the high 



394 



