to say the least. Furthermore if they were capable of 

 such bending, then they would be brought into alignment 

 when placed under tension, and not into a zig-zag con- 

 figuration. On the other hand if it results from a swivel- 

 ing of the atoms about their valence-bonds, then the side- 

 chain connections to the surrounding structures would 

 be destroyed during such swiveling movement, and fur- 

 thermore there would be nothing to cause a return to the 

 original configuration upon release of the tension. 



In the double-strand spiral structure these difficulties 

 are entirely eliminated. As the above diagrams clearly 

 show, the spiral structure can be elastically stretched to 

 exactly twice its original length without appreciably 

 changing the positions of the hydroxyl groups by which 

 it is anchored to adjacent spirals. 



The spiral structure also accounts for the identity in- 

 tervals of 3.5, 4.5, and 10 Angstroms along mutually per- 

 pendicular axes, such intervals having been determined 

 experimentally for proteins {e.g., beta-keratin) in the 

 stretched condition. Since the 4.5 and 10 Angstrom spac- 

 ings have already been accounted for elsewhere, it re- 

 mains to consider only the 3.5 Angstrom spacing. Ac- 

 cording to W. T. Astbury, this is supposed to be the 

 length of one amino acid residue when stretched to the 

 hreahing -point. It will be the length of one such residue 

 only as long as the valence-bonds remain at angles of 109 

 degrees, but there is an abundance of evidence that they 

 can flex through moderate angles and are not rigidly 

 fixed at 109 degrees. In order to account for the transi- 

 tion from alpha- to beta-keratin, Astbury assumes that 

 the valence-bonds can flex elastically through angles of 

 180 degrees — although in order to account for the length 

 of one amino acid residue, he assumes that they are in- 

 capable of any flexing whatever ! 



In the spiral structure shown in the above diagrams, 

 the 3.5 Angstrom interval represents the distance be- 

 tween successive diketopiperazine rings while the system 

 is under tension as shown in the diagram at the right. 



