MOLECULAR AND MACROMOLECUL AR STRUCTURE 



205 



the double orientation. The devising of a molecular structure capable of 

 accomplishing this change is a difficult problem. 



Rudall has offered two interpretations which, however, he finds only 

 partially satisfactory. The first considers straight a-helices of diameter 

 10*3 A hexagonally packed. Spacings of 31 A and 17*8 A at right angles 

 are thus achieved (Fig. 87). The a ±? j8 transformation presents no special 

 difficulty since the helices are straight. The diagonal lines crossing the 

 ribbons (as seen electron-microscopically) are accounted for by supposing 

 a superficial layer of fibrils making an angle of 20° lying across the main 

 sheets. Crick's argument suggests a good fit would be achieved between 



31 A 



A B 



Fig. 87. Proposals for the packing of a-helices to give rise to the spacings 



observed in X-ray patterns fron mantis ootheca (Rudall, 1955-56) and 



to the diagonal lines observed electron-microscopically crossing ribbons 



of the material at an angle of 20°. 



(a) hexagonal packing of straight a-helices of diameter 10*3 A; 



(b) two superposed layers of a-helices inclined at about 20°. Repro- 

 duced by permission. 



fibrils placed together in this way (p. 183). The other interpretation 

 is based on a two-stranded rope of a-helices. A layer of such ropes, 

 in which the pitch of the supercoils is chosen to be 180-190 A, has a 

 pattern of well-marked grooves running parallel across the layer making 

 an angle of 20° with its length. The similarity to the pattern seen in the 

 ribbons is close, but the grooves in the model are separated by 30 A while 

 those in the ribbons are separated by about 120 A indicating an axial 

 period of 360 A or double that of the model. A model involving coiled 

 helices also meets some difficulty in explaining how a double-oriented 

 condition can be maintained during extension to the j8-form. 



Feather keratin 



The X-ray patterns yielded by feathers are much more detailed than 

 those given by any mammalian keratin (Plate 2A). The best patterns are 

 given by the calamus and rachis (Fig. 30, p. 70) and Rudall has shown 



