MOLECULAR AND MACROMOLECULAR STRUCTURE 185 



molecules to twist around a seventh straight a-helix to form a " seven- 

 strand cable " (Fig. 78). Further, a closely-filled structure of hexagonally- 

 packed seven-strand cables with individual a-helices occupying the 

 interstices, would improve the fit to the density (Fig. 78). This solution 

 gives a good fit for the many strong reflections (Pauling and Corey, 1956) 

 but is very much a crystallographers' solution and lacks support from other 

 directions. A subfilament of three a-helices with irregularities about 27 A 

 apart (" segmented three-stranded cable ") is indicated by the most 

 recent analysis of the keratin diffraction data (Fraser and MacRae, 1961). 



: ... 



,5, .< pj : , ■"% 



•-•-•.. ..—••. &?&'■ / \ #i"i-"A- '" "••••• \ 





r}ks?A ?" y \ M&! \ 



; :V:rr-X> '""' v - : :. : .' ; v;'- c \ - s ••' 



"<$*&*' 



Fig. 78. A suggestion by Pauling and Corey for the macromolecular 



structure of a-keratin in terms of hexagonally-packed seven-stranded 



cables in the interstices of which are packed single helices (C) to correct 



the density deficit. 



The a-helix of diameter (c. 10 A) is a structural element rather on the 

 small side for current electron microscopy. With the concept of super- 

 helices we enter a domain of dimensions which should be accessible to 

 microscopy. The diameter of the seven-strand cable is 20-30 A. Such a 

 dimension has not yet been observed in cross-sections of keratin in hair 

 and wool. The actual cross-section of what seem to be elementary fila- 

 ments of keratin in hair and skin is of the order of 60 A (Figs. 79 and 80) 

 and for myosin a similar value is reported. These figures would seem to 

 demand much more than seven-component spirals, the width of the 

 cylindrical filament being approximately that of six a-helices suggesting a 

 rope of between twenty and thirty component helices as indicated in Fig. 79. 



