170 KERATIN AND KERATINIZATION 



calculated from, the X-ray patterns (198 A). In thin sections even well- 

 formed materials, such as feather and porcupine quill, fail to show 

 unambiguous evidence of longitudinal spacings although the expected 

 values, 96 A and 198 A respectively, are well within the instrumental range. 

 This further emphasizes the different organization of the collagens and 

 keratins. The system of long equatorial side spacings could very likely 

 arise as scattering from the quasi-regular packing of filaments embedded 

 in a matrix of different scattering power (p. 247). Significantly, Fraser 

 et al. (1957 and 1959) have observed intensity changes in this group 

 of spacings when fibres are treated with osmium tetroxide in a manner 

 known by electron microscopy to lead to strong deposits of osmium 

 compounds in the matrix (p. 248). The 84 A lateral spacing probably 

 corresponding to the interlayer spacing of filament and matrix is strongly 

 enhanced. 



Speculation on the nature of these long meridional spacings has tended 

 to take the form of either of two extreme theories which may be called (a) 

 long chain theories, and (b) corpuscular-aggregate theories. In the first 

 theory the larger fibril is pictured as being built up from many parallel 

 chains (or other thin linear elements) and the periodicities along the 

 macroformation arise from the repetition of structure in the basic chain. 

 This mode of construction seems to apply to collagen. Many of the electron- 

 microscopical images can be accounted for in terms of such a unit (Hodge, 

 1960). Solutions of both types have been proposed at one time or another 

 for the keratins. The elaborate equatorial reflections given by feather 

 suggested an aggregation of corpuscles to Astbury and Marwick (1932). 

 Macarthur (1943) considered the possibility of the long-chain model; 

 and the nodular appearance of the fibrils released from reduced wool by 

 enzymatic digestion suggested a linear aggregation of corpuscles to 

 Farrant et al. (1947). There is other evidence that many other fibres 

 are formed by the aggregation of particles (Astbury, 1949 and 1958; 

 Jeffrey et al, 1956). 



The wide-angle patterns. The wide-angle patterns yielded by the fibrous 

 proteins (see Chapter 1) contain far less information in the crystallo- 

 graphic sense than those now available from crystalline proteins. Never- 

 theless, the first progress towards an understanding of the arrangement of 

 the polypeptide chains in the solid state came from a study of mammalian 

 hair and silk fibroin. This progress was made possible by an ingenious 

 integration of data derived from a variety of sources: chemical com- 

 position, X-ray diffraction, physicochemical and elastic behaviour. We 

 owe the development of these methods principally to Astbury (1933) and 

 Astbury and Woods (1933), and in spite of an enormous increase in the 

 precision of the crystallographic side of the work, the principles of the 

 methods remain essentially as devised then. See also Kendrew (1954). 



