KERATIN AND MOLECULAR BIOLOGY 19 



certain structural regularities arising from the configurations of their 

 main polypeptide chains. For example, a-type patterns, very nearly 

 identical at large angles, are given by a whole class of proteins, including 

 keratin, myosin, fibrin, etc., which, for this reason, is referred to 

 as the kmf etc. . . . family. The specific differences between proteins, 

 which determine their function, concern superficial chemical groups whose 

 presence usually affects little the main chain structures yielding the 

 patterns described above. 



More recently it has been found that some fibres have a characteristic 

 fine structure visible in the electron microscope and this may be used to 

 identify them. Of particular value in this respect is the appearance of 

 some kinds of collagen fibril which typically display a longitudinal spacing 

 of 640 A. A recent demonstration of collagen in the neural tissues of 

 certain insects was based on their electron-microscopic appearance 

 (Gray, 1959 and Hess, 1958) and confirmed the earlier reports by Rudall 

 (1955) based on X-ray evidence. 



It is currently believed now that the fundamental configurations of the 

 main chains of polypeptides are determined in advance by the stereo- 

 chemical characteristics of the component residues (see Chapter 5), which 

 permit a limited number of stable configurations, and that these are 

 spontaneously assumed in solution by free chains if circumstances permit 

 it. Since an almost unlimited variety of side-chain composition is com- 

 patible with the main-chain configurations, these stereochemical demands 

 place little limitation on the functional possibilities of proteins. 



Stabilization 



The fibrous macromolecules forming the protective coatings of organisms 

 are usually subjected to a stabilizing process which may take various 

 forms and be developed to varying degrees. As a result of this process 

 the protective layer is hardened and insolubilized. The simplest means 

 of effecting changes of this sort is by crystallization between the long 

 polymer chains. When the chains are of a simple character, or of such a 

 regular shape that they readily fit together, localized crystallization may 

 develop with the formation of crystallites, which, when stabilized by 

 sufficiently large energy of crystallization, virtually lock the chains together 

 and thus render the network insoluble. The effectiveness of this device 

 is apparent in such materials as silk or cellulose which are very insoluble ; 

 yet when sufficient of the hydroxy groups of the pyranose rings of cellulose 

 are methylated to prevent crystallite formation, readily soluble methyl- 

 celluloses result. 



A further very common method of insolubilizing proteins is the chemical 

 process known as tanning (Gustavson, 1956) in which covalent chemical 

 cross-linkages between the polypeptide chains are introduced by a reaction 



