128 KERATIN AND KERATINIZATION 



Fibrous insulin. An example of fibre formation from a more nearly 

 spherical type of molecule is provided by fibrous insulin (F-insulin) which 

 has been extensively investigated by Waugh (1954). Insulin molecules are 

 small and display a marked tendency to form small aggregates in solution. 

 When acid solutions are heated, the insulin separates out in the form of 

 long stiff fibrils (Farrant and Mercer, 1952); the suspension displays 

 strong birefringence and may gel. Oriented fibre-type X-ray patterns can 

 be obtained (Bear, 1955). The reaction is reversible in alkaline media and 

 soluble, biologically-active insulin may be regenerated. Since the insulin 

 molecule is cross-linked internally by disulphide bridges, it is unlikely to 

 be grossly distorted when entering the fibril. All these observations suggest 

 a simple aggregation of the insulin units. 



NATIVE 



Bill II ill Hi: ill ill 



li HMlH|ill =//\ ^\ 



If 



/ 



Fig. 55. Diagrammatic illustration of patterns of aggregation of tropo- 



collagen macromolecules in native, FLS and SLS types. Polarization of 



macromolecules indicated by arrow. (After Schmitt, 1958). 



There is an important general geometrical principle, pointed out by 

 Crane (1950) that the structures which result from the successive addition 

 of asymmetrical units are always helices. The linear aggregate formed by 

 the addition of large molecules should conform to this principle, and 

 accordingly we may expect to find that many protofibrils are helices. 

 Pauling (1953) has described F-insulin as a helix of this type and it would 

 seem not unlikely that some of the coiled-coiled models proposed for 

 keratin, which appear as fine filaments in electron micrographs, have such 

 an origin (Chapter 5). 



