MACROMOLECULES 95 



the collagen of connective tissue, myosin of muscles, fibrin present in 

 blood clots, keratin and epidermin of vertebrate skin, the gorgonins of 

 coral, the conchiolines of mollusc shells, the sclerotins of the teguments of 

 arthropods, etc. 



Apart from the determination of their amino acids, fibrous proteins have 

 chiefly been studied by X-ray diffraction methods. Astbury, applying the 

 results obtained by this method, classifies fibrous proteins into two groups : 

 the k-m-e-fgroup(keratin-myosin-epidermin-fibrin) and the collagen group. 



X-ray diffraction spectra show that stretched fibres of keratin (/S-keratin) 

 are formed by the repetition of units 3-3 A in length, a figure very near the 

 calculated length (3-6 A) for the distance — NHCHCO — . The amino acid 



R 



Fig. 8 (Springall) — A sheet of polypeptide ribbons in zig-zag form (P form). 



side-chains project on alternate sides of the main chain. The peptide 

 chains in j8-keratin are separated by a distance of 9.7 A. This distance is 

 near that calculated for the longest side chain of an amino acid such as 

 arginine (8-4 A). 



Unstretched keratin (a-keratin) gives a different X-ray pattern. Fibrous 

 proteins of the k-m-e-f group give patterns similar to either that of a- 

 keratin or of /3-keratin, whilst the fibrous proteins of the collagen group 

 give patterns of another type. 



The most generally (but not unanimously) accepted view is that the /3 

 form of the k-m-e-f group corresponds to a zig-zag structure (Figs. 7 and 8), 

 and the a form of the k-m-e-f group and the collagen group represents a 

 helical twisting, the axis of this twist corresponding to the fibre axis. The 

 most generally accepted helical structure is that in which there are 3-69 

 amino acid residues per turn (Fig. 9). In certain fibrous proteins the 



