368 FINE-STRUCTURE OF PROTOPLASMIC DERIVATIVES III 



diffraction. The results of such studies are collected in Table XXXII. 

 In silk fibroin Friedrich-Freksa, Kratky and Sekora (1944) found 

 a period of 70 A, which corresponds to 20 amino acid residues. In 

 feather keratin a somewhat longer spacing of 95 A is reported. In 

 porcupine quill there is a long-range spacing of 198 A and, as its 

 keratin is present in the a-form, where 3 amino acid residues cover 

 5.1 A, 116 residues (which is near to 2^ x 3^ = 108) would constitute 

 such a period. In the adductor muscle of the mollusc Venus mercenaria 

 a small-angle spacing of even 725 A has been reported, which seems to 

 be divided into four subspacings of 145 A. In this spacing 426 amino 

 acids can be lodged (which can be associated with 2^ X 3^ = 432). 



Bear (1944), who has measured these long fibre periods, discovered 

 transverse long-range spacings as well; they amount to about 0.4 of 

 the fibre spacings reported (Table XXXII). Consequently, there is not 

 only a repetition of definite sequences of amino acids in the main 

 chains, but at the same time distinct numbers of polypeptide chains 

 are collected into crystallographic units. 



In contrast to this behaviour, which shows that the proteins of the 

 keratin-myosin group exist in a three-dimensional crystalline state, 

 BoLDUAN and Bear (1950) have only found a long-range fibre spacing 

 in the collagen group but no transverse spacings. The inference is, 

 therefore, that there is no true crystal pattern in the collagen proteins, 

 but simply an arrangement of parallel chains, similar to that of liquid 

 smectic crystals with only a unidirectional periodicity. 



The long-range fibre period seems to be the same for all collagen 

 proteins investigated ; it measures 640 A (Marks, Bear and Blake, 

 1949) and agrees with the striation seen in the electron microscope, 

 corresponding to 228 amino acid residues. There seems to be more 

 uniformity in the proteins of the collagen group than in those of the 

 keratin-myosin-epidermis-fibrinogen group. 



General occurrence of striated protein fibrils. Microscopic histology 

 considered the striated muscle fibrils as a special case of protein fibres. 

 The electron microscope has, however, revealed the fact that banding 

 is a general feature of fibrillar proteins, the period of this striation 

 being submicroscopic. It has been found in smooth muscle fibrils 

 (Hall, Jakus and Schmitt, 1945), collagen fibrils (Fig. 174, p. 349), 

 precipitated blood fibrin (Wolpers and Ruska, 1939), ejected tricho- 

 ■cysts oi Paramecium (Jakus, 1945; Wohlfahrt-Bottermann, 1950; 



