2 PROTEINS 337 



added, is very unequal. In order to preserve rigid regularity in this 

 respect as well, Meyer and Mark (1950) assumed, as mentioned 

 above, that only glycine and alanine residues form crystallized silk 

 iibroin. It does not seem likely that this view can be maintained, for 

 up to the present it has not been possible to define an undoubted 

 elementary cell of the crystal lattice as in cellulose (Kratky and 

 KuRiYAMA, 1951; Sakurada and Hutino, 1935; Brill, 1943). The 

 reason may be a certain irregularity caused by the side chains of other 

 amino acids. Friedrich-Freksa, Kratky and Sekora (1944) treated 

 silk fibroin with iodine and found by X-ray analysis a new period of 

 70 A perpendicular to the fibre axis. As it is likely that the iodine is in- 

 troduced into the tyrosine residue, every 20th amino acid of the poly- 

 peptidic chain should be tyrosine. This would agree with the statement 

 of Bergmann and Niemann (1937) that out of 16 amino acid residues 

 one is tyrosine. It is therefore probable that tyrosine belongs to the 

 crystallizing polypeptide chains. The primary valence chains are held 

 together by hydrogen bonds (see p. 148) forming a chain lattice 

 (Brill, 1941). 



In the glands of the silkworm the fibroin exists probably as globular 

 protein called fibroinogen. Kratky, Schauenstein and Sekora (1950) 

 find that air-dried glands yield an X-ray diagram similar to F-actin 

 (see p. 352). It is called silk I, whereas the usual diagram is silk II. 

 By stretching, silk I can be transformed into silk II of the spun thread. 

 Only the transition silk I -> silk II has been observed, the reverse 

 being apparently impossible. This favours the view that the formation 

 of the silk thread consists in the denaturation of an originally soluble 

 globular protein. If these protein molecules contain a certain pro- 

 portion of all the amino acids found in silk fibroin, it would be likely 

 that the polypeptide chains formed by denaturation comprise not only 

 glycine and alanine, but also the other amino acids. The portions of 

 the chains with unwieldy side branches would then not crystallize 

 (Fig. 54b, p. 70) and might therefore be more easily susceptible to 

 hydrolysis than the smooth glycine and alanine portions of the chain 

 which can crystallize. 



Mercer (195 i) finds that microfibrils of fibroin (100 A thick and 



3 500 A long) are formed spontaneously from a solution of fibroinogen 

 in water. This seems to be a favourable object for studying the trans- 

 formation of a globular protein into fibrils with the electron microscope. 



