2 PROTEINS 355 



disintegration in the blender. There is a similarity here with rayon in 

 which different types of fibrillar strands are visible in the electron 

 microscope (Frey- Wyssling and Muhlethaler, i 949 c) without any 

 evidance of individual microfibrils. It seems that during the spinning 

 process a less regular, more compact body is formed than during 

 o-rowth, when innumerable uniform microfibrils originate from a 

 living matrix. There is therefore a pronounced difference between the 

 submicroscopic texture of grown and spun fibres. 



Molecular structure. Silk fibroin consists of expanded polypeptide 

 chains which crystallize in a chain lattice. This is why silk has a high 

 tensile strength and a large intrinsic double refraction, similar to those 

 of the chain lattice of cellulose. It is noteworthy that this similarity 

 has no chemical background whatsoever, since silk fibroin and cellu- 

 lose belong to quite different classes of chemical compounds. It is only 

 the fundamental morphological principle of parallel macromolecular 

 chains with a high polarizability parallel to the fibre axis which is 

 responsible. This shows how important morphological considerations 

 are for the analysis of the properties of high-polymer substances. 



According to Bergmann and Niemann (1957) silk fibroin consists 

 of 2^ X 3^ = 2592 amino acids (mol.wt. --^-^ 220,000). Half of these 

 are considered to be glycine, 1/4 alanine, 1/16 tyrosine, 1/2 16 arginine, 

 1/648 lysine and 1/2592 histidine. In addition to these constituents 

 Drucker and Smith (1950) have found by paper chromatography 

 small amounts of aspartic acid, glutamic acid, serine, threonine, valine, 

 leucine, phenylalanine and proline. From viscosity measurements they 

 assign a molecular weight of 33,000 to fibroin, which is almost one 

 order smaller than that of Bergmann and Niemann (1937). This 

 discrepancy is no doubt due to the fact that the determination of 

 amino acids in very small quantities is open to considerable error. 



Since three quarters of silk fibroin consists of the smallest amino 

 acids glycine and alanine, a relatively simple chain lattice can be 

 derived from X-ray analysis (Meyer and Mark, 1928), if the other 

 amino acids (tyrosine etc.) are considered to exist as amorphous 

 substances without participating in the crystal lattice. This view is 

 supported by the observation of Drucker and Smith (1950) that 

 tryptic hydrolysis of short duration leaves glycine, alanine and serine, 

 i.e. the simplest amino acids, undissolved, whereas all the other amino 

 acids are found in the hydrolysis liquor. 



