CHEMICAL COMPOSITION OF PROTEIN MOLECULE 49 



The Scleroproteins. 



The scleroproteins, which in their physical properties comprise a 

 heterogeneous collection of proteins, give on hydrolysis, as would be 

 expected, results which support their classification. 



Of the proteins in this group those of silk have been most 

 thoroughly investigated. Silk is a mixture of two proteins silk- 

 fibroin and silk-gelatin ; the latter is extracted from raw silk by boiling 

 out with water under pressure when it loses 1 5-20 per cent in weight ; 

 the insoluble portion which has the structure of the original silk forms 

 the silk-fibroin. 



Both silk-fibroin and silk-gelatin were analysed by Fischer and by 

 Fischer and Skita when the ester method was first introduced. Silk- 

 fibroin is composed of practically only three amino acids, glycine, 

 alanine, and tyrosine, and is probably the simplest protein known. It 

 contains more tyrosine than any other protein except that of the cara- 

 pace of the tortoise and is the best source of tyrosine. Silk-fibroin 

 differs very markedly in composition from silk-gelatin ; this substance 

 contains more serine than any other protein. 



The composition of silk-fibroin and silk-gelatin from different 

 sources is under investigation by Abderhalden and his pupils. The 

 present data show that the composition of the various silk-fibroins 

 is fairly similar, although many differences can be noted. The New 

 Chwang, Schantung, and Chefoo varieties are rather peculiar in 

 leaving a somewhat large residue after hydrolysis, which seems to be 

 connected with the food-stuff of the silk- worms. The New Chwang 

 and Schantung worms are fed on oak leaves. The Canton and Bengal 

 silks are most like the Italian ; Indian Tussore silk contains a 

 considerably smaller amount of glycine. No striking differences are to 

 be noted in the various silk-gelatins. Abderhalden and Dean and 

 Abderhalden and Weichardt have tried to ascertain whether the amino 

 acids composing the silk are elaborated at the moment of spinning or 

 whether they are selected out of the protein material by the spinning 

 gland. Since the composition of the moth and cocoon together is 

 about the same as the silk- worm, it seems most likely that the spinning 

 gland selects the constituents in making the silk and does not syn- 

 thesise them from other products. Whether the silk-worm can syn- 

 thesise tyrosine and other amino acids still remains for decision when 

 the analysis of the food has been accomplished. 



The silk of other arthropods has also been examined. The silk- 

 fibroin from spider's silk, except for its high content in glutamic acid 



FT. i. 4 



