SPECIAL REACTIONS OF THE ALBUMINS. 41 



.As regards the manner in which the various radicles of the albu- 

 minous molecule are linked together, our knowledge, while still 

 imperfect, has been materially advanced within recent years through 

 the researches of E. Fischer and his pupils. As a result there is 

 evidence to show that the a-amino acids exist in combination with 

 each other as so-called peptids, which have the general structure 

 NH 2 .(CH 2 .CO.NH) n .CH 2 .COOH, the amino group of one amino 

 acid having united with the carboxyl group of the other (with coinci- 

 dent loss of water). The simplest product of this order, glycyl-glycin, 

 would be formed according to the equation : 



NH 2 .CH 2 .COOH + HNH.CH 2 .COOH = NH 2 .CH 2 .CO.NH.CH 2 .COOH -f H 2 .O. 

 Glycocoll. Glycocoll. Glycyl-glycin. 



That the albuminous nitrogen is largely present in the intact mole- 

 cule as an imido group is shown by the fact that only a small frac- 

 tion can be readily split off as ammonia, while approximately 90 

 per cent, remains and on hydrolytic decomposition appears in the 

 form of amido-acids. On treating with nitrous acid similarly only 

 a small amount of nitrogen is split off, while a body remains in 

 which the albuminous character is still preserved to a large extent, 

 but which only gives an imperfect biuret reaction the desamido- 

 albumin of Schiff. 



Fischer's theory of the presence in the albuminous molecule of 

 the amino-acids in the form of peptids, constructed on the plan just 

 outlined, is strengthened by his successful synthetic preparation not 

 only of dipeptids (glycyl-alanin, alanyl-leucin, leucyl-glycin, etc.), 

 but also of tripeptids (leucyl-glycyl glycin, leucyl-alanyl alanin), 

 tetrapeptids (dileucyl-glycyl-glycin), and even pentapeptids (penta- 

 glycin, leucyl-tetraglycin, etc.). The higher peptids, it is interesting 

 to note, give the biuret reaction (e. g., the tetrapeptid of glycin, 

 dialanyl cystin, leucyl-pentaglycin). On careful hydrolysis of silk 

 collagen with cold hydrochloric acid and subsequent digestion with 

 pancreatin Fischer obtained a peptone-like body, moreover, which 

 could be shown to be glycyl-a-alanin. 



Still more recently Fischer and Abderhalden have shown that on 

 digestion of various albumins (casein, edestin, serum-globulin, egg- 

 albumin, hemoglobin, and fibrin) with pancreatin a polypeptid-like 

 body remains, which, on hydrolysis with acids, yields a large 

 amount of a-pyrrolidin carbonic acid, phenylalanin, and all the 

 glycocoll which was originally present in the albuminous molecule. 



Through the union of the a-amino-peptids with diamino-acids, 

 the dicarbonic acids, the sulphur-containing cystin, and the glucos- 

 amin group, etc., still more complex radicles result, of which we are 

 in comparative ignorance as yet. But we can conceive that these 

 various complexes may then be further united to form still more 

 complicated radicles, until at last the complete molecule is con- 

 structed. In the typical albumins we can distinguish three such 

 complicated radicles, two of which, in conformity with the nomen- 



