CHEMICAL CONSTITUENTS OF BODY AND FOOD? 33 



ure^ radical like creatin and histldin. The introduction of a second 

 ammo or XH 2 group confers basic properties upon the acid; hence 

 the name hexone bases hexone because all contain six atoms of 

 carbon. 



(3) Aromatic ammo-acids, such as phenylalanin, tyrosin, and 

 tryptophan, the mother substance of indol and skatol. 



(4) Pyrimidin bases, such as thymin (cytosin) and pyrrolidin 

 derivatives. 



(5) The sulphur-containing substance, cystin. 



(6) Ammonia. 



Some of the amino-acids belong to the aliphatic series, others 

 to the aromatic and some to the heterocyclic compounds. Nearly 

 all the amino-acids have been made synthetically. All of the amino- 

 acids from proteid, except glycocoll, contain one or more asym- 

 metrical carbon atoms, and hence are optically active. Thus, the 

 leucin of protein is 1-leucin; the alanine rotates the ray to the 

 right, hence it is d-alanine. The same amino-acids are present in 

 nearly all vegetable and animal proteids, though certain exceptions 

 exist. Thus, gliadin from wheat flour contains no lysin, and gelatin 

 has no tyrosin and tryptophan. 



Glycocoll is not found in egg albumin, casein or oxyhaemo- 

 globin, whilst it makes nearly one-third the weight of silk fibroin. 

 Glutaminic acid is not found in silk fiber, whilst in gliadin there 

 is about 36 per cent., and arginin varies from 2 per cent, in zein 

 to 84 per cent, in salmin. 



The amido-acids have been shown by Fischer to possess the 

 property of combining with one another to make complex molecules 

 containing two, three, or more groups of amido-acids. Thus, two 

 molecules of amido-acid (glycocoll) may be made to unite to form 

 a compound, glycylglycin, which Fischer calls a peptid. When 

 formed from the union of two amido-acids, they are called dipeptids ; 

 from three, tripeptids; from more complicated compounds of this 

 kind, the polypeptids, which have a reaction similar to that of pro- 

 teids. The polypeptids occupy in proteolysis a stage between the 

 peptones and the final simpler amido-acids, and can be found in 

 peptic and tryptic digestion of albumin. 



We may write the formulae for the three typical amino-acids as 

 follows : 



Glycin HKH, CH 2 , COOH (amino-acetic acid). 



Alanin HNH, C 2 H 4 , COOH (amino-propionic acid). 



Leucin HNH, C 5 H 10 , COOH (amino-caproic acid). 



