CH. IX] PKOTEINS AND AMINO- ACIDS 133 



of the globulins, unlike the corresponding animal products, are only 

 imperfectly coagulated on heating and some not at all. The precipitate 

 formed when coagulation is complete will not go into solution again 

 either in water, acid, alkali or salts. Alcohol precipitates the proteins; 

 in the case of animal proteins, the precipitate becomes coagulated and 

 insoluble if allowed to remain in contact with the alcohol but this does 

 not appear to be so with plant proteins. 



In addition, certain neutral salts, the chlorides and sulphates oi 

 sodium, magnesium and ammonium, have the property of precipitating 

 proteins (except peptones) from solution when added in sufficient quan- 

 tity. The protein is quite unchanged in precipitation and can be made 

 to go into solution again. The various proteins are precipitated by 

 different concentrations of these salt solutions (see p. 138). 



The salts of calcium and barium and the heavy metals produce 

 insoluble precipitates with the proteins, and in this case the reaction is 

 irreversible. 



In regard to chemical composition, the proteins contain the elements 

 carbon, hydrogen, nitrogen, oxygen and sulphur. There is every reason 

 to believe tfiat the protein molecule is constituted of amino-acids con- 

 densed, with elimination of water, on the plan which may be depicted 

 as follows: 



Ri Rii Riii 



NH2— CH— COjOH HjNH— CH— COiOH H:NH— CH— COiOH HiNH- 



-COjOH H;NH— CH— COOH 



Conversely, when the proteins are acted upon by hydrolyzing en- 

 zymes, a series of hydrolytic products are formed which have smaller 

 molecules than the original proteins. They may be enumerated as: 



1. Albumoses. 



2. Peptones. 



3. Amino-acids. 



In the same way when proteins are boiled with acids, a number of 

 the amino-acids are obtained as an end-product. 



The above amino-acids may be either aliphatic or aromatic, and they 

 are characterized by having one or more hydrogen atoms, other than 

 those in the carboxyl groups, replaced by the group — NHg. Thus they 

 are acids by virtue of the carboxyl groups, and bases by virtue of the 

 — NH2 groups: towards strong acids they act as bases, and towards 



