THE PROTEINS 365 



THE GENERAL REACTIONS OF PROTEINS. 



Though proteins are classified into so many groups, they give a 

 series of reactions which are very characteristic and which serve for 

 their identification. 



The exact group to which a protein belongs is more difficult to 

 determine, but it can be ascertained by reference to the physical pro- 

 perties of the members of the different groups. 



The general reactions depend upon : 



(1) The constituent units (the colour reactions). 



(2) The basic character of the units, especially the diamino acid 

 units (the precipitation by alkaloidal reagents). 



(3) Their colloidal nature and high molecular weight (coagulation 

 reactions). 



All proteins do not give all the reactions ; if a particular unit is 

 missing the colour reaction for that unit will be absent ; some of the 

 coagulation reactions are negative with some of the groups. Con- 

 sequently it is necessary to perform several of the reactions before the 

 presence of protein is verified. 



The general reactions of a protein are given by a solution of egg- 

 white, which is very conveniently prepared as follows : 



Egg-white is beaten to break up the membranes, filtered through 

 calico and diluted with nineteen times its volume of water. A pre- 

 cipitate of ovomucin (formerly regarded as globulin) separates out, 

 but it passes into solution on adding a little salt solution (NaCl, 

 Am 2 SO 4 ). 



Undiluted egg-white has a faintly alkaline reaction and contains 

 about 10 or 12 percent, of protein. The solution may show very 

 faint alkalinity to litmus and contains 0-5 to I per cent of protein. 



A. COLOUR REACTIONS. 



(i) Biuret. 



To some of the protein solution is added caustic soda and then, 

 drop by drop, dilute copper sulphate solution (j per cent), mixing 

 after each addition : a violet colour appears. Excess of copper sulphate 

 must be avoided as its blue colour masks the reaction. 



This reaction is due to the presence of at least two CO NH 

 groups. (See under urea.) 



