PROPERTIES 433 



Chick and Martin,* however, the conversion of albumin into 

 globulin may be explained merely by assuming a difference 

 in the state of aggregation. 



THE PHYSICAL AND CHEMICAL PROPERTIES OF PROTEINS. 



A. Physical Properties. 



1. Indiffusibility. 



Proteins are colloids and therefore are unable to diffuse 

 through a parchment or animal membrane ; it is thus fre- 

 quently possible to purify a protein from salts by dialysis. 

 The purification is, however, not complete, and it has, hitherto, 

 not been found possible to remove from any protein the 

 last traces of adhering inorganic salts, so that a perfectly pure 

 protein, which on ignition yields no ash, has not as yet been 

 obtained by this means. 



2. Optical activity. 



The solutions of all proteins are laevo-rotatory, the degree 

 varying from —33-5° in the case of egg albumin to —80° in 

 the case of casein. 



3. Irreversible precipitation. 



Soluble proteins by the action of various agents may 

 undergo a physical change whereby their solubility properties 

 are altered without any demonstrable chemical change. This 

 is known as denaturing. 



The change may be effected by [a] heat and by [b] alcohol. 



(a) The solutions of all animal albumins or globulins may 

 be coagulated by heating ; the temperature at which the 

 change takes place is characteristic for each substance, and 

 varies from 56° C. in the case of fibrinogen to 70-80° C. for 

 serum albumin. f The reaction of the solution as well as the 

 presence of dissolved salts are factors which exercise a powerful 

 influence, a slightly acid solution being most favourable for the 

 phenomenon, whereas an alkaline reaction may prevent co- 

 agulation entirely. 



The plant globulins, on the other hand, are less readily 



* Chick and Martin : " Journ. Physiol.," 1912, 45, 261. 

 f The coagulation temperature is not sufficiently well defined to be 

 employed as a means of identification. 



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