312 PROTEINS 



A. PHYSICAL PROPERTIES. 



I. Indiffusibility. 2. Coagulation. 3. Optical activity. 

 4. Precipitation without change. 



B. CHEMICAL PROPERTIES. [Shared by all proteins.] 

 I. Precipitation reactions. 2. Colour reactions. 



A. Physical Properties. 



I. Indiffusibility. 



Unaltered or native proteins * belong to that class of bodies 

 known as colloids (see p. 283) which 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 separation is, however, not quantitative, 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. 



Although all proteins are more or less colloidal in nature, 

 they possess this property of diffusion in a varying degree; 

 thus, for example, the albumoses and peptones, which are 

 derived from the more complex proteins by the action of 

 certain ferments, diffuse with comparative ease. In view, 

 however, of the fact that these substances have never as yet 

 been obtained in crystalline form, and that such typical colloids 

 as oxyhaemoglobin and serumalbumin have been crystallized, 

 the rigid distinction between colloid and crystalloid can no 

 longer be upheld. 



2. Coagulation. 



All genuine or native proteins on keeping undergo a 

 curious change known as coagulation ; the nature of this 

 change is at present not understood, but as a result of it, such 

 proteins lose their distinctive properties of solubility, and can 

 no longer be dissolved without first decomposing them into 

 simpler substances, as, for example, the albumoses or peptones. 



Coagulation may be brought about by (a) Heat, (b) Fer- 

 ments, (<:) Alcohol. 



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

 coagulated by heating ; the temperature at which the change 



*The term native protein is applied to proteins which have been isolated 

 from the tissues by some simple process which does not involve any alteration in 

 {heir original properties, , 



