CHAPTER IV 



THE PROPERTIES OF PROTEINS IN ISO-ELECTRIC 



REACTION 



THE alterations in properties which natural proteins display 

 in the iso-electric region can, generally speaking, be explained 

 by the difference in physico-chemical behaviour between 

 neutral and ionised protein particles. The relation between 

 the alterations in natural proteins and their electric charge has 

 been worked out in a series of researches by Pauli and his 

 collaborators, with which we shall now deal in detail. We 

 propose briefly to anticipate here such results as appear neces- 

 sary for the understanding of the discussions which follow. 



Neutral albumin shows lower hydra tion or imbibition, lower 

 viscosity and lower osmotic pressure than ionised albumin. In 

 contrast with electrically charged albumin, it is coagulated both 

 by heat and by the addition of alcohol, and, similarly, only 

 neutral particles form the substratum of coagulation by salts, 

 acids and bases. A small addition of neutral salts stabilises 

 neutral albumin against both spontaneous coagulation and that 

 caused by alcohol or by heating ; whereas it causes ionised 

 albumin to be discharged and dehydrated, and to become 

 sensitive to coagulating agents. 



The methods which employ the properties of neutral albumin 

 for the determination of iso-electric behaviour work most 

 simply for those natural proteins of which the neutral particles 

 are not stable in solution owing to their lyophobe character. 

 Casein and the globulins are the most important representatives 

 of this class, and in consequence they show a maximum of 

 precipitation at the iso-electric point. The location of this 

 maximum in mixtures of regulators of varied hydrion concen- 

 tration is easily carried out. The values given in the following 

 table were obtained in this way. 



