54 BIOLOGICAL CHEMISTRY 



will come into contact with a colloidal particle is for trivalent 



ions and for monovalent ions. Therefore to have equal 

 x 3 



effects the monovalent ions must be present in proportion to 

 the square of the divalent ions and to the cube of trivalent ions 

 which is expressed by the ratio x : Vx ' tfx. 

 CATAPHORESIS 



Owing to the electrical charge colloidal particles, placed in 

 an electrical field, travel to either one pole or the other. This 

 movement is spoken of as cataphoresis. 



Proteins behave in a peculiar manner. In alkaline solution 

 they behave like negatively charged colloids and in acid 

 solution like positively charged colloids. They behave in fact 

 like colloidal amino acids. The amino acids can act either as 

 acids by combining with bases, in which case the amino acid 

 exists as a negatively charged ion or as bases by combining 

 with acids, in which case the amino acid exists as a positively 

 charged ion. 



According to the Law of Mass Action these two conditions 

 are represented by the equations 



Ka= C^xC* and K 6 =^^ where K a = 



^1 X ^\X 



the dissociation constant of the amino acid acting as an acid 

 and K 6 that of the amino acid acting as a base and C H , C OH , 

 C and Ci_^ are the concentrations of hydrogen ion, hydroxyl 

 ion, amino acid ion and unionised amino acid respectively. 



THE ISOELECTRIC POINT 



The least amount of dissociation occurs near the neutral 

 point either slightly to the acid or basic side. At this point 

 there is the least amount of movement in the electrical field, 

 and it is spoken of as the isoelectric point.* The isoelectric 

 point is determined by the ratio K a /K b so that by adjusting 

 the hydrogen ion concentration to the isoelectric point the 

 minimal ionisation of the protein can be obtained. 



The isoelectric point is said to be the point at which proteins 

 are most easily precipitated from their solutions f but Walpole 

 finds that this does not always hold .J 



* L. Michaelis and H. Davidsohn, Biochem. Zeitschr., 1910, vol. 



30, P- 143- 



f P. Rona and L. Michaelis, Biochem. Zeit., 1910, vol. 28, p. 193. 

 j G. S. Walpole, Biochem. Journ., 1914, vol. 8, p. 170. 



