436 



PROTEINS 



The Isoelectric Points of Proteins. 



The proteins having both amino and carboxyl groups are 

 amphoteric electrolytes and can function either as bases or as 

 acids according to circumstances. Thus in acid solution the 

 proteins tend to assert their basic function, occupying the 

 position of kations I., while in alkaline solutions they tend to 

 function as acids :• — • 



Pr 



+ 

 COOH 



\nh,h . 



+ 

 Na 



CI 



OOCv 



H^N- 



>Pr 



II. 



It is therefore evident that the behaviour of proteins is 

 largely influenced by the reaction, or, in other words, the 

 hydrogen ion concentration of the solution.* Assuming that 

 a given protein in neutral solution is functioning as the anion, 

 as indicated by formula II., the addition of acid will gradually 

 tend to reduce this state and to drive over the protein into 

 the position of cation ; as the acidity is gradually increased, a 

 certain hydrogen concentration will be attained at which the 

 number of protein anions and kations are exactly equal. This 

 hydrogen ion concentration is known as the isoelectric point ; 

 it does not coincide with the point of neutrality Pg = 7 and 

 will vary for different proteins. 



Appended are some values for the isoelectric points of 

 various plant materials taken from a paper by Pearsall and 

 Ewing : — f 



Substance. 

 Edestin 

 Legumin 

 Globulin (yeast) 

 Albumin (yeast) 

 Glutenin (wheat) 

 Leucosin (wheat) 

 Tuberin (potato) 

 Globulin (carrot) 



,, (tomato) 



Nitella extract 

 Yeast cells . 



Isoelectric Point. 



5-3-5-6 

 4-4-4-6 



4-6 



4-6 

 4-4-4-5 



4-5 



4-4 

 4-I-4-4 



4-0 

 4-6-4-7 

 3-I-3-3 



* See Appendix I. 



t Pearsall and Ewing : " Biochem. Journ.," 1924, 18, 329. 

 Atkins : " Proc. Roy. Dublin Soc," 1922, 16, 414. 



See also 



