and basic groups of the constituent amino acids. In this state of dissoci- 

 ation, the protein molecule exists as a doubly charged ion or zwitterion. 

 The basic group which contributes most to the positive charge of proteins 

 is the substituted ammonium (NH;i+) group and is present in the amino 

 acids, lysine, histidine, and arginine. The acidic carboxyl group of glu- 

 tamic and aspartic acid and the acidic hydroxyl group of tyrosine and 

 serine are primarily responsible for giving proteins their negative charge. 

 The influence of pH on the dissociation of free basic and acidic groups 



pH 



Figure 11-5. Variations in Acid (Fast Green) and Basic (Methylene 

 Blue) Dye Uptake by Fixed Films of Fibrin, with Changes in pH of the 

 Staining Solution. Fixation, 10 per cent formaldehyde for 10 hrs; dye con- 

 centration. 5 X 10-"'M; ionic strength, 0.02. (From Singer, M., 1952. "Fac- 

 tors which Control the Staining of Tissue Sections with Acid and Basic Dyes," 

 Intern. Rev. CytoL, 1, Fig. 1, p. 220.) 



of the amino acids is shown by the addition of either acid or base to a 

 protein solution at its isoelectric point. At pH's acid to the isoelectric 

 point of proteins, the dissociation of free basic groups of the protein 

 is enhanced to give it a positive charge and to permit its combination 

 with acid dyes. At pH's basic to the isoelectric point, the dissociation 

 of free acid groups is favored to make the proteins negative in charge 

 and capable of reacting with basic dyes. 



The net electric charge of nucleic acids is determined primarily by 

 the dissociation of the phosphoric acid group of the sugar moiety and the 

 substituted ammonium group of the pyrimidine and purine base moie- 

 ties in the molecule. The effect of pH on the dissociation of nucleic acids 

 may be shown as follows: 



SURVEY OF CYTOLOGICAL TECHNIQUES / 217 



