192 DYEING 



The amino-acid side-groups that can give positive charges are 

 lysine, arginine, and histidine, and the terminal amino-group of 

 the chain is also available. 



The majority of the amino-acids (glycine, alanine, leucine, 

 phenylalanine, etc.) cannot provide electric charges unless they 

 happen to lie at the end of a chain. 



Since different proteins contain different proportions of acidic 

 and basic amino-acids, the position of the iso-electric point 

 varies. 



The tissues provide a number of other charged groups beyond 

 the amino-acid residues of proteins. Negative charges can occur on 

 the phosphoric acid groups of nucleic acids and phospholipids, and 

 on the uronic and sulphuric groups of mucopolysaccharides. 



The various tissue-constituents that are visible under the micro- 

 scope vary in their content of these positively and negatively 

 charged groups. Some are preponderatingly positive or basic, 

 others negative or acidic, others again amphoteric or easily swayed 

 by changes in pH. Some characteristic examples are these: — 



Acidic . . DNA and chromatin 



RNA and ribonucleoprotein 



matrix of cartilage 



many mucous secretions 



most Hpids other than triglycerides 



Amphoteric . cytoplasm of most cells 



contractile substance of muscle 



Basic . . collagen 



cytoplasm of red blood-corpuscles 



granules of eosinophil leucocytes 



nuclei of the spermatozoa of certain fishes. 



The substances here listed as acidic and basic act as such within 

 the range of pH at which dyeing usually takes place in micro- 

 technique, but if the dye-solution be made sufficiently acid or 

 alkaline it will be found that they are in fact amphoteric. 



Since both dyes and tissue-constituents are electrically charged, 

 it is natural that they react with one another. The acidic constitu- 

 ents have an affinity for basic dye-ions and are therefore called 

 hasiphil (see p. 329), while the basic attract acid dye-ions and are 

 called acidophil. The fact that basic dyes show a 'most striking con- 

 formity with one another' in their reactions with tissue-constitu- 



