STAINING, PRACTICAL AND THEORETICAL 



with those offered by basic dyes. Proteins also possess carboxyl 

 groups identical with those offered by carboxylated (acid) dyes. 

 In other words, most basic dyes, and carboxylated dyes devoid 

 of sulphonic groups, have nothing to offer relatively simple 

 proteins which are not already possessed by the latter. This may 

 be the reason why most basic dyes, used without mordants, and 

 carboxylated acid dyes (devoid of sulphonic groups) can be 

 readily extracted from most tissue elements, coloured by them, 

 with ordinary solvents. In short, as the proteins already possess 

 acidic and basic groups identical with those offered by basic 

 dyes and carboxylated acid dyes (devoid of sulphonic groups), 

 no impulse, therefore, exists on the part of the protein for an 

 exchange of ions to be made, and any coloration that occurs 

 would be due to physical and not chemical factors. Not all 

 basic dyes, however, can be completely removed from tissues 

 by ordinary solvents. In such cases presumably the basic dye 

 is bound to the strongly acidic groups of tissues by chemical 

 affinity. On the other hand it might be that the molecules of 

 the dye have penetrated deep into the tissues and have been 

 trapped in narrow crevices from which they cannot be dislodged 

 by ordinary solvents. Here it should be mentioned that all basic 

 dyes are of relatively low molecular weight (E. Gurr, i960, 

 pages 423-432). They are presumably, but not necessarily, of 

 small molecular dimensions or volume which would enable 

 their molecules to penetrate deeper into " close-knit " tissue 

 elements than might the majority of acid dyes, most of which 

 are of higher molecular weight than any of the basic dyes. If 

 the basic dye is in chemical union with acidic groups of tissues, 

 then it could not have united with carboxyl groups, if the hypo- 

 thesis put forward above is valid. 



As previously stated, proteins are found in intimate association 

 with other substances in tissues to form protein complexes. 

 When ordinary solvents will not remove an unmordanted basic 

 dye, for example, from tissue elements that have been stained by 

 it, then the dye may have entered into chemical union with 

 tissue-elements. Such tissue elements might contain chemical 

 groups that are more strongly acid than the carboxyl radical. 

 Examples of such tissue groups might be the phosphate and the 

 sulphate radicals. 



When the two basic dyes, methyl green (molecular weight 458) 



