46 



activity of other substances containing S, C, and N atoms in prox- 

 imity is actually due to the action of this atom combination on £*. 

 Thiourea and thiouracil contain 



S 



II 

 N— C— N 



6-Mercaptopurine, known for its anticarcinogenic activity, has no 

 action on excitation of rhodamin or fluorescein, but completely 

 quenches the phosphorescence of riboflavine in a 6.10"'* M concen- 

 tration, and has a moderate quenching action on the phosphores- 

 cence of acrid ine. Its action shows thus a certain measure of speci- 

 ficity. It is believable that a mercaptopurine molecule built into 

 the nucleic acid cuts the energy transmission along the column of 

 purine and pyrimidin bases which forms the core of DNA in the 

 Watson-Crick model. Such an action would be analogous to the 

 action of hydroquinone which cuts the energy transmission through 

 Scheibe's columns of isocyanine dyes (see pp. 16, 17) . 



Another atomic group known to abolish fluorescence, if intro- 

 duced into the molecule of fluorescent substances, is the N=N. 

 Accordingly, we can expect to find strong quenchers among azo 

 compounds. McLaughlin and the author, while finding diazoben- 

 zene a poor quencher, found aminoazotoluene (toluazotoluidine) 

 highly active. The latter substance, as well as butter yellow are 

 difficult to evaluate, owing to their insolubility in water. Both seem 

 to quench the phosphorescence of riboflavine but were inactive 

 with rhodamin. 



