VOL. 12 (1953) CARCINOGENIC COMPOUNDS, PURINES AND NUCLEIC ACID 



85 



10:, 



I 



61 



— ~o__ 



70 80 90 



% Ionization 



Fig. 10. lonisation of 3 :4-6:7-dibenzacridine in purine solutions. 

 X X, in 0.072 71/ tetramethyluric acid; O 



-0, in 0.1% DNA. 



, in 0.063 -^^ caffeine; 



/O 



TABLE IV 



pKa VALUES OF THE DIBENZACRIDINES IN PURINE SOLUTIONS 



Dibenzacridine 



Solvent 



pKa 



3:4-6:7 DBA ^ 



1:2-6:7 DBA 

 1:2-8:9 DBA 



25 %. 50% or 75% ethanol 3.8 



0.1 0/0 DNA 6.8 



50% ethanolic 0.1% DNA 6.1 



0.063 -^^ caffeine 3.5 



0.072 M tetramethyluric acid 3.1 



0.072 M tetramethyluric acid 2.2 



0.072 M tetramethyluric acid -0.2 



DISCUSSION 



The molecular compounds formed between caffeine (or tetramethyluric acid) and 

 one of the dibenzcarbazoles (or dibenzacridines) , which have so far been isolated, resemble 

 the hydrocarbon-purine complexes isolated by Weil-Malherbe, in that only small 

 changes in light absorption are associated with their formation. In this way they differ 

 from the molecular compounds formed between nitro-compounds and hydrocarbons or 

 amines. The change or intensification of colour which takes place in these cases has been 

 ascribed to the formation of a complex molecule, essentially ionic in character, involving 

 an electron transfer from the unsaturated hydrocarbon to the poly-nitro compound 

 (Weiss^^; Kronberger and Weiss^^). 



The infrared absorption spectra of the complexes of purines with hydrocarbons and 

 with aromatic nitrogenous substances (Booth, Boyland and Orr^^) are similar to those 



References p. 8y. 



