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



83 



The ultraviolet absorption spectra of 3:4-benzpyrene, 3 : 4-5 : 6-dibenzcarbazole and 

 3 : 4-6 : 7-dibenzacridine in saturated purine solutions, were compared with the absorption 

 of the same compounds in 50% ethanol alone and saturated with caffeine or tetra- 

 methyluric acid. In each case there was a depression of the maxima and a bathochromic 

 shift in the purine solution {e.g. see Fig. 8). The greatest shift occurred in 3 : 4-benzpyrene 

 when the maximum at 384 mpi was shifted to 391 m/u in tetramethyluric acid. 



350 



360 



370 



380 



390 



UQO UIO 



Wavelength (/im) 



i,20 



Fig. 8. Absorption spectra of 3 : 4-6: 7-dibenzacridine in purine solutions. 



in 50% ethanol; 



5070 ethanol saturated with caffeine; ■ , in 50% ethanol saturated with tetramethyluric 



acid. 



Similar changes in the absorption spectra of acriflavine dissolved in nucleic acid 

 have been described (Oster^). Such modifications of the spectra may be similar to the 

 changes produced by using different solvents. 



The purine components of the molecular compounds do not absorb at wavelengths 

 above 300 m/^ (caffeine) or 330 m/< (tetramethyluric acid), whereas all the dibenz- 

 carbazoles and dibenzacridines studied have well defined maxima between 340-400 m/^. 

 As the molecular extinctions of dilute solutions of the molecular compounds in this 

 range are identical with those of the pure dibenzcarbazoles or dibenzacridines, such 

 maxima are convenient for the estimation of the aromatic component in molecular 

 compounds. Ethanolic solutions of known concentrations of the molecular compounds 

 were prepared and optical density (Log I,,/!) at suitable maxima determined. The con- 

 centration, and hence the proportion of aromatic component in the complex could then 

 be calculated by comparison with a similar determination on the pure aromatic sub- 

 stance, since if the concentrations used are such that the optical density readings are of 



References p. 8j. 



