84 



J. BOOTH, E. BOYLAND 



VOL. 12 (1953) 



the same order they are directly proportional to the concentration. The results of these 

 estimations are shown in the last column of Table IIL 



Fluorescence of solutions. Weil-Malherbe^" noticed that the fluorescence of solutions 

 of hydrocarbons in aqueous caffeine was quenched by the addition of acid, the quenching 

 agent being the caffeine ion and not the hydrogen ion, whereas substituted uric acids 

 alone quenched the fluorescence in aqueous solutions and this quenching was inde- 

 pendent of the pH of the solution. Similar results were obtained with the dibenz- 

 carbazoles, the aqueous caffeine solutions possessing an intense blue fluorescence which 

 was quenched on acidification and restored on neutralisation, whilst the aqueous tetra- 

 methyluric acid solutions were not fluorescent. Caffeine solutions of the dibenzacridines 



were colourless and possessed an intense blue 

 fluorescence; acidification of these solutions 

 quenched the fluorescence and produced a 

 bright yellow colour owing to the ionisation 

 of the dibenzacridine ; the tetramethyluric 

 acid solutions again showed no fluorescence 

 in acid or alkali, but acidification produced 

 bright yellow solutions. 



Ionisation of the dibenzacridines. When 

 3:4-6:7-dibenzacridine was dissolved in a 

 neutral solution of the sodium salt of DNA 

 (pH 6.5) a bright yellow colour and a change 

 in the absorption spectra were observed. 

 These changes were similar to those brought 

 about by the addition of acid to an ethanolic 

 solution of the dibenzacridine (Fig. 9). The 

 absorption curves in DNA show the typical 

 bathochromic shift and reduction of maxima 

 produced by dissolving these compounds in 

 purines. 



The degree of ionisation at any pH value 

 is directly proportional to the height of the 

 absorption maximum of the dibenzacridine 

 ion at 435 m/< for solutions of the same 

 concentration since this value is negligible 

 for the unionised molecule. The p/v^ values 

 were determined spectrophotometrically for 

 3:4-6:7-dibenzacridine in 25%, 50% and 

 75% ethanol, aqueous caffeine, tetramethyl- 

 uric acid and 50% ethanoHc DNA solutions 

 (Fig. 10). The DNA apparently increases the pi^^of the dibenzacridine from approxi- 

 mately 3.5 in aqueous ethanol, caffeine and tetramethyluric acid to 6.8 in aqueous DNA 

 and to 6.1 in 50% ethanolic DNA. Determination of the \)Ka, values of the other two 

 isomers of dibenzacridine in tetramethyluric acid showed that they were weaker bases 

 (Table IV). 



The dissociation curve for 3:4-6:7-dibenzacridine dissolved in DNA is abnormal 

 in that the slope is less steep than that of a normal ionisation curve. 

 References p. 8y. 



360 



380 



AGO 



i20 



1,1,0 460 



Wavelength (/im) 



Fig. 9. Effect of DNA on the absorption curve 



of 3:4-6: 7-dibenzacridine. , in 50% 



methanol pH = 6.2; , in 50% 



methanoHc 5iV HCl pH < i ; ■, in 0.1% 



DNA solution pH = 6.5; in 0.1% 



DNA + o.i ml N NaOH, pH = 9.0. 



