298 ZACHARIAS DISCHE 



by extraction with chloroform. The absorption curve has a sharp peak at 490 m/x and a 

 second small but constant peak at 460 m/i. 



Specificity of the reaction according to Ceriotti.'—Apurimc as well as thy- 

 midylic acid according to our observations produce the same color as DNA; 

 the molar extinction coefficient of thymidylic acid, however, is about 20% 

 that of DNA. Free 2-deoxyribose gives a much lower color intensity than 

 DNA. The same is true of arabinal. Glucosamine, laevulic and uric acids, 

 as well as creatine and ascorbic acid, do not give any color. Hexoses, on the 

 other hand, give a pink color which is completely extracted by chloroform. 

 Arabinose, and probably also other pentoses as well as RNA, however, 

 produce a brown color in the water phase which corresponds to 8 % of the 

 color produced by an equivalent amount of DNA. Other pentoses and RNA 

 were not investigated but probably behave similarly. Galacturonic acid 

 also produces a considerable yellow-brown color in the water phase. These 

 two types of substances, therefore, may interfere with the determination 

 of DNA, if present in excess of DNA, and must be accounted for after 

 determination of their concentration in the unknown. 



Mechanism of the reaction: — Not only tryptophan but other /3-substituted 

 derivatives of indole give color reactions when heated with HCIO4 and 

 DNA. This, and the fact that hydroxy and keto aldehydes produce more 

 or less intensive colors^® in both modifications of the indole reaction as well 

 as in the tryptophan reaction, suggests that the first reaction resembles 

 the second one in its mechanism. Furfuryl alcohol, however, does not pro- 

 duce any color in the water phase in the Ceriotti modification, and a very 

 faint one only in the original form of the indole reaction. This, like the weak 

 reaction of free deoxyribose, may be due to a rapid destruction of these 

 compounds by HCl at 100°. 



e. Reaction with Carhazole and Sulfuric Acid. 



This reaction, described in 1930^ is best carried out according to the 

 modification of Gurin and Hood.^^ DNA shows a purple color with an ab- 

 sorption maximum at 530 m/x. The maximum of the color is already reached 

 after 2 minutes at 100°. 



Specificity of the reaction: — A characteristic feature of this reaction is 

 that after destruction of the sugar of the purine nucleotides by heating for 

 2 hours wuth 2 % H2SO4 , the remaining pyrimidine polynucleotides react 

 with about the same intensity as the whole DNA molecule, in spite of the 

 fact that the free purine nucleotides also react with great intensity.-^ This 

 indicates a considerable influence of the linkages between the nucleotides 

 on the intensity of the reaction. Free thymidylic acid reacts about 10% 



" S. Gurin and D. B. Hood, J. Biol. Chem. 139, 775 (1941). 



28 H. Angermann and F. Bielschowsky, Z. -phijsiol. Chem. 191, 123 (1930). 



