COLOR REACTIONS OF NUCLEIC ACID COMPONENTS 295 



intensive absorption between 305 and 330 m/z,'^ or at 405 m^x.^^ This ab- 

 sorption appears immediately after the addition of the sulfuric acid to the 

 solution of the sugar. With DNA or its nucleotides, no such absorption 

 appears before addition of cysteine. Subsequently, a compound is slowly 

 formed which shows a sharp absorption maximum at 375 m^. The maximum 

 absorption is reached at room temperature only after 48 hours, but 80% 

 of the maximum is already obtained after 20 hours. Apurinic acid, thy- 

 midylic acid, arabinal, and furfuryl alcohol, according to our more recent 

 observation, give the same reaction and the slope of the absorption curve, 

 between 350 and 380 m;u, is identical in all these cases with that obtained 

 with DNA. The molar extinction coefficients at 375 m^u, however, are con- 

 siderably decreased, viz., for apurinic acid by 30%, for arabinal by 57%, 

 for furfuryl alcohol 53%, and by 40% for thymidylic acid. These values 

 indicate that the mechanism in this reaction is identical or similar to that 

 in the cysteine reaction of DNA with 75 vol. % H2SO4 , and that either 

 furfuryl alcohol itself or a furan derivative closely related to it is an inter- 

 mediate in this reaction. This conclusion is supported by the fact that 

 apurinic acid gives lower values than equivalent amounts of DNA, as it 

 can be assumed that in APA at least a part of the sugar is present as a 

 straight-chain compound. The lower values for furfuryl alcohol do not 

 exclude the possibility that this compound itself is an intermediate, as here 

 again side reactions may occur when the compound is present from the 

 beginning, and may be lessened or altogether voided when it is formed 

 slowly from another compound. 



Interference from other siihstances: — As all carbohydrates give absorption 

 spectra in this cysteine reaction, with peaks around 400 m^u, they will all 

 interfere with the determination of DNA. As the reaction product of pen- 

 toses, however, is unstable and most of the absorption due to it disap- 

 pears after 24 hours' standing at room temperature,^- the interference from 

 small amounts of ribonucleic acid in general will not be significant. As far 

 as the interference from hexoses is concerned, it may be possible to elim- 

 inate it by dichromatic readings at 375 m/i, and a higher wave length chosen 

 in such a way that the absorption due to hexoses which has a maximum 

 at 414 m^, is equal to the absorption at 375 m/x. The difference in the op- 

 tical density of these two wavelengths will then be a measure of the amount 

 of DNA or one of its breakdown products. On the other hand, aliphatic 

 aldehydes and hydroxy aldehydes such as glycolic aldehyde, trioses, tet- 

 roses, and pyruvic aldehyde do not react at all in the general cysteine re- 

 action of carbohydrates and will not interfere with it. The same is true 

 of amino acids and pure proteins in considerable amounts, exceeding several 



» Z. Dische, J. Biol. Chem. 204, 983 (1953). 



