COLOR REACTIONS OF NUCLEIC ACID COMPONENTS 293 



dine diphosphate is then obtained by comparing the optical density of the unknown 

 with that of the standard closest to it. As the latter does not differ by more than 10%. 

 the deviation from proportionality can be neglected. 



Neither aldopentoses or hexoses, nor short-chain sugars, glycolic al- 

 dehyde, trioses, erythrose, nor ahphatic aldehydes give any reaction with 

 the cysteine reagent, even in much higher concentrations than used for 

 DNA. The same is true of hexuronic acids, and amino, alpha-keto, and 

 alpha- and beta-hydroxy acids, which occur in significant quantities in 

 animal tissues. Fructose gives a yellow color; 2-deoxyhexoses and hexals a 

 yellow-red, and digitoxose a yellow color with completely different spectra 

 from that produced by 2-deoxyribose. The only compound so far investi- 

 gated related to saccharides which produces a cysteine reaction product 

 with a characteristic maximum at 490 m/u, is arabinal which, of course, 

 under conditions of the reaction should be partly hydrated to 2-deoxy- 

 pentose. Even in this case, however, the absorption curve differs con- 

 siderably from that of DNA insofar as it shows two more maxima— one 

 at 450 and one at 415 m^u. The molar extinction coefficient at 490 m/x is 

 only about one-third of that of DNA. Furthermore, arabinal produces a 

 yellow color with an absorption maximum at 470 m^t with sulfuric acid 

 alone in the absence of cysteine. This maximum at 470 mn disappears when 

 cysteine is added. 



Mechanism of the reaction: — The cysteine reaction resembles the diphenyl- 

 amine reaction as far as its possible mechanism is concerned, insofar as, 

 according to our observation, furfuryl alcohol which gives the characteristic 

 blue color in the diphenylamine reaction, produces also a compound with 

 the characteristic maximum at 490 mju in the cysteine reaction. However, 

 while the absorption curve of the furfuryl alcohol in the first reaction was 

 found to be identical with that for 2-deoxyribose or DNA, this is not the 

 case in the cysteine reaction. Here the absorption curve shows again two 

 additional maxima which were found in the reaction of arabinal at 450 m/x 

 and 415 m^, and these two maxima are still more pronounced in the case of 

 furfuryl alcohol. This compound also gives the yellow color with the ab- 

 sorption maximum at 470 m/z with sulfuric acid alone. The molar extinction 

 coefficient at 490 m/i for furfuryl alcohol in both modifications of the cyste- 

 ine reaction, is considerably lower than that of DNA and arabinal. These 

 observations indicate that either furfuryl alcohol itself or a heterocyclic 

 derivative of it is an intermediate in the cysteine reaction of 2-deoxyribose. 

 It seems much less probable that w-hydroxylseviilic aldehyde or a similar 

 aliphatic aldehyde should be the intermediate in this reaction because 

 hydroxy aldehydes and keto aldehydes, in general, do not react with the 

 cysteine reagent while they give color reactions of varying intensities with 

 the .diphenylamine reaction. Furfural, on the other hand, gives a violet 



