COLOR REACTIONS OF NUCLEIC ACID COMPONENTS 299 



less than half the equivalent amount of DNA. The reaction is not specific 

 as practically all saccharides give purple colors with slightly different ab- 

 sorption maxima. Aliphatic aldehydes and hydroxy aldehydes also give 

 intensively colored products. 



/. Reaction of DNA with Schiff's Reagent 



The presence of considerable amounts of the aldehydic form of 2-deoxy- 

 ribose in its solutions manifests itself by its ability to produce a characteris- 

 tic red color with Schiff's reagent. Feulgen-^ found in 1924 that after a 

 brief hydrolysis with 1 N HCl, DNA reacts with Schiff's reagent. Tobie^" 

 introduced a modified reagent characterized by a much higher concentration 

 of SO2 which increases considerably the sensitivity of the reaction. Wid- 

 strom^' adapted the reaction with Schiff's reagent for quantitative pur- 

 poses. While the reaction proved of great importance for cyto- and histo- 

 chemical studies on nucleic acids [Cf. Swift, Chapter 17], it cannot compare 

 in sensitivity with the other methods here described, as far as quantitative 

 determinations are concerned. 



g. Evaluation of Various Methods of Quantitative Determination of DNA 

 and Its Constituents 



Of all the reactions here described, that with cysteine and 75 vol. % 

 sulfuric acid appears to be the most specific, as even a compound as closely 

 related to 2-deoxyribose as arabinal, produces a color which can be dis- 

 tinguished by its absorption spectrum from that of 2-deoxyribose. It will 

 be of particular interest to see whether 3-deoxypentose and 2,3-deoxy- 

 pentose which, according to Overend,^ show a weak blue color in the di- 

 phenylamine reagent, react at all with the cysteine reagent. For quanti- 

 tative determination, however, the difference in specificity between this 

 reaction and that with diphenylamine appears negligible and the sensitivity 

 of the latter reaction is about twice that obtained with the modification of 

 Stumpf of the cysteine reaction with 75 vol. % H2SO4 . The cysteine reaction 

 of DNA, furthermore, may prove valuable for the determination of free 

 thymidylic acid in the presence of other deoxynucleotides or DNA, par- 

 ticularly in its free form. However, more investigations appear necessary 

 to test the application of the cysteine-H2S()4 reaction for determinations 

 in living tissues. The tryptophan-perchloric acid reaction, when measured 

 with the Beckman spectrophotometer, appears to have about one-third of 

 the sensitivity of the diphenylamine reaction. Its use for quantitative pur- 

 poses in living cells has not sufficiently been studied, and the data so far 



" R. Feulgen and H. Rosenbeck, Z. physiol. Chcrn. 135, 203 (1926). 

 30 W. C. Tobie, Ind. Eng. Chem., Anal. Ed. 15, 405 (1942). 

 3' G. Widstrom, Biochem Z. 199, 298 (1928). 



