CHEMISTRY OF PURINES AND PYRIMIDINES 121 



uracil to isobarbitiiric, isodialuric, and oxalic acids and urea.^^^ When ex- 

 posed to a solution of ferrous sulfate and sodium bicarbonate in the presence 

 of oxygen, thymine is oxidized and yields urea, acetol, and formic and 

 pyruvic acids?^" The acetol condenses with o-aminobenzaldehyde, yielding 

 the strongly fluorescent 3-oxyquinaldine which turns a deep red upon the 

 addition of ferric chloride, thus furnishing a sensitive color test for thymine. 

 The same oxidation mixture converts cytosine and uracil into dihydroiso- 

 barbituric acid, and this compound forms a red complex with ferrous sul- 

 fate.^" Thymine is also oxidized bj'- peroxide in the presence or absence of 

 ferrous sulfate and acetol is obtained upon heating the reaction mixture.^*^^ 



The mechanism of this oxidation was investigated by Baudisch and 

 Davidson^^^ with the possibility in mind that the intermediate oxidation 

 product might be thymine glycol (XXXV). Thymine was converted quan- 

 titatively ,^° upon treatment with bromine water, into 5-bromo-4-hydroxy- 

 hydrothymine (XXXVI), and this in turn was transformed^*^^ into thymine 

 glycol by shaking with moist silver oxide. Acetol (arbitrarily shown in the 

 keto form, XXXVII) and urea were obtained by boiling the glycol in either 

 sodium bicarbonate or barium hydroxide solution and it was concluded^^^ 

 that thymine glycol was indeed the intermediate in the peroxide oxidation of 

 thymine. (Thymine can be regenerated from its glycol by hydriodic acid 

 reduction.) Advantage has been taken of this elegant degradation proce- 

 dure to locate the isotope (C^*) in the methyl group of biologically labeled 

 thymine ;'^^ the resulting acetol was converted by hypoiodite into iodoform. 

 Acetol is also obtained when 5-methylcytosine is subject ed^^^ to the Baudisch- 

 Davidson procedure. 



Uracil, cytosine, and thjonine absorb iodine in aqueous bicarbonate solu- 

 tion; urea is formed when the reaction mixtures are heated, and, in the 

 case of thymine, acetol is also formed.^^^ Uracil, xanthine, and guanine 

 each absorb four equivalents of iodine from an alkaline solution,^" ■'®^ 

 whereas uric acid absorbs only one mole^^'^ and adenine does not react; 

 although the chemistry of these reactions has not yet been clarified, they 

 serve as analytical procedures. [Cf. Dische, Chapter 9.] 



2*^ C. R. Schwob and L. R. Cerecedo, Proc. Am. Soc. Biol. Chemists, J. Biol. Chem. 105, 

 Ixxvi (1934). 



360 T. B. Johnson and O. Baudisch, J. Am. Chem. Soc. 43, 2670 (1921). 



361 O. Baudisch, ./. Biol. Chem. 60, 155 (1924). 



3«2 O. Baudisch and L. W. Bass, ./. Am.. Chem. Soc. 46, 184 (1024). 



363 o. Baudisch and D. Davidson, /. Biol. Chem. 64, 233 (1925). 



364 D. Elwyn and D. B. Sprinson, J. Am. Chem. Soc. 72, 3317 (1950). 



365 H. H. Harkins and T. B. Johnson, J. Am. Chem. Soc. 51, 1237 (1929). 

 3.66 L. W. Bass and O. Baudisch, J. Am. Chem. Soc. 46, 181 (1924). 



367 M. Z. Grynberg, Biochem. Z. 253, 143 (1932). 



368 W. Klein, Z. physiol. Chem. 231, 125 (1935). 



