CHEMISTRY OF PURINES AND PYRIMIDINES 123 



dized by oxygen in the presence of catalytic amounts of copper ;^^* the 

 spectral changes accompanying this oxidation are identicaP^^ with those 

 that occur when uric acid is oxidized by uricase (cf. Bentley and Neu- 

 berger."^). 



In addition to the alterations brought about by the oxidizing agents 

 discussed above, certain pyrimidines and purines undergo profound, but 

 poorly understood, changes when their solutions are exposed to ultraviolet 

 light. For example, thymine,^^*^ uracil,^^^ isoguanine,^*^ and adenylic and 

 uric acids^*^ show extensive loss of their ultraviolet absorption spectra 

 after such exposure. In the case of thymine,^^" the pyrimidine ring is dis- 

 rupted with the formation of urea and pyruvic acid. When uracil and uri- 

 dine solutions are irradiated in the 230-280 m/x region, the loss in absorption 

 is largely restored by acidification of the irradiated solutions.^^^ 



d. Color Tests 



Although they have been largely superseded by the modern and more 

 specific chromatographic, spectrophotometric, and polarographic tech- 

 niques, color reactions are often employed to follow the course of a chemical 

 reaction, the effectiveness of a fractionation or isolation procedure, or for 

 the identification and quantitative estimation of pyrimidines and purines. 

 Reference has already been made in the previous three sections to a number 

 of color tests. The Wheeler-Johnson test^^'' for uracil and cytosine depends 

 upon the conversion of these compounds to 5,5-dibromo-4-hydroxyhy- 

 drouracil, the bromine atoms of which are replaced by treatment with 

 barium hydroxide solution to give isodialuric and dialuric (2,4,5,6-tetra- 

 hydroxypyi'imidine) acids. The barium salts of these acids are purple. The 

 test is not given by thymine, 5-methylc5rtosine, or pyrimidine nucleosides.^^ 

 In an adaptation of this reaction, ^^^ uracil, cytosine, isocytosine, 5-bromo- 

 cytosine, and thiouracil are also converted with bromine water to the di- 

 bromohydroxyhydrouracil ; the latter reduces the uric acid reagent, lithium 

 arsenotungstate, to produce a blue color which is measured quantitatively. 

 Uracil, cytosine, and thymine, but not N-3-substituted pyrimidines, couple 

 with the Pauly reagent, diazobenzenesulfonic acid, to give a red diazo 

 dye following alkalinization''*'^*^'^^^ (cf. Lythgoe et alP^). This reaction 

 serves as the basis of a microestimation technique for thymine.'^"^^" 



378 M. Griffiths, /. Biol. Chem. 197, 399 (1952). 



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380 L. W. Bass, /. Am. Chem. Soc. 46, 190 (1924). 



3" M. M. Stimson, J. Am. Chem. Soc. 64, 1604 (1942). 



382 D. Rapport and A. Canzanelli, Science 112, 469 (1950). 



383 R. L. Sinsheimer and R. Hastings, Science 110, 525 (1949). 



384 H. L. wAeeler and T. B. Johnson, J. Biol. Chem. 3, 183 (1907). 



385 M. Soodak, A. Pircio, and L. R. Cerecedo, /. Biol. Chem. 181, 713 (1949). 



386 T. B. Johnson and S. H. Clapp, J. Biol. Chem. 5, 163 (1908). 



