110 AARON BENDICH 



many of the hydroxy and amino derivatives have a limited water solubihty 

 (Table I). The parent compounds are very soluble in water at ordinary 

 temperatures; the introduction of hydrogen-bonding groups ( — OH or 

 — NH2) into pyrimidine or purine results not only in a reduction in water 

 solubility, but in an increase in melting point as well. (An analogous situa- 

 tion is seen in the pteridine system.^"'') This effect is especially true of the 

 purines: the least soluble is 250,000 times less soluble than purine itself. 

 The replacement of the 2-hydroxyl of xanthine, or of the 6-hydroxyl of 

 uric acid, by an amino group is manifested by a further marked decrease 

 in water solubility (cf. Albert^"'*). However, all the compounds listed in 

 Table I are easily soluble in dilute aqueous alkali or mineral acid (or both) . 

 A common practice in the purification of the less-soluble compounds in- 

 volves their solution in dilute acid or alkali; crystallization often follows 

 neutralization. Whereas pyrimidine, purine, and the simple alkyl, aryl, 

 aryloxy, diarylamino, or halo derivatives are more or less soluble in com- 

 mon organic solvents, the other compounds listed in Table I are, in general, 

 extremely insoluble. 



For these reasons, preference has been given to the use of aqueous sys- 

 tems for the purpose of separation, purification, and characterization of 

 pyrimidines and purines of biological interest. For example, the counter- 

 current distribution technique^^^ has been successfully applied to such 

 compounds^"^ in a system consisting of n-butanol and M phosphate, pH 

 6.5. The coefficients of distribution of pyrimidines and purines between 

 the two phases of this system (Table I) are very useful in identification and 

 characterization. The method is also valuable in the separation of mixtures 

 of these compounds and for the estimation of homogeneity. Another ex- 

 ample is the use of a variety of aqueous systems in column and paper 

 chromatography. These techniques are discussed in detail in Chapters 6 

 and 7. The Rf values of some pyrimidines and purines in an isopropanol- 

 water-HCl system are listed in Table I. 



h. Criteria of Purity and Identity; the Value of Ultraviolet Absorption Spectra; 

 Ionization; Tautomerism; the Value of the Isoshestic Point 



For discussions of the principles of the purity and identity of organic 

 compounds see Pirie^"^ and Eyring.^°^ 



Table I enumerates the melting and decomposition points of a few pyrimi- 

 dines and purines, and in some cases those of a salt. The substituted com- 



304 A. Albert, Quart. Revs. (London) 6, 197 (1952). 



306 L. C. Craig, Fortschr. chem. Forsch. 1, 292, 302, 312 (1949). 



306 J. F. Tinker and G. B. Brown J. Biol. Chem. 173, 585 (1948). 



307 N. W. Pirie, Biol. Revs. 15, 377 (1940). 



308 H. Eyring, Anal. Chem. 20, 98 (1948). 



