346 ERWIN CHARGAFF 



this brief section I shall limit myself to those aspects that are of importance 

 for the succeeding discussion of composition and structure. 



1. Sugar 



As regards the type of sugar occurring in deoxypentose nucleic acids, 

 very much has been taken for granted owing no doubt to the great difficulty 

 of isolating the free deoxysugars. Only for the nucleosides and nucleotides 

 obtainable from thymus nucleic acid, and strictly speaking, only for the 

 purine derivatives, is the chemical evidence decisive. There exist, however, 

 no indications that of the possible 2-deoxypentoses, 2-deoxyribose and 

 2-deoxyxylose, any but 2-deoxy-D-ribose occur in nucleic acids, though 

 proof by actual isolation has been scanty (see Chapter 2); nor has an 

 authentic case of the coexistence of both pentoses and deoxypentoses in the 

 same nucleic acid been described. The lack of information on the optical 

 activity of nucleosides and nucleotides isolated from other nucleic acids 

 than that of calf thymus is regrettable. 



The chromatographic investigation of the sugars present in different 

 deoxypentose nucleic acids, or at any rate in their purine nucleotide moie- 

 ties, has, however, been pursued on a quite extensive scale in the past few 

 years. It is based on the enzymic degradation of the nucleic acid to the 

 nucleoside stage, on the release of the sugar by controlled heating of the 

 nucleosides at pH 1.5, and the chromatographic comparison, in at least 

 three different solvent systems, of the unknown sugar with 2-deoxyribose 

 liberated under identical conditions from calf thymus deoxyribonucleic 

 acid.'^'*'^^^ In all instances examined heretofore, the sugars were identical 

 and agreed in their chromatographic behavior with that of 2-deoxyribose. 

 This tentative identification has been performed with deoxypentose nucleic 

 acid preparations from the following sources: ox (thymus, spleen),"'' sheep 

 (thymus, liver), "^ pig (thymus, liver), "^ man (thymus, liver, carcinoma),"^ 

 salmon sperm," sperm from four sea urchin genera,'"* yeast, and avian 

 tubercle bacilli.'^' 2-Deoxyribose has also been demonstrated in apurinic 

 acid.224 



2. Nitrogenous Constituents 



With very few exceptions, the bulk of the nitrogenous constituents (aboul 

 98 to 99%) of all deoxypentose nucleic acids is composed of two purines 

 adenine and guanine, and two pyrimidines, cytosine and thymine. The 

 principal exceptions are the deoxypentose nucleic acids of the bacterio- 

 phages T2, T4, and T6 of E. coli, in which 5-hydroxymethylcytosine takes 

 the place of cytosine,'*'-^' and the nucleic acid of wheat germ, in which 



"2 E. Chargaff, C. Levine, and C. Green, J. Biol. Chem. 175, 67 (1948). 

 2" G. R. Wyatt and S. S. Cohen, Nature 170, 1072 (1952). 



