CHEMICAL BONDS IN NUCLEIC ACIDS 443 



No attempts have been made to piece together information based on the 

 structure of oligonucleotides found in deoxyribonuclease digests; for rea- 

 sons similar to those discussed in connection with the ribonucleic acids (p. 

 437) it is, in any case, unlikely that such attempts would lead to a unique 

 solution. Chargaff and his co-workers, however, have sought to ascertain 

 whether there is any recognizable order in the nucleotide sequence of typi- 

 cal deoxyribonucleic acids. Thus Zamenhof and Chargaff'^ ''^^'^^^ have 

 studied the distribution of purine and pyrimidine bases in the dialyzable 

 and nondialyzable fractions of deoxyribonuclease digests. Their results 

 suggest a very complex pattern; it should be remembered, of course, that 

 the as yet unknown specificity of deoxyribonuclease and the effect of differ- 

 ent dialysis rates of the fragments must be taken into account in drawing 

 any definite conclusions from such studies. The separation of six dinucleo- 

 tides from such digests^ ^- seems to indicate that no simple regularity is to 

 be expected in the nucleotide distribution. 



When deoxyribonucleic acids are subjected to mild acid hydrolysis, 

 materials are formed which were formerly described by the generic term 

 "thymic acid."^" Although most of the earlier preparations were highly 

 degraded, Chargaff and his co-workers have reinvestigated their prepara- 

 tion and obtained an interesting group of substances of molecular weight 

 ca. 15,000 which they term apurinic acids. ^^^ As their name suggests, they 

 are produced by removal of all purine residues from the polynucleotide, 

 through fission of the labile purine A'^-glycosidic linkages, and thus have 

 deoxyribose phosphate residues in place of the purine nucleotide residues 

 originally present in the nucleic acid. It is obvious that such products are 

 of great structural interest and their hydrolysis would merit close study. 



Relatively vigorous acid hydrolysis of deoxyribonucleic acids yields, as 

 already indicated, the 3 ',5 '-diphosphates of thymidine and deoxycytidine. 

 Brown and Todd*'- originally suggested that a certain amount of these di- 

 phosphates might be expected as a result of random fission of internucleo- 

 tidic linkages. Dekker, Michelson, and Todd,'^^ however, have commented 

 on the unexpectedly large amounts of the diphosphates in acid hydroly- 

 sates and suggest that they owe their origin to some other mechanism 

 operating at those positions in the polynucleotide chain where pyrimidine 

 and purine residues are adjacent to one another in view of the above-men- 

 tioned evidence^" that the initial action of acid is to remove purine residues 

 from deoxyribonucleic acids. A probable mechanism is that discussed by 

 Brown, Fried, and Todd^^' as a basis for stepwise degradation of polyribo- 



'" S. Zamenhof and E. Chargaff, /. Biol. Chem. 178, 531 (1949). 



if^^S. Zamenhof, Phosphorus Metabolism 2, 301 (1952). 



'" See footnote 158 for bibliography. 



1*8 C. Tamm, M. E. Hodes, and E. Chargaff, J. Biol. Chem. 195, 49 (1952). 



