CHEMICAL BONDS IN NUCLEIC ACIDS 411 



different workers of a variety of rather ill-characterized nucleic acid prep- 

 arations. 



I 

 Base— sugar— PO(OH) P0(0H)2 



I 1 



Base — sugar — PO — sugar — Base 



Base— sugai — PO (OH) 2 

 la 



No direct chemical evidence has been adduced in support of this type of 

 branched structure; it would be expected to be relatively unstable both to 

 acid and alkali and might, therefore, have been missed in chemical degrada- 

 tions. More recent evidence for this type of linkage derived from enzyme 

 experiments will be considered later in connection with the general problem 

 of chain-branching in ril)onucleic acids. 



Electrometric titration of deoxyribonucleic acids of high molecular weight 

 shows only small amounts of secondary phosphoryl dissociation. -° These 

 acids are thus generally considered to be straight-chain polydiesters of 

 type I, a structure which also accords with their other physical properties.^' 

 [Cf. Jordan, Chapter 13.] Nevertheless, Lee and Peacocke-^ have inter- 

 preted their titration data on the basis of a branched structure including 

 phosphotriester linkages, a conclusion also reached from dye adsorption 

 studies.'^ 



Euler and Fono'^ have observed the liberation of base-binding groups 

 when deoxyribonucleic acid is treated with alkaH (pH 11.5). They suggest 

 that this represents the fission of linkages between phosphate and the 

 enolic hydroxyl groups of purine or pyrimidine residues. A similar explana- 

 tion has been suggested by Little and Butler-^ to account for their observa- 

 tion that during the action of deoxyribonuclease on deoxyribonucleic acids 

 groups of pK 9-10 are liberated, in addition to secondary phosphoryl 

 groups. It seems more likely that these represent enolic hydroxyl groups 

 on the purine and pyrimidine residues, which are masked by hydrogen- 

 bonding and are set free during degradation of the molecule, than that they 

 originate in covalent internucleotidic linkages. Since the specificity and 

 mode of action of deoxyribonuclease has not been clearly defined, it is pos- 

 sible, as Zamenhof and Chargaff-^ point out, that two effects may be super- 



" J. M. Gulland, D. O. Jordan, and H. F. W. Taylor, J. Chem. Soc. 1947, 1131. 

 ^' D. O. Jordan, Progr. Biophys. and Biophys. Chem. 2, 51 (1951). 

 " W. A. Lee and A. R. Peacocke, J. Chem. Soc. 1951, 3361. 



23 L. F. Cavalieri and A. Angelos, J. Am. Chem. Soc. 72, 4686 (1950). 



24 H. von Euler and A. Fono, Arkiv. Kemi Mineral. Geol. 25A, No. 3 (1947). 

 "J. A. Little and G. C. Butler, J. Biol. Chem. 188, 695 (1951). 



2« S. Zamenhof and E. Chargaff, J. Biol. Chem. 187, 1 (1950). 



