CHEMICAL BONDS IN NUCLEIC ACIDS 435 



analogous to the adenosine diphosphates obtained by degradation of tri- 

 phosphopyridine nucleotide^-* and coenzyme A.^^^ 



I OPO3H., OH 

 R-CH 



-CHo-OPOsH, 



I OH OPO3H2 



•CHo-OPOsHj R-CH \ \ 



H H H H H H 



XXIVa XXIVb 



The pyrimidine nucleoside diphosphates do not appear to be produced 

 from the end-groups of the ribonucleic acids bearing monoesterified phos- 

 phate since they are obtained in undiminished yield from nucleic acids pre- 

 viously treated with bone phosphomonoesterase. This enzyme removed 

 about 10 % of bound phosphate, considered on other grounds to represent 

 end-group monoesterified phosphate. Oligonucleotides produced by ribo- 

 nuclease action from the nucleic acids were attacked very slowly, if at all, 

 by the purified venom. 



Volkin and Cohn^^* explain these observations on the basis of a branched- 

 chain structure based on XXIII which can be represented by XXIIIa. They 



C,.— C3.— C,. 



C,. C3.— C,. a 



e c ^. 



€2- Cj. ^Cj. C3. Cj, d 



\ \P "^P 

 / \ 

 C3.1 C3. Cg. C3. Cj. 6 



xxnia 



suggest that the diesterase causes fission of the internucleotidic linkages at 

 the Czr — 0- — P bonds (i.e., at the broken lines in XXIIIa) liberating nu- 

 cleoside-5 '-phosphates (from residues a, 6, and c) as in the case of the poly- 

 deoxy ribonucleotides.^^ The diphosphates would arise from branch points 

 (residue d) and the (purine) nucleosides from the terminal residue of the 

 branch (residue e). The purine nucleoside produced in their experiments was 

 in fact equivalent in amount to the nucleoside diphosphates. For this ex- 

 planation to hold good, they had to postulate that the residue at the branch- 

 ing point {d) is a pyrimidine nucleoside residue. The first residue (c) in the 

 branch must also be a pyrimidine nucleoside residue to accomodate the 



'25 A. Romberg and W. E. Pricer, Jr., J. Biol. Chem. 186, 557 (1950). 



'26 T. P. Wang, L. Shuster, and N. O. Kaplan, J. Am. Chem. Soc. 74, 3204 (1952). 



