CHEMICAL BONDS IN NUCLEIC ACIDS 



431 



hydrolyzed by ribonuclease, it is clear that the 6(3') -position of the pyrimi- 

 dine nucleoside residues is involved exclusively in the internucleotidic 

 linkage. Only in the case of branched-chain structures could the a (2') -posi- 

 tion be involved and then only as a branching point; this exceptional case 

 will be discussed later. 



•0- 



OCH,Ph 

 I 

 OHOPO-OH 



R-CH- 



H H 



H 



CH,-OH 



CHo-OH 



R-CH 



CHo-OH 



H H 



In discussing the intimate, action of ribonuclease, Brown and Todd*^ 

 suggest that the fundamental catalytic action is the formation and subse- 

 quent rupture of Ca{2-) — — P bond as indicated schematically below. 

 The elimination of the residue R as the alcohol ROH is a necessary conse- 

 c^uence of this action; the close similarity between ribonuclease and chemi- 

 cal hydrolytic agents is apparent. 



•OH .0- 

 P^O - 

 O OR 



R Nase 



(fast) 



.^' 



— o ^0- 



+ R-OH 



(slow) 



-OH 

 OPO(OH)j 



This interpretation gives a simple explanation for the apparently two- 

 fold action of ribonuclease on ribonucleic acids. The initial "depolymeriza- 

 tion" without liberation of acid functions, followed by the slow liberation 

 of acidic secondary phosphoryl groups observed b}^ Chantrenne, Linder- 

 str0m-Lang, and Vandendriessche^^^-^^ using a dilatometric method is 

 undoubtedly related to the above formation and cleavage of cyclic phos- 

 phoryl groups. Vandendriessche"' has indeed shown that the volume 

 changes observed during the action of ribonuclease on ribonucleic acids are 

 closely parallel to those observed in the ribonuclease hydrolysis of the 

 mononucleotide esters described above. 



Although the study of ribonuclease action on mononucleotide esters es- 



"^ H. Chantrenne, K. Linderstr0m-Lang, and L. Vandendriessche, Nature 159, 877, 



(1947). 

 '•' L. Vandendriessche, Acta Chem. Scand. 7, 699 (1953). 



