56 E. MARKHAM 



these linkages presented quite considerable difficulty in the first place, 

 however, because it had to be accomplished by degradation methods of 

 uncertain specificity. 



One of the main peculiarities of ribonucleic acids is their extreme lability 

 to the action of alkali. Phosphate diesters are usually very stable to the 

 action of alkali, but ribonucleic acids are converted quantitively to mono- 

 nucleotides by iV-KOH at 20°C. for 18 hoiu's. Originally these mononucleo- 

 tides were considered to be nucleoside 3'-phosphates (largely because the 

 latter are less soluble than the corresponding 2'-phosphates and so are more 

 readily precipitated), but work done by Cohn (1950) and Carter (1950) 

 showed that the adenylic nucleotides were in fact two in number, and it was 

 later found that all the mononucleotides in the alkaline digest occurred in 

 pairs. The nature of these nucleotides and the reason for the labihty of 

 ribonucleic acids were eventually clarified in an interesting theoretical 

 discussion by Brown and Todd (1952). Arguing from the known behavior of 

 other phosphate diesters, they showed that, if in ribonucleic acid the phos- 

 phate was linked to C-2' or C-3', and esterified elsewhere as well, the presence 

 of the adjacent hydroxyl group at C-3' or C-2' would aid the degradation. 

 The phosphate first esterifies the adjacent hydroxyl group which is in the 

 cis relationship, so forming a nucleoside 2' : 3'-cyclic phosphate, as shown in 

 Formula III. 



CH2O 

 I /0\ BASE 



1^ 



O OH 



I 

 OH— P=0 



I 

 OR (HI) 



The latter is then hydrolyzed randomly to give either the nucleoside 2'- 

 or 3'-phosphate. (It will be noted that the first stage in the degradation is 

 not hydrolytic but a transesterification. The second, and slower, step is 

 hydrolytic.) 



Brown and Todd were of the opinion that the cyclic nucleotide would only 

 be a transient intermediate, but Markham and J. D. Smith (1952a) showed 

 that it could be isolated from alkaline digests, and also that compounds of 

 this type were mtermediates in the digestion of the nucleic acid by pancreatic 

 ribonuclease, which attacks them relatively slowly. 



