CHEMICAL BONDS IN NUCLEIC ACIDS 437 



polynucleotides, followed by structural determinations on these and re- 

 construction to yield a unique solution. The fact that only four mononu- 

 cleotides are involved makes this type of approach in some respects less 

 likely to succeed than in the case of the proteins, as is evident when one 

 considers the results obtained by analyzing the oligonucleotides isolated 

 from ribonuclease digests of ribonucleic acids. ^^•"•^•^29a /^\\ ^\^q possible 

 residue arrangments are found in the di- and trinucleotides (with terminal 

 pyrimidine residues) obtained in this way, and there is little reason to be- 

 lieve that the same will not also hold for the mixture of larger nondialyzable 

 fragments which are obtained. The problem would be even more compli- 

 cated if, as Markham and Smith'" suggest, ribonucleic acids are to be re- 

 garded as mixtures of large numbers of relatively small molecules rather 

 than single species of very high molecular weight. These authors arrived 

 at this suggestion on the basis of a method of end-group determination de- 

 pending on removal of terminal monoesterified phosphate with prostatic 

 phosphatase; using it, they found all four nucleotides as end-groups in 

 addition to nucleotides bearing a cyclic phosphoryl group. Their conclu- 

 sions seem open to criticism, however, firstly because chain-branching, 

 which they do not consider, could account for increased numbers of end- 

 groups, and secondly because the presence of traces of diesterases in their 

 enzyme preparations would lead to high apparent figures for end-groups 

 and correspondingly low estimates of molecular weight. 



At the present time, available evidence does not indicate anything more 

 than a rather random sequence of residues, and it seems clear that progress 

 must depend (a) on the development of methods of stepwise degradation 

 and (b) some certainty as to the individuality of a given ribonucleic acid — 

 clearly if it were a mixture of different molecular species"" efforts to de- 

 termine sequence would be useless. A method of stepwise degradation is 

 in any case of considerable value, since quite apart from its use on intact 

 ribonucleic acids, such a method is necessary if the structure of the larger 

 oligonucleotides is to be determined. The essential requirement for such a 

 method is that the terminal internucleotidic linkage in a chain should be 

 rendered more labile than any other similar linkage in the molecule so as to 

 permit removal (and identification) of that residue, leaving the remainder 

 of the polynucleotide intact and ready for a repetition of the same process. 

 The lability of the internucleotidic linkage severely limits the choice of 

 possible methods. Brown, Fried, and Todd'^' have proposed a method 



129a 'YYiQ recent demonstration (L. A. Heppel and P. R. Whitfeld, Proc. Biochem. Soc. 

 56, ii, 1954; L. A. Heppel, P. R. Whitfeld, and R. Markham, ibid. 56, iii, 1954) 

 that ribonuclease acts reversibly must render ineffectual any attempt to derive 

 the structure of a polynucleotide from a consideration of the products of its action. 



'■'n V. Desreux and J. M. Ghuysen, Bull. soc. chim. Beiges 60, 410 (1951). 



"1 D. M. Brown, M. Fried, and A. R. Todd, Chemistry & Industry 1953, 352. 



