412 D. M. BROWN AND A. R. TODD 



imposed — ^an enzymic cleavage of covalent links followed by a spontaneous 

 rupture of secondary valence bonds. Similar observations have been made 

 by Vandendriessche^^ in a study of the degradation of yeast ribonucleic acid 

 by ribonuclease. He, too, points out that the liberation of such titratable 

 groups does not necessarily indicate that they were originally present in 

 covalent linkages. Cavalieri, Kerr, and Angelos'^ report discrepancies be- 

 tween base content and titratable groups in the so-called "core" of ribo- 

 nucleic acid, and suggest that in this material (see later), which constituted 

 some 3-10% of the original nucleic acid, the enolic hydroxyl of uracil and 

 guanine may be involved in internucleotidic linkages. Corresponding 

 changes in ultraviolet absorption when this material'^ '^^ and intact ribo- 

 nucleic acid^^ are hydrolyzed by alkali, or when deoxyribonucleic acids are 

 degraded by deoxyribonuclease^^ or snake venom,'" suggest, however, that 

 the changes are more likely to originate in cleavage of secondary valence 

 bonds, which interfere with the resonating system of purine and pyrimi- 

 dine rings,^^''^ than in rupture of covalent linkages. [Cf. Beaven, Holiday, 

 and Johnson, Chapter 14.] 



Based on the reaction of deoxyribonucleic acids with Feulgen's reagent 

 after very mild treatment with acid and alkali, or even simple dialysis, 

 Stacey et al.^^'^^ have proposed that a small number of labile phosphoryl 

 linkages attached to Ci of deoxyribose residues are present. This type of 

 structure would, of course, be at variance with the generally accepted poly- 

 nucleotide structure of nucleic acids, although it could be argued that a 

 very small proportion of such links in a nucleic acid of high molecular weight 

 might make little difference to the nitrogen-phosphorus ratio as determined 

 by analysis. 



Consideration of the various types of linkage which have been proposed 

 in recent years leads to the conclusion that the phosphodiester linkage 

 between the sugar residues of the individual nucleosides remains at least 

 the major one. The others, apart possibly from the phosphotriester link, 

 may have been advanced on reasonable grounds, but the absence of cor- 

 roborative chemical evidence and, in general, their apparently very small 

 occurrence in proportion to the others, suggests that they are of minor 



" B. Magasanik and E. Chargaff, Biochim. et Biophrjs. Acta 7, 396 (1951). 



" M. Kunitz, /. Biol. Chem. 164, 563 (1946). 



" M. Kunitz, J. Gen. Physiol. 33, 349, 363 (1950). 



3" R. L. Sinsheimer and J. F. Koerner, J. Biol. Chem. 198, 293 (1952). 



" B. Commoner, Science 110, 31 (1949). 



« C. F. Li, W. G. Overend, and M. Stacey, Nature 163, 538 (1949). 



" W. G. Overend, M. Stacey, and M. Webb, J. Chem. Soc. 1951, 2450. 



»* W. G. Overend, A. R. Peacocke, and M. Stacey, J. Sci. Food Agr. 3, 105 (1952). 



« W. A. Lee and A. R. Peacocke, J. Chem. Soc. 1952, 130. 



