478 



D. O. JORDAN 



Fig. 16. The titration curve of sodium deoxypentose nucleate (Lee and Pea- 

 cocked"*). 



I Titration with alkali from pH 6.4 to 12.0 

 II Back-titration with acid from pH 12.0 



III Titration with acid from pH 6.4 to 2.4 



IV Back-titration with alkali from pH 2.4 



purine-pyrimidine amino and the — NH — CO- — dissociations of guanine 

 and thymine, respectively. The number of these dissociating groups is 

 known with some accuracy from the analytical results of Chargaff^° and 

 Wyatt*'-^"^ [cf. Chargaff, Chapter 10], and in Table VI the results of Lee and 

 Peacocke^"^ are summarized for three deoxypentose nucleic acids. The 

 analysis'** of the sodium salt of the deoxyribonucleic acid from calf thymus 

 for sodium shows that there is one sodium ion for every phosphorus atom, 

 and, in view of the fact that the amount of the secondary phosphoric acid 

 dissociation is small (Table VI), these must be combined largely or entirely 

 with the primary phosphoric acid dissociations. The presence of the theo- 

 retical number of amino, primary phosphoric acid, and — NH- — CO — dis- 

 sociations is in agreement with the view that the deoxypentose nucleic 

 acids have a long-chain structure in which the internucleotide bond is 

 through a phosphoester linkage. The difficulty in accepting this type of 

 linkage in view of the very different behavior of deoxypentose nucleic acids, 

 compared with ribonucleic acids, towards alkali has now been overcome by 

 the conclusion of Brown and Todd^^^ that the lability of the latter acid is 

 due to the ease of formation of a cyclic intermediate involving the adjacent 



1" D. M. Brown and A. R. Todd, J. Chem. Soc. 1952, 52. 



