476 D. O. JORDAN 



different observers and the results interpreted in terms of an open-chain and cyclic 

 structure, respectively. The samples studied, however, were considerably degraded 

 owing to the methods of extraction that were employed, and the conflicting results have 

 been ascribed to the different degrees of degradation of the samples studied.^'- ss-ioo 

 The first study of an acid isolated by a mild procedure was that of Hammarsten,'"! 

 who concluded from conductivity titrations on the free acid that the latter showed 

 four acid dissociations for every four atoms of phosphorus, having the very approxi- 

 mate pK'a values of 2.4, 3.7, 4.3, and 5.2. Incomplete electrometric titration results 

 were obtained on similar preparations by Jorpes'"'' and by Stenhagen and Teorell,^"' 

 which indicated the absence of any appreciable secondary phosphoric acid dissocia- 

 tion since the solutions were found to be almost entirely unbuffered in the region 

 pH 6.0-9.0. 



More extensive studies on a carefully prepared high-molecular-weight 

 sample of the sodium salt of the deoxyribonucleic acid of calf thymus have 

 been made by Gulland et al.,^^ whose results have been confirmed by Signer 

 and Schwander,^^ Cosgrove and Jordan,^"* and Lee and Peacocke'"^ for the 

 deoxypentose nucleic acids from lamb thymus, herring sperm, wheat germ, 

 and mouse sarcoma. The titration curve of the sodium salt of the deoxy- 

 ribonucleic acid of calf thymus is shown in Fig. 15. The addition of acid 

 or alkali to the solution in water does not at first bring about the ionization 

 of groups between pH 5.0 and 11.0, but outside these limits there occurs a 

 rapid liberation of groups titrating in the ranges pH 2.0-6.0 and pH 9.0- 

 12.0. On back-titration, either with acid from pH 12.0 or with alkali from 

 pH 2.5, a curve is obtained which is different from that representing the 

 initial (forward) titration and which exhibits a well-defined point of inflec- 

 tion in the neutral region and shows incipient points of inflection in the 

 regions of pH 12.0 and 2.0 corresponding, respectively, to approximately 

 2.0 equivalents of alkali and 3.0 equivalents of acid for each four atoms of 

 phosphorus. Gulland et al}^ found that the same back-titration curve was 

 obtained irrespective of whether the titration was commenced at pH 12.0 

 or 2.5. Lee and Peacocke,^^^ however, found that this was not so and that 

 slightly different curves were obtained on back-titrating from the different 



s^K. Makino, Z. physiol. Chem. 232, 229 (1935). 



95 H. Bredereck, M. Kothnig, and G. Lehmann, Ber. 71, 613 (1938). 



98 H. Bredereck, M. Kothnig, and G. Lehmann, Ber., 72, 121 (1939). 



" L. Ahlstrom, H. von Euler, I. Fischer, L. Hahn, and B. Hogberg, Arkiv. Kenii. 



Mineral. Geol. A20, No. 15 (1945). 

 "» G. Schmidt, E. G. Pickels, and P. A. Levene, J. Biol. Chem. 127, 251 (1939). 



99 S. S. Cohen, J. Biol. Chem. 146, 471 (1942). 



"» J. M. Gulland, G. R. Barker, and D. O. Jordan, Ann. Rev. Biochem. 17, 175 (1945). 



'»! E. Hammarsten, Biochem. Z. 144, 383 (1924). 



"2 E. Jorpes, Biochem. J. 28, 2102 (1934). 



'"3 E. Stenhagen and T. Teorell, Trans. Faraday Soc. 35, 743 (1939). 



1"^ D. J. Cosgrove and D. O. Jordan, /. Chem. Soc. 1949, 1413. 



'9* W. A. Lee and A. R. Peacocke, /. Chem. Soc. 1951, 3361. 



