PHYSICAL PROPERTIES OF NUCLEIC ACIDS 481 



of fairly strong reagents and the nucleic acid has consequently suffered deg- 

 radation. It is not known, therefore, whether the pentose nucleic acids 

 exist in a similar hydrogen-bonded structure to that of the deoxypentose 

 nucleic acids. 



Early work by Levene and Simms,^^ Jorpes,^°- Makino,^^ and Bredereck 

 et a/."^'''^ had shown that the ribonucleic acid of yeast contained three to 

 four ionizable groups per four atoms of phosphorus on titration to pH 8.0. 

 The first systematic study was that of Allen and Eiler,i'^ who were the first 

 to observe that the titration curve indicated the presence of three amino 

 groups for every four atoms of phosphorus, together with one group titrat- 

 ing in the range pH 5.0-7.5 and which they regarded as a primary phos- 

 phoric acid group that had been weakened by virtue of the ionization of 

 the remaining phosphoric acid dissociations. Fletcher et al.,^^^ who obtained 

 titration curves (Fig. 17) very similar to those of Allen and Filer, inter- 

 preted their results as indicating the presence, in the ribonucleic acid of 

 yeast, of three amino groups, one secondary phosphoric acid, two — NH — 

 CO — -, and three primary phosphoric acid dissociations for every four 

 atoms of phosphorus. The pK'a values of the nucleotides were assumed to 

 be unchanged in the nucleic acid. This conclusion was confirmed by the 

 titration curve of the deaminated acid (Fig. 17); the removal of the amino 

 groups permits the direct titration of the primary phosphoric acid dissocia- 

 tions, and the curve indicates the presence of three primary and one sec- 

 ondary phosphoric acid dissociations, the — NH — -CO — dissociation of 

 xanthosine which titrates in the range pH 5.0-7.0 (pK'a 6.0), and three 

 — NH— CO — dissociations titrating in the range pH 8.0-12.0. These re- 

 sults suggest that, on average, three secondary and one primary phosphoric 

 acid groups are utilized in yeast ribonucleic acid for every four atoms of 

 phosphorus, in forming the phosphoester internucleotide bond. This con- 

 clusion indicates that one in every four atoms of phosphorus (on average) is 

 triply esterified. The shape of the electrometric titration curve has been 

 confirmed by several workers, '^^"'", who have generally interpreted their 

 results in the same way although there is no general agreement about the 



"2 H. Bredereck and M. Kothnig, Ber. 72, 121 (1939). 

 "3 H. Bredereck and I. Jochman, Ber. 75, 395 (1942). 

 "4 F. W. Allen and J. J. Eiler, J. Biol. Chem. 137, 757 (1941). 

 "5 W. E. Fletcher, J. M. Gulland, and D. O. Jordan, J. Chem. Soc. 1944, 34. 

 ^'^ H. Chantrenne, Bull. soc. chim. Beiges 55, 5 (1946). 



1" H. Chantrenne, K. Linderstr0m-Lang, and L. Vandendriessche, Nature 159, 877 

 (1947). 



118 L. Vandendriessche, Com-pt. rend. trav. lab. Carlsberg, Ser. chim. 27, 341 (1951). 



119 Y. Khouvine and J. Gregoire, Bull. soc. chim. biol. 28, 424 (1944). 



120 C. A. Zittle, J. Biol. Chem. 166, 491 (1946). 



121 G. Wiener, E. L. Duggan, and F. W. Allen, /. Biol. Chem. 185, 163 (1950). 



1" L. F. Cavalieri, S. E. Kerr, and A. Angelos, /. Am. Chem. Soc. 73, 2567 (1951). 



