196 HUBERT S. LORINQ 



d. Formation of Di- and Oligonucleotides 



Considerable attention is given in the earlier literature'^^*^^ to the occurrence of di- 

 and oligonucleotides as hydrolytic products of nucleic acids. The existence of such 

 compounds, however, was largely disproved when the acid-resistant pyrimidine nu- 

 cleotide fraction, thought to be a cytidylic uridylic dinucleotide, was successfully 

 separated into cytidylic and uridylic acids. Accordingly the reported isolation bj' 

 Thannhauser^® of a trinucleotide after enzymic hydrolysis was never accepted by the 

 Levene school. Recent evidence of the occurrence of products larger than mononu- 

 cleotides in ribonuclease digests of PNA and deoxyribonuclease digests of DNA is 

 considered in subsequent sections of the book. [Cf. Chapters 6, 8, 10, 11, and 15.] 



It has, in addition, been shown" that several dinucleotides and one trinucleotide 

 are formed when yeast PNA is treated at room temperature with 15 parts of 6 A^ HCl 

 for 3 minutes at room temperature. The products were successfully fractionated on 

 Dowex 1 (chloride) and were characterized as 5'-[(6 or a)-guanylyl]cytidylic acid b, 

 5'-[(b or a)-adenylyl]cytidylic acids a and b, 5'-[(b or a)-cytidylyl]cytidylic acid b, 

 uridylic acid-cytidylic acid dinucleotide, and adenylic acid diguanylic acid trinucleo- 

 tide. 



2. Acid Hydrolysis of DNA 

 a. Liberation of Purine Bases 



The purine bases of DNA are easily removed by mild acid treatment 

 (heating the free nucleic acid in 2 % solution at boiling water bath tempera- 

 ture for 10 minutes) apparently without complete degradation of the origi- 

 nal polynucleotide structure. The material remaining was early recognized 

 as a complex substance. '^^®^ It was believed free of cytosine as well as ade- 

 nine and guanine and was named thymic acid^^ because presumably the 

 only base remaining in the original polynucleotide structure was thymine. 

 Feulgen, in a highly important paper,^^ noted that a less degraded product 

 could be obtained under somewhat milder conditions than those mentioned 

 above, namely heating for 40 minutes in slightly acid solution below 80°. 

 This material gave N:P ratios corresponding to approximately equimolar 

 quantities of cytosine and thymine, which were both also isolated after 

 acid hydrolysis. Of special interest to cytology^" was the demonstration 

 that thymic acid gave a positive fuchsin test which later was correlated 

 with the occurrence of DNA in the cell nucleus. In more recent experiments 

 it was shown that the cytosine-thymine ratio of the original DNA was not 

 altered when DNA was dialyzed at 37° and pH 1.6 and the name "apurinic 

 acid" was proposed for the resulting product.^' [Cf. Chargaff, Chapter 10.] 



26 S. J. Thannhauser, Z. physiol. Chem. 91, 329 (1914). 



" R. B. Merrifield and D. W. Woolley, Federation Proc. 11, 258 (1950); J. Biol. Chem. 



197,521 (1952). 

 28 A. Kossel and A. Neumann, Z. physiol. Chem. 22, 74 (1896-97). 

 " R. Feulgen, Z. Physiol. Chem. 101, 296 (1918). 



^^ J. Brachet, "Chemical Embryology." Interscience Publishers, New York, 1950. 

 31 C. Tamm, M. E. Hodes, and E. Chargaff, /. Biol. Chem. 195, 49 (1952). 



