ISOLATION AND COMPOSITION OF DEOXYPENTOSE NUCLEIC ACIDS 341 



The controlled action of deoxyribonucleases leads to breakdown products 

 of an entirely different character, namely, to a mixture of large oligonucleo- 

 tides, more resistant to enzymic action than the bulk of the nucleic acid 

 molecule and originally designated as the "core."-^- This name has given 

 rise to some criticism; but, since it seems to be more often used by its 

 critics than by its proponents, it may at least have the virtue of terseness.^^' 

 Other possible designations would be "limit polynucleotide" or "enzyme- 

 resistant residue." 



2. Preferential Removal of Purines 



a. Thymic Acid 



The action of dilute mineral acid at 80° or 100° on deoxy pentose nucleic 

 acid yields a rather ill-defined degradation product which, as its original 

 discoverers believed it to contain only thj'mine, was designated "thymic 

 acid."-''' It was later, however, shown to contain also cytosine.^'* -'^ Prod- 

 ucts of this type, often of varying, always of incompletely known, composi- 

 tion have played an important part in discussions of the structure of 

 deoxypentose nucleic acid.''*^-^'^---" Their role in the nucleal reaction of 

 Feulgen also has often been considered. [Compare Swift, Chapter 17.] In 

 the course of their studies on partial degradation products and on the 

 formation of apurinic acid Tamm et a/.'" '^^-^ prepared and analyzed a 

 series of partial degradation products and compared their composition with 

 that of the parent calf thymus sodium deoxyribonucleate. Their studies 

 should be consulted for details. The rate of dialysis of the liberated purines 

 and the composition of the dialysate, when calf thymus nucleic acid is 

 exposed to the conditions leading to the formation of apurinic acid (pH 1.6, 

 37°), are shown in Fig. 2. 



b. Apurinic Acid 



A typical preparation of apurinic acid from the sodium deoxyribonucleate 

 of calf thymus will be described here. 



(/) Preparation.^'"' To a solution of 105.0 mg. of the sodium nucleate (lyophilized, 

 moisture content 13%) in 42.5 cc. of water a total of 12.5 cc. of 0.1 A'^ aqueous HCl was 



«'2 S. Zamenhof and E. ChargafT, J. Biol. Chem. 178, 531 (1949). 



^^^ CORE: "A central part of different character from that which surrounds it." 



(Oxford English Dictionary, Vol. II, p. 990.) 

 2'^ A. Kossel and A. Neumann, Ber. 26, 2753 (1893) ; Z. physiol. Chem. 22, 74 (1896-97). 

 2'^ H. Steudel and P. Brigl, Z. physiol. Chem. 70, 398 (1910-11). 

 "« R. Feulgen, Z. physiol. Chem. 101, 296 (1917-18). 

 2" H. Steudel and E. Reiser, Z. physiol. Chem. Ill, 297 (1920). 

 2'8 S. J. Thannhauser and B. Ottenstein, Z. phijsiol. Chem. 114, 39 (1921). 

 2" H. Bredereck and G. Miiller, Ber. 72, 115 (1939). 



"0 J. M. Gulland, m Nucleic acid. Symposia Soc. Exptl. Biol. 1, 1 (1947). 

 «' C. Tamm, H. S. Shapiro, R. Lipshitz, and E. Chargaff, J. Biol. Chem. 203, 673 



(1953). 



