322 ERWIN CHARGAFF 



proteins, and most likely as the prosthetic groups of conjugated proteins, 

 the .danger of secondary changes attending their liberation is particularly 

 great, since whatever forces had anchored them to the protein may now 

 have become free to interact. The series of degradative changes to which 

 a nucleic acid is exposed in the course of its isolation will, however, usually 

 be gradual; and, while it may not yet be possible to define the perfect 

 compound, the badly degraded one will, as a rule, be recognized.^" As in 

 all such cases, one must be satisfied with avoiding the avoidable. 



Several physical criteria (viscosity, ultracentrifugal and electrophoretic 

 behavior, light scattering, ultraviolet absorption, etc.) that are in certain 

 cases useful for a decision on the integrity of a preparation are discussed 

 in Chapters 13 and 14. Unfortunately, from most of these tests we may 

 learn how bad a given specimen is, but only rarely how good it is. This 

 applies even more to the use of biological criteria of testing, the retention 

 or loss of transforming activity''^ (see Chapter 27) ; this expedient is, in its 

 present form, far from being a generally useful guide. ^^ 



While the early workers in this field, such as Miescher and Hoppe-Seyler 

 (see Chapter 1), conjectured the macromolecular and complex character of 

 the nucleic acids that they were the first to isolate, many of the precepts 

 and safeguards of biochemical etiquette were forsaken by the succeeding 

 generations. This may have been due in part to the powerful influence that 

 the great successes of the organic chemistry of small molecules had on the 

 development of biochemistry.'' A passage from Levene's book^ (p. 251) may 

 be instructive: "At that period of their work these investigators were still 

 under the influence of the traditional belief in the unusual lability of the 

 nucleic acids, and they accordingly avoided the use of heat for the libera- 

 tion of the nucleic acid from the protein." A revival of this, by no means 

 erroneous, traditional belief took place in the last thirty years.^"'®-'^^ 



Deoxypentose nucleic acids usually are isolated from tissues in the form 

 of their sodium salts. The requirements for a satisfactory preparation may 

 be listed as follows: (a) absence of proteins, polysaccharides, lipids, etc.; 

 (b) absence of pentose nucleic acids; (c) freedom from inorganic salts and 

 other easily dialyzable impurities; (d) a phosphorus content not far from 

 9.2%; (e) a maximum of absorption in the ultraviolet at pH 7 between 

 257 and 261 mn with an 6(P) around 6600 (see Chapter 14); (f) fibrous 

 character, a high and nonthixotropic viscosity,^" presence of streaming 

 birefringence (see Chapter 13). Several other requirements could be added 

 e.g., monodispersity in the ultracentrifuge, electrophoretic homogeneity, 



90 S. Zamenhof and E. Chargaff, J. Biol. Chem. 186, 207 (1950). 



9' S. Zamenhof, H. E. Alexander, and G. Leidy, J. Exptl. Med. 98, 373 (1953). 



92 R. Feulgen, Z. physiol. Chem. 237, 261 (1935) ; 238, 105 (1936). 



" R. Signer, T. Caspersson, and E. Hammarsten, Nature 141, 122 (1938). 



