392 B. MAGASANIK 



the possible degradation of the material by acid can be avoided. More- 

 over, sodium nucleate dissolves in water to give a clear, colorless solution, 

 whereas free PNA is almost insoluble. 



The purity and homogeneity of PNA preparations can be determined 

 by physical and chemical methods. The former are discussed in Chapters 

 13 and 14; so far they have not been used to any great extent for the char- 

 acterization of PNA. The behavior of a number of PNA preparations in 

 the ultracentrifuge has been recorded. Cohen and Stanley observed that 

 PNA, freshly isolated from tobacco mosaic virus, sedimented from a 1 .0 % 

 solution in 0.2 M NaCl at pH 4.9 with a constant S2o,w of 5.9." The mate- 

 rial was found to be inhomogeneous. Similarly high sedimentation con- 

 stants were obtained by Grinnan and Mosher''^ with a preparation of rat 

 liver PNA, and by Kay and Bounce with rabbit liver PNA.^^ Comparison 

 of these results is difficult as different concentrations of PNA as well as 

 solvents of different ionic strength and pH were used. The importance of 

 the composition of the solvent is shown by the observations of Grinnan 

 and Mosher: their preparation was polydisperse in sodium chloride solution 

 but monodisperse in water. Volkin and Carter-* observed a doubling of the 

 sedimentation constant of a preparation of rabbit liver PNA when the pH 

 was lowered from 6.7 to 4.8. At the higher pH, several of their preparations 

 from different animal organs appeared to sediment as a single boundary 

 with Sao.w of about 2.3. 



The behavior of liver PNA, yeast PNA, and pancreas PNA on dial- 

 ysis has been studied by Magasanik and Chargaff*'* and by Kerr and 

 Seraidarian." In all cases the isolated PNA preparations contained dialy- 

 zable fractions amounting to 10-25% of the preparation. The dialyzable 

 material consisted of polynucleotides. In the case of yeast PNA the compo- 

 sition of this fraction did not differ materially from that of the undialyzable 

 material, while in the case of the animal PNA a marked difference was 

 observed.^* 



These few observations indicate that PNA preparations are far from 

 homogeneous. The lack of homogeneity of these preparations may be due 

 in part to their decomposition on storage. The stability of PNA has not 

 been carefully investigated, but isolated observations indicate that they 

 are indeed very labile substances. Some preparations cannot be stored 

 even in the cold. Thus, freshly isolated PNA of tobacco mosaic virus 

 (Nucleate A) whose molecular weight was estimated at 300,000, spon- 

 taneously decomposed at 4° and pH 7.0 in less than one week to give 

 material with a molecular weight of approximately 60,000." Similarly, the 

 viscosity of freshly prepared solutions of highly polymerized rat liver PNA 

 decreased with time, indicating spontaneous degradation. ^^ Beef pancreas 

 PNA isolated by Kerr and Seraidarian could not be dried without partial 

 decomposition." It has been mentioned before that this pancreas PNA 



