ISOLATION AND COMPOSITION OF PENTOSE NUCLEIC ACIDS 393 



TABLE II 

 The Ultraviolet Extinction at 260 mp of PNA 



preparation contained nucleotide fragments which could be removed by 

 dialysis in the cold. Continuation of the dialysis at room temperature 

 resulted in the appearance of more polynucleotides in the dialysate, indi- 

 cating lack of stability at room temperature. 



The extinction of ultraviolet light is a characteristic property of nucleic 

 acids. [Cf. Beaven, Holiday, and Johnson, Chapter 14.] The chromophores 

 are the conjugated double bond systems of the purines and pyrimidines. 

 The ultraviolet extinctions of nucleic acids and nucleotides are conveniently 

 expressed as e(P), the extinction of a solution containing one gram-atom 

 of nucleotide phosphorus per liter." The e(P) values for a number of 

 different PNA preparations are summarized in Table II. It can be seen 

 that the ultraviolet extinction of the PNA preparations is less than the 

 sum of the extinctions of the mononucleotides to which they can be hydro- 

 lyzed by alkali. It seems that the polymerization of mononucleotides to 

 form PNA is accompanied by the suppression of certain chromophores. 

 The level of polymerization at which this effect becomes noticeable cannot 

 be defined. It is not due to the simple union of a few mononucleotides, 

 since the dialyzable polynucleotides show the same extinction as the mix- 

 ture to which they can be hydrolyzed by alkali. On the other hand, the 

 highly polymerized PNA preparations obtained by the use of sodium 

 dodecyl sulfate do not show a greater degree of suppression of extinction 

 than the less highly polymerized preparations obtained by salt extraction. 

 Still, the measurements of the increase in ultraviolet absorption obtained 

 by exposure of PNA to alkali would appear to be of value in the charac- 

 terization of such preparations. 



The chemical methods used to characterize PNA include the estimation 

 of the nucleotide composition, which will be discussed separately, the 

 determination of nitrogen, of total and acid-hydrolyzable phosphorus, and 

 of pentose. The nitrogen and total phosphorus content differs with the 

 nature of the sample. Free PNA contains about 15-16% nitrogen and 

 8.5-9% phosphorus, while the values for sodium nucleates range around 



" E. Chargaff and S. Zamenhof, J. Biol. Chem. 173, 327 (1948). 



