ISOLATION AND COMPOSITION OF DEOXYPENTOSE NUCLEIC ACIDS 339 



The observations of the spectral changes attendmg the denaturation and 

 subsequent degradation of the nucleic acids start from the fact that the 

 extinction of intact preparations is lower than would correspond to the 

 sum of their constituent mononucleotides. The hyperchromic effect of 

 degradation has been discussed for pentose nucleic acids by Magasanik 

 and Chargaff.''^ [Compare Chapters 11 and 14.] As regards deoxypentose 

 nucleic acids, it was Kunitz-^* who first described the intensification of the 

 extinction brought about by deoxyribonuclease. Similar effects produced 

 by acid, alkali, heat, or the addition of salts have been studied fre- 

 quently, '^"-'^^ in greatest detail by Thomas.'*' Owing to the lability of the 

 pentose nucleic acids, the optical effects accompanying their denaturation 

 and their degradation can hardly be separated; and in the deoxypentose 

 nucleic acids, too, a further-reaching chemical degradation, namely, the 

 removal of the purines, has been shown to have a hyperchromic effect.'^" 

 But it may be concluded that for such effects to become noticeable in 

 deoxypentose nucleic acids a relatively mild trea.tment affecting only 

 secondary valence bonds is sufficient.'*' 



The transforming activity of certain bacterial deoxypentose nucleic acids 

 is discussed in Chapter 27. Reference may be made here, however, to the 

 curious observation by McCarty that the transforming substance of Pneu- 

 mococcus Type III is inactivated reversibly by ascorbic acid.'''' 



The end results of chemical and enzymic degradation need not concern 

 us here; they are discussed, in different contexts, in Chapters 5, 12, and 15. 

 Certain aspects will also later be touched upon in this chapter as far as they 

 bear on questions of composition and structure. What should be mentioned 

 here — but it can only be done in the briefest form^ — is the existence of 

 numerous studies on the degradation of deoxypentose nucleic acids by 

 irradiation with ultraviolet light, "^-'^ by treatment with X-rays,^""-^"* 



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