56 



D. SHUGAR 



12 - 



220 



240 



X (nuij 



260 



280 



Fig. 8. Hyperchromicity resulting from specific polymer interaction, in this case 

 an equimolar mixture of poly-A and poly-U: (a), spectrum of mixture of mononucleo- 

 tides; (b), calculated spectrum for mixture of poly-A and poly-U; (c), observed spec- 

 trum for mixture of poly-A and poly-U; [R. C. Warner, J. Biol. Chem. 229, 711 (1957)]. 



nucleotides is 10,100 so that the hyperchromicity should be about 70 % 73 ; 

 an experimentally observed value 74 based on exhaustive digestion with 

 DNase and phosphodiesterase is 80 % ; this may be somewhat on the high 

 side because of the fact that following DNase digestion, the pH was raised 

 to 9.2 for diesterase digestion. 



It may, however, be concluded from the above that about 40-50% of 

 the hyperchromicity of a DNA chain is due to hydrogen bonding between 

 base pairs in the twin-stranded Watson-Crick structure. 



Insofar as single chains are concerned, the hyperchromicity at neutral 

 pH varies from 6% for poly-U to 40% for polycytidylic acid (poly-C), 

 50-55 % for poly-A, and 63 % for poly-I. 70 However, Shapiro and Chargaff 75 

 have shown that the diphosphate of thymidine (pTp) exhibits an extinction 

 coefficient about 8 % lower than that of thymidine phosphate (Tp) ; a simi- 

 lar effect most likely prevails for pUp, so that this alone may fully account 

 for the small hyperchromicity of poly-U. Such an explanation cannot apply 

 to other polymers since, e.g., diphosphates of deoxycytidine possess an ex- 

 tinction coefficient 8 % lower than for monophosphates, whereas the hyper- 

 chromicity of poly-C is about 40%. The increase in absorption of such 

 polymers under the influence of urea (see Fig. 7) or heat has been taken as 

 evidence for involvement of hydrogen bonding 72 which, in the case of poly- 



73 P. D. Lawley, Biochim. el Biophys. Ada 21, 481 (1956). 



74 R. L. Sinsheimer and J. F. Koerner, /. Biol. Chem. 198, 293 (1952). 



76 H. S. Shapiro and E. Chargaff, Biochim. et Biophys. Acta 26, 596 (1957). 



