30. PHOTOCHEMISTRY OF NUCLEIC ACIDS 



55 



« 8 



2 - 



240 260 



Fig. 7. Absorption spectrum of poly-I, illustrating hyperchromicity in a single 

 polynucleotide chain under various conditions [R. C. Warner, J. Biol. Chem. 229, 

 711 (1957)]. 



in attempts to interpret the photochemical behavior of model oligonucleo- 

 tides. 68 ' 69 



The magnitude of the hyperchromic effect exhibited by high molecular 

 weight polymers is illustrated by Fig. 7 for polyinosinic acid (poly-I) under 

 various conditions. 70 The hyperchromicity of polyuridylic acid (poly-U) is 

 small (see below) so that the difference between curves a and b in Fig. 8 

 is equal to the hyperchromicity of polyadenylic acid (poly-A). 



Certain pairs of homopolymers also interact specifically under defined 

 conditions, 71 ■ n the resulting complexes exhibiting an additional decrease 

 in extinction (Fig. 8). In fact such interactions were first discovered by 

 spectral methods which are still the most suitable for quantitative studies 

 of polymer interactions. 



From Fig. 8 it will be seen that the total hyperchromicity of a twin strand 

 chain is almost 100% (actually 96%), i.e., this is the increase in absorption 

 on quantitative hydrolysis of the complex to mononucleotides. For highly 

 polymerized DXA with an e(P) of 6000, the calculated extinction of mono- 



68 K. L. Wierzchowski and D. Shugar, 4th Intern. Congr. Biochem., Vienna, 1958, 

 No. 3-35. • 



69 K. L. Wierzchowski and D. Shugar, Acta Biochim. Polon. 6, 313 (1959). 



70 R. C. Warner, J. Biol. Chem. 229, 711 (1957). 



71 D. R. Davis and A. Rich, J. Am. Chem. Soc. 80, 1003 (1958); G. Felsenfeld and 

 A. Rich, Biochim. et Biophys. Aria 26, 457 (1957). 



72 R. C. Warner, 4th Intern. Congr. Biochem., Vienna, 1958 Symposium No. 9. 



