30. PHOTOCHEMISTRY OF NUCLEIC ACIDS 57 



A, may be between amino groups and phosphate oxygens, 76 but this is not 

 the complete explanation. 



Additional information is forthcoming from studies on smaller oligonu- 

 cleotides. A number of observers have confirmed the observations of Sins- 

 heimer 77 and deGarilhe and Laskowski, 78 that even dinucleotides exhibit a 

 hyperchromic effect, the latter authors demonstrating how this varies quan- 

 titatively from 2-11% for various pairs of nucleotides (Table I). These 

 findings have been extended in a rather elegant manner by Michelson 79 to 

 a number of small oligonucleotides with chain lengths of from 2 to about 

 13 residues. The hyperchromicity was found to increase with chain length 

 and attain a limiting value for a chain with 5 to 6 residues (Table I). From 

 a study of the variation of hyperchromicity with pH, as well as the changes 

 in dissociation constants of ionizable groups as a function of chain length, 

 it was concluded that the major cause of hyperchromicity is due to the 

 ordered stacking of the aromatic rings parallel to each other, even in di- 

 nucleotides; the resultant interaction between the ir-electron orbitals of 

 adjacent rings leads to the formation of electron orbitals extending over 

 several rings with a lower total energy, and, from a geometrical point of 

 view, also a smaller chromophoric area (cf. Lawley 73 and Laland et a/. 80 ). 



The behavior of an aromatic ring in a polynucleotide chain, under the 

 influence of absorbed radiation, will consequently depend on its neighbors 

 as well as whether it is near one end of the chain or in the interior. The 

 merging of electron orbitals of adjacent rings would also be expected to 

 lead to a transfer of absorbed energy; such a transfer has actually been 

 demonstrated in DPNH, which is analogous to a dinucleotide and which 

 also exhibits appreciable hyperchromicity (see Section V, 3). 



The very low hyperchromicity of poly-U, and the absence of any altera- 

 tion of its extinction by heat, urea, or ionic strength, has been interpreted 

 by Warner 72 as due to the absence of interaction between the bases of this 

 polymer. Photochemical studies on oligo- and polynucleotides of Up, on 

 the other hand, suggest that such interaction does exist. 69 



Use of e(P): The introduction by Chargaff and Zamenhof 81 of the term 

 e(P) for characterization of extinction coefficients of natural nucleic acids 

 was (and still is) due to the difficulty of characterization of molecular 

 weights; it was based on the fact that there is one atom P per purine or 

 pyrimidine residue. This expression has proved its usefulness, particularly 

 in the characterization of the degree of degradation of a given preparation. 



76 R. F. Beers, Jr., and R. F. Steiner, Nature 179, 1076 (1957); 181, 30 (1958). 



77 R. L. Sinsheimer, J. Biol. Chem. 208, 445 (1954). 



78 M. P. deGarilhe and M. Laskowski, J. Biol. Chem. 223, 661 (1956). 



79 A. M. Michelson, Nature 182, 1502 (1958); J. Chem. Soc. p. 1371 (1959). 



80 S. G. Laland, W. A. Lee, W. G. Overend, and A. R. Peacocke, Biorhim. et Biophys. 

 Acta 14, 356 (1954). 



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



