34. THE RIBONUCLEIC ACIDS OF VIRUSES 



267 



200 



W 



wo ■ 



TIME (MIN) 



Fig. 5. Alteration of the optical rotation and ultraviolet absorption' of TMV- 

 RNA by ribonuclease degradation; # — rotatory constant [a] at 546 m/x, X = UV 

 absorption e at 260 im* (in per cent of starting material). [A. Gierer, Z. Naturforsch. 

 13b, 477 (1958).] 



of the total hyperchromic effect of the intact molecule. From this observation, 

 Reddi 74 concluded that the molecular size is not critical for the hyperchromic effect 

 since the core has an average chain length of only 6 nucleotides. He suggested that 

 the chemical composition (purine-rich core) may influence the hyperchromic effect. 

 This effect may also be seen in synthetic oligonucleotides 77 from dinucleotides to 

 polynucleotides of an average chain length of 13. It is probably essentially due to 

 the interaction of ^-electrons of adjacent rings. Electron orbitals extending over more 

 than one unit are formed, and these are oriented perpendicular to the planes of the 

 ring systems stacked in layers above each other. This represents a new electronic 

 species. Such bond interaction between bases exists even at the dinucleotide level. 

 Since the ionizable groups participate directly in the electron system giving rise to 

 the absorption, this interaction also has an effect on the ionizable groups which is 

 expressed in a displacement of the pK value of mononucleotides, in contrast to oligo- 

 nucleotides. 



Optical rotation. High molecular weight RNA, in comparison to smaller 

 polynucleotides, has a very high optical rotation (Table VII) which is 

 larger than that due to the sum of the nucleotides. 68 This property cannot 

 be due to the individual nucleotides or to the internucleotide bonds, but 

 rather to a structure found in larger RNA molecules in solution. 



Ribonuclease degradation causes a decrease in the optical rotation which 

 is scarcely observable during the early drop in viscosity, as is also true for 



77 A. M. Michelson, Nature 182, 1502 (1958). 



