34. THE RIBONUCLEIC ACIDS OF VIRUSES 265 



phosphates by paper electrophoresis is about 1 diphosphate per 500 nucleo- 

 tides. The nucleosides and nucleoside diphosphates, which had been found 

 in earlier studies after alkali degradation of TMV-RNA, 72 could not be 

 demonstrated after the virus was dialyzed at 37°C. versus 0.1 M citrate 

 buffer at pH 7 to remove any adsorbed polynucleotides prior to the extrac- 

 tion of RNA. 73 



In the case of an RNA of mean molecular weight 1-2 X 10 6 , it is most 

 difficult, if not impossible, to decide whether the RNA is of type (I) or (II). 

 For example, in order to detect one micromole of nucleoside or nucleoside 

 diphosphate, it would be necessary to employ 1-2 gm. of RNA. This indi- 

 cates how difficult it is to determine the molecular weight of a high molecu- 

 lar weight RNA by chemical methods such as "end-group" analysis. 



(4) Properties of Ribonucleic Acid in Solution. Dependence of RNA struc- 

 ture on ionic strength and pH. TMV-RNA in solution has the typical prop- 

 erties of a polyelectrolyte. The molecular shape is strongly dependent on 

 the medium, especially on the H ion and salt concentration. Low pH values 

 and high salt concentrations decrease repulsion between charged PO~ 

 groups of the RNA molecule, and hence, increase molecular coiling and 

 association; the association may develop end to end as well as side by side. 

 This effect leads to an increase in sedimentation velocity and a decrease in 

 viscosity. The opposite situation holds for RNA in a solution free of ions 

 or in a solution of very low ionic strength, where the viscosity is extremely 

 high due to the mutual repulsion of PO~ groups, which leads to a stretching 

 of the molecules. The sedimentation constant is accordingly much lower. 

 Although infectious RNA prepared with detergents has a much lower 

 sedimentation constant (s 2 o = 6-10 *S) 4 than that prepared with phenol 

 (S20 > 20 <S), 68 it is most likely that the molecular weights are identical 

 when the above mentioned effects are considered since the detergent-ex- 

 tracted RNA was studied in nonionic solution. 



Further, as to be expected for a polyelectrolyte, the sedimentation con- 

 stant of RNA increases with decreasing solute concentration since the 

 mutual interaction of the polymer particles decreases with decreasing con- 

 centration. 



The viscosity [77] is approximately proportional to the average molecular 

 weight and to the 1.8 power of the sedimentation constant in 0.02 M phos- 

 phate buffer (see Fig. 3). Now in the case of a random coil, viscosity is 

 directly proportional to the sedimentation constant, whereas for a rigid 

 rod a reduction in molecular weight leads to a marked decrease in viscosity, 

 approximately as the square of molecular weight with little change in 



72 R. Markham, R. E. F. Matthews and J. D. Smith, Xature 173, 537 (1954). 



73 R. E. F. Matthews and J. D. Smith, Xature 180, 375 (1957). 



