28 



VIRUSES 



1.1 1.2 



Density ot' solvent 



FIGURE 23 - SEDniENTATION RATES IN SVEDBERG UNITS OP PR 8 

 INFLUENZA A VIRUS IN SUCROSE SOLUTIONS OF DIFFERENT DENSITIES. 

 (K.A- Lauffer and W.LI. Stanley, J. Exp. V.ed. 80, 531 (194-4 J ;. 



Here is plot 

 solvent. Wh 

 ments toward 

 to l.lB, it 

 moves in a n 

 clearly that 

 sugar soluti 

 state, the d 

 virus must c 



ted the sedimentation rate of the virus against the density of the . 

 en the density of the solvent is less than l.lS, the virus sedi- 



the periphery of the centrifuge, in a solvent with a density equal 

 does not sediment at all, and in solvents more dense than 1.18, it 

 egative direction - toward the axis. This experiment shows quite 



the density of a virus particle is l.lfi when it is suspended in a 

 on with a density of l.lo. It must be remembered that in the dry 

 ensity of the virus particle is 1.26. Therefore, in solution the 

 ontain some v/ater. 



There is something peculiar about these results, however. If the density 

 of the virus particle remains constant, one should get a straight line when 

 sedimentation rate is plotted against solvent density. But here a curved line 

 was obtained. This is evidence that the density of influenza virus is not a 

 constant, but changes as the sugar content of the surrounding medium changes. 

 This suggests that, in solvents containing very little sugar, the density of 

 the virus Is even less than l.l8. In other words, in the presence of low sugar 

 concentrations, the virus contalnp more water thsm in the presence of high sugar 

 concentrations. The density of the virus particle in a solvent free of sugar 

 can be estimated by drawing a straight line tangent to the curve at the point 

 representing zero concentration. This line will intersect the zero level of 

 sedimentation rate at a point corresponding to the density of the sugar solu- 

 tion In which the virus would float if the virus did not increase in density as 

 the sugar concentration increases. One finds a value of about 1.1, showing 

 that the density of the virus in a solvent free of sugar is about 1.1. A part- 

 ial reason that the virus density increases as the sugar content is increased 

 is obvious. The sugar solution has a high osmotic pressure, and it accordingly 

 draws water out of the virus. The higher the sugar content, the more water is 

 drawn out. A more accurate way of determining the density of the virus in 

 solution is to utilize a material for varying the density of the medium which 

 does not affect the osmotic pressure. Sharp and other associates of ileard at 

 Duke University utilized a protein, serum albumin, for this purpose. Their data 

 =ire shown in Figure 24. 



