THE SIZE AKD SHAPE OP VIRUSES 2? 



In Figure 22 the results of an ultracentrifugation experiment on influenza 

 7irus are shown. 



^1' 



5.8 6,0 6.2 6.4 66 6,8 



Distance from ixis of rotation in cm. 



FIGURE 22 - TRACINGS OF SEDIMENTATION DIAGRAMS OBTAINED 

 BY SCHLIEREN METHOD WITH PR 8 INFLUENZA A VIRUS. (M.A.Laxif f er 

 and W.M. Stanley, J. Exp. Med. 80, 53I (1944) ). 



The boundary between virus solution and solvent is represented by a peak. The 

 center of the peak tells where the center of the boundary is at a given time 

 and the sharpness of the peak tells how foggy the boundary is. The successive 

 peaks represent the boundary at five minute intervals after the experiment was 

 started. From the displacement of . the peak centers and the speed of the centri- 

 fuge, the sedimentation constant can be calculated to be 722 x 10"13 cm. per 

 second per unit field, or 722 Svedberg units. From the progress- 



ively greater breadth of the curves, it is possible to determine the degree of 

 boundary fogging, and from that to get an idea of the degree of inhomogeneity 

 of the virus. The partial specific volume is 0.79 c c /grams . This corresponds 

 to a dry density of 1.26 g/cc , With this figure and the value of the sedimen- 

 tation constant, it Is possible to calculate, using equation (6), that the 

 disuneter of the influenza virus particle is 70 millimicrons. This must be com- 

 pared with the value of 115 obtained with the electron microscope. Obviously 

 something is wrongi The difficulty could be attributed to either of two possi- 

 ble causes. First, the assumption that the particles are spherse could be wrong. 

 However, this is unlikely in view of the results obtained with the electron mi- 

 croscope. Second, the density of the particles in solution may not be as great 

 as in the dry state. That is, the influenza particle in suspension might soak 

 up a certain amount of water, thereby increasing its size and decreasing its 

 density. This idea seems plausible, and it can be tested by using the ultra- 

 centrifuge. 



The rate of sedimentation of a particle suspended in a solvent is direct- 

 ly proportional to the differences between the density of the particle and the 

 density of the solvent. This follows from equation (6) or from a slightly re- 

 arranged form of equation (10). Thus, if influenza virus particles were 

 suspended in a solvent with density equal to that of the virus, they would fail 

 to sediment, no matter how fast the centrifuge was run. Also, if the particles 

 were suspended in a medixim more dense, sedimentation would take place in a re- 

 verse direction. Just as in the case of cream in a separator. In fact, if the 

 sedimentation of the particles is measured in several solvents of different 

 densities, one should get a straight line when the sedimentation rate is plotted 

 against the density of the meditim, provided the particle has the same density 

 In all the media. Figure 23 presents the results of an experiment in which 

 influenza virus was dissolved in sugar solutions of various densities and then 

 studied in the ultracentrifuge. 



