THE PHYSICAL PROPERTIES OF INFECTIVE PARTICLES 



265 



Fig. 4. Diagrams of Separation Cells. 



a. Fixed partition cell (Tiselius, Pedersen and Svedberg, 1937). In this cell a fixed, porous, 

 metal plate is built into the cell at a position about 2/3 of the distance from the centri- 

 petal surface. Filter paper is overlaid on the metal plate, and this unit acts as a barrier 

 against mixing of the contents of the upper and lower compartments at the conclusion 

 of the experiment. The sedimentation of the macromolecules occurs through the filter 

 paper-plate combination. 



b. Movable partition cell (Yphantis and Waugh, 1956). The partition m this cell is sup- 

 ported by two synthetic rubber strips which act as a spring. Under the influence of the 

 centrifugal field developed during acceleration of the rotor, the plate, which is solid and 

 plastic, settles to the bottom of the cell. Sedimentation then occurs imdisturbed by the 

 presence of the barrier at the bottom of the cell. At the conclusion of the run, during 

 deceleration, the rubber springs cause the plate to rise slowly to its rest position thereby 

 effecting a separation of the contents of the cell into centripetal and centrifugal fractions 

 which can be removed readily without intermixing. 



c. Sedimentation Equilibrium. As early as 1938 careful sedimentation 

 equilibrium experiments were performed with bushy stunt virus (McFarlane 

 and Kekwick, 1938). Actually some work had been conducted prior to that 

 time with tobacco mosaic virus (Eriksson-Quensel and Svedberg, 1936), but 

 details of these studies were not given. Despite the promismg results obtained 

 in the experiments with bushy stmit virus, no other studies appear to have 

 been made in the ensuing twenty years. This work with bushy stunt virus is 

 particularly mteresting, even though the results seem to be erroneous, 

 because it illustrates some of the important aspects of experimentation by 

 the sedimentation equihbrium method. Very low centrifugal fields must be 

 employed; otherwise the solute molecules (if they are of the size of most 

 viruses) will be sedimented to the bottom of the cell. Also extremely short 

 columns of solution are necessary if prohibitively long experiments are to be 

 avoided. With a column height of only 2 mm. (as contrasted with the colunms 

 of about 15 mm. height that are used for sedimentation velocity experiments) 

 sedimentation equilibrium was attained in about 48 hours. As a result of 

 recent developments in instrumentation, routine operation of the ultracentri- 

 fuge for such time periods is now commonplace. Moreover, the risk of in- 

 activation of the virus is minimized by operation at low temperatures. 



