THE PHYSICAL PROPERTIES OF INFECTIVE PARTICLES 269 



clieniical composition and the characteristics of the ionizable groups of the 

 macromolecule. Second, the hydrodynamic resistance to motion is compli- 

 cated by the fact that the layers of liquid immediately surrounding each 

 macromolecule are themselves subject to electric forces. The flow patterns 

 are not governed solely by viscous drag of the type considered by Stokes 

 (1851) [or by Perrin (1936) in the modified treatment for nonspherical 

 particles]. Consequently, the hydrodynamic behavior observed in diffusion 

 and sedimentation is not exhibited in electrophoresis. Owing to the ambigui- 

 ties introduced by the screening effect and the distortion of the flow patterns, 

 the net charge on a macromolecule is not evaluated directly from electro- 

 phoresis studies. Empirical correlations often prove very informative, 

 however. 



Electrophoretic investigations are of three general types, each of which 

 possesses special features commending it for use. The so-called micro- 

 electrophoresis method (Abramson et at., 1942) is based on the use of an 

 ordinary light microscope for the direct visualization of the migrating 

 particles. Since viruses or proteins are too small to be seen directly, objects 

 capable of optical resolution in the microscope are introduced into the 

 solution. For this purpose glass beads or even droplets of oil are frequently 

 employed. These objects become coated with a film of the protein or other 

 macromolecules, and the electrophoretic migration is then determined by 

 the charged groups of the macromolecules of the film. The method is simple 

 and precise, and data are obtained rapidly. In studies of the dependence of 

 electrophoretic mobihty on pH this method proves most useful. The micro- 

 electrophoresis technique, to a large extent, became obsolete with the 

 development (Tiselius, 1930) of the moving boundary method. Here the 

 motion of the macromolecules is followed by optical methods which register 

 the position and shape of the boundary between the solvent and the solution. 

 The presence of two types of molecules with different electrophoretic 

 mobilities is clearly demonstrable as two boundaries. Moreover, the amounts 

 of the two components can be evaluated with some accuracy from analyses 

 of the electrophoresis pattern. With the microelectrophoresis apparatus such 

 an analysis is virtually impossible. If each of the two components coats the 

 microscopic particles, an average mobility is obtained which often is not 

 meaningful. The moving boundary method has also found wide application 

 in the isolation and purification of macromolecules since the components in 

 a mixture became separated, depending upon their different migration rates. 

 Such preparative procedures are limited, however, because of the absolute 

 requirement that the system have gravitational stability at all times. This 

 precludes the complete separation of two species into two different zones. 

 Were this to occur, a region would be created which possessed a lower density 

 than the zone above it, i.e., somewhere in the cell there would be liquid 



