268 H. K. SCHACHMAN AND R. C. WILLIAMS 



the ultracentrifuge at extremely low speeds should soon allow the Archibald 

 method to be applied as routinely to virus particles as it is now applied to 

 virus subunits (Hersh and Schachman, 1958). 



6. Electrophoresis 



Electrophoresis, Hke sedimentation and diffusion, involves the measure- 

 ment of the migration of molecules (or ions) under the influence of a driving 

 force. In diffusion and sedimentation the driving force stems from a con- 

 centration gradient and from a centrifugal field, respectively, while in electro- 

 phoresis an electric field is employed to cause the directed movement of the 

 charged molecules. Unlike the other two methods discussed above, the 

 results of electrophoresis experiments cannot be interpreted in an entirely 

 satisfactory maimer in terms of the molecular parameters of the migrating 

 particles. For this reason, only a brief discussion of electrophoresis is pre- 

 sented here. The brevity of the remarks should not be taken as implying 

 that electrophoresis is only of moderate usefulness to virologists or protein 

 chemists. Indeed, in the study of the purity of macromolecules, for example, 

 electrophoresis has proved to be a powerful tool comparable in sensitivity to 

 the sedimentation velocity method. Electrophoretic investigations comple- 

 ment the other hydrodynamic methods by virtue of their utilizing not the 

 size and shape of the migrating particles but, rather, the nature and number 

 of the iouizable groups on their surfaces. Whereas a mixture of strains of 

 tobacco mosaic virus may appear homogeneous by the criteria of sedimenta- 

 tion and electron microscopy, electrophoretic examination reveals distinct 

 molecular species (Smger et at., 1951). Also electrophoresis provides im- 

 portant information relevant to the location of nucleic acid within a virus 

 particle. 



The appHcation of an electric field to a solution of charged molecules 

 creates a force on the molecules which is directly proportional to their net 

 charge and to the strength of the electric field (the potential gradient). This 

 force causes the molecules to move, rapidly attaining a limiting velocity 

 that is fixed by the driving force and the frictional resistance experienced by 

 the moving particle. It is tempting, therefore, to write equations similar to 

 those employed in the treatment of sedimentation. However, this approach 

 is not fruitful for two reasons. First of all, the effective net charge on the 

 macromolecule is not dictated solely by its own chemical composition. 

 Electrophoretic experiments are conducted in salt solutions. As a result of 

 electrostatic forces, ions from the medium are attracted toward the surface 

 of the charged macromolecules with ions of charge opposite to the macro- 

 molecule predominating in the neighbourhood of the material in question. 

 This ionic cloud causes a partial screening of the total charge of the macro- 

 molecule. Thus the effective charge is less than that expected from the 



