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



containing solute molecules lying above the pure solvent. Convective stirring 

 would occur immediately. Attempts to circumvent this limitation have led 

 to the development of zone electrophoresis, the third principal type of electro- 

 phoresis teclmique. Here the charged molecules migrate through an aqueous 

 solution which is held partially immobilized in the interstices of paper or 

 in the capillary space between starch granules. Paper and starch electro- 

 phoresis are examples of zone electrophoresis. The success of these methods 

 is attributable to the fine capillary spaces which are sufiiciently narrow that 

 bulk flow of liquid is effectively prevented. Many ingenious designs of 

 apparatus have been proposed and tested during the past decade, and zone 

 electrophoresis has become one of the more powerful tools available for the 

 detection and isolation of biologically interesting substances. Although zone 

 electrophoresis can be used in studies of mobilities, empirical corrections are 

 required because of the electroosmotic flow of the liquid itself through the 

 supporting medium. 



Most quantitative electrophoretic investigations have involved the 

 moving boundary method in a form somewhat like that employed by Tiselius 

 (1930), Longsworth (1945), Alberty (1953), and their collaborators. By this 

 method both large and small molecules can be examined with ease. The 

 number of components in a mixture is evaluated readily and fractionation 

 can be effected so as to identify the component responsible for a given 

 biological activity. Results are reported as the electrophoretic mobility, 

 which is the velocity of the charged particles per unit potential gradient, 

 and has the units, cm./sec. /volt/cm. By convention the mobility is given a 

 positive or negative sign in accord with the sign of the net charge of the 

 migrating ions. Perhaps the most popular application of electrophoresis is 

 the determination of the isoelectric point, the pH at which the molecules do 

 not migrate in an electric field. In this regard electrophoresis yields valuable 

 information about the interaction of macromolecules with specific ions like 

 phosphate and chloride ions. This is revealed by the dependence of the iso- 

 electric point on the amount and nature of the salts present during the 

 electrophoresis experiments. 



Despite inadequacies in the theoretical treatments for the interpretation 

 of electrophoretic mobilities, there has been spectacular success in explaining 

 boundary anomalies by means of the moving boundary theory (Longsworth, 

 1945; Dole, 1945). These anomalies include (1) different mobilities in the 

 ascending and descending limbs of the electrophoresis cell; (2) different 

 shapes of the boundary in the two limbs, with the rising boundary being 

 sharper than the descending one; (3) the presence of additional boundaries, 

 the so-called 8 and e boundaries in the ascending and descending limbs, 

 respectively; and (4) the lack of correspondence of the amount of the migrat- 

 ing material in the two limbs. Any precise electrophoretic studies must 



