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



disappears and there is only one position in the cell at which the concentration 

 is equal to the initial concentration. Finally, after a considerable length of 

 time, an equilibrium state is attained and the concentration distribution 

 becomes independent of time. From measurements of the concentration as 

 a function of distance, the molecular weight is calculated. Except for the 

 partial specific volume, no auxiliary information about the macromolecules 

 is necessary. In contrast, molecular weight determinations by the sedimenta- 

 tion velocity method are dependent upon knowledge of additional physical 

 properties such as the diffusion coefficient. Despite some obvious virtues of 

 a theoretical and experimental nature, the equilibrium method has been 

 eniployed only infrequently, and there are scarcely any recorded applica- 

 tions of it for the study of viruses. This can be attributed to a variety of 

 factors, principal among them being the lack of apparatus capable of sus- 

 tained, continuous operation for the long periods (days or even weeks) 

 required before equilibrium is attained. Recent innovations from the stand- 

 point of technique have altered this situation markedly. Although these 

 modifications have not as yet been applied in the study of viruses 

 and some experimental difficulties may be encountered, it is likely 

 that the sedimentation equilibrium method soon will find wide application 

 for molecular weight determinations of viruses and especially of their 

 subunits. 



An additional type of ultracentrifuge experiment has achieved wide 

 popularity in the past three years. This, Hke the sedimentation equilibrium 

 method and in contrast with sedimentation velocity studies, gives molecular 

 weights directly. We refer, here, to the Archibald method (Archibald, 1947) 

 by which molecular weights are determined from data obtained in the 

 transient states during the approach to sedimentation equilibrium. Owing 

 largely to the theoretical studies of Archibald and certain technical improve- 

 ments especially with regard to optical methods, precise determinations of 

 molecular weights are now feasible in experiments of very short duration 

 (Klainer and Kegeles, 1955; Ginsburg et al., 1956), Although equilibrium is 

 not attained throughout the cell until after many hours or days of centri- 

 fugation, the conditions for equilibrium (transport of solute by sedimenta- 

 tion equals transport by diffusion) are fulfilled at the two end surfaces of the 

 cell at all times during an experiment. Therefore, molecular weights can be 

 calculated from the approximate data obtained immediately after the ultra- 

 centrifuge rotor attains the desired speed. Numerous technical problems 

 have been solved satisfactorily, and this method appears to be among the 

 best available for molecules of the size of proteins. For macromolecules as 

 large as viruses, difficulties still exist, and the method thus far has been 

 applied to materials no larger than 4 X 10® in molecular weight. Despite 

 this, it seems likely that studies now in progress will eliminate some of the 



