THE PHYSICAL PROPERTIES OF INFECTIVE PARTICLES 255 



existing complications, thereby permitting tlie Archibald method to be used 

 routinely with viruses. 



6. Sedimentation Velocity Method, i. Sedimentation Coefficient. Just as in 

 diffusion, so in sedimentation, a transport equation can be written for the 

 amount of material crossing a given surface in a centrifuge cell which is 

 rotating at some fixed angular velocity. This transport equation relates the 

 mass transported per unit time to the area of the surface, the concentration 

 of the solute, the magnitude of the centrifugal field, and the velocity of the 

 molecules per unit field, this last term usually being described by s, the 

 sedimentation coeflicient. In effect, the transport equation serves as a 

 definition of the sedimentation coefficient which is written: 



. = ^-^' (20) 



where x is the distance in centimeters from the axis of rotation, t is the time 

 in seconds, and lo is the angular velocity in radians per second. Dimensional 

 analysis show that the sedimentation coefficient has the units of seconds, 

 but it is more meaningfid to consider the units as cm./sec./dyne/gm. Sedi- 

 mentation coefficients are now reported in terms of svedbergs (S) where 

 1 S = 10-13 sec. 



Sedimentation coefficients are evaluated from measurements of the 

 position of the boundary as a function of time. For this calculation it is 

 customary to consider the integral form of Equation 20 and plot log x versus 

 t. Except for unusual materials, this plot is a straight line whose slope gives 

 s. It is interesting to note that tobacco mosaic virus (Lauffer, 1944b) serves 

 as the most prominent substance for which a plot of log x versus t is not 

 linear. Special treatments which are beyond the scope of the present review 

 are required for these materials. Usually, for convenience in the comparison 

 of results, sedimentation coefficients are reported as s^q, w which is the sedi- 

 mentation coefficient that would have resulted had the experiment been 

 conducted in a solvent having a viscosity and density equal to those of 

 water at 20°C. Actually, electrolytes must be present in the solution during 

 the ultracentrifuge experiments, if serious errors are to be avoided. Macro- 

 molecules of biological interest have ionizable groups and frequently possess 

 a net charge under the conditions of the experiment. As a result of the 

 difference in the sedimentation rate of the macro-ion and its counter-ions in 

 the solution, a potential gradient is established during sedimentation. 

 This potential gradient is fuUy equivalent to an externally applied electric 

 field, and allowances for it must be made in the force equation for sedimen- 

 tation. Analysis of this problem (Pedersen, 1958) has shown two separate 

 effects, the primary and secondary charge effects, both of which can be 

 minimized if a neutral electrolyte is present to the extent of about 0.1 molar 



VOL. I — 18 



