THE PHYSICAL PROPERTIES OF INFECTIVE PARTICLES 253 



in which the solute molecules are present in uniform concentration, almost 

 equal to the initial concentration; this zone is called the plateau region. 

 Between these two is a transition zone in wliich the concentration varies 

 with distance from the axis of rotation. This transition zone is called the 

 boimdary; the sedimentation velocity method is based generally on observa- 

 tions, by optical methods, of the rate of movement of such boundaries (see 

 Fig. 3). Although it Avill not be demonstrated here, it can be shown that the 

 movement of a bomidary in such an experiment provides a direct measure- 

 ment of the net migration of solute molecules in the plateau region. Thus, the 

 directed movement of individual molecules can be measured conveniently 

 and accurately despite the absence of techniques for the visualization of 

 individual molecules. From the rate of movement of a botmdary, a quantity 

 termed the sedimentation coefficient is calculated. This is related to the size, 

 shape, and other properties of the solute molecules. In conjunction with 

 other data, particularly the diffusion coefficient, the sedimentation velocity 

 method serves as one of the more popular techuiques now available for mole- 

 cular weight determinations. Moreover, the presence of separate, discrete 

 components is detected easily by the appearance of several boundaries, each 

 representing a different molecular species. Finally, the amount of each com- 

 ponent can be evaluated and an approximation can be made of their mole- 

 cular weights. Owing to diffusion, the bomidaries widen continuously in a 

 predictable manner during a sedimentation velocity experiment. For pure 

 materials, this blurring of the boundary can be employed for the calculation 

 of diffusion coefficients. Alternatively, the spreading of the boundary can be 

 used as an extremely sensitive indicator for the study of the homogeneity 

 of the sedimenting material. In this respect, the sedimentation velocity 

 method is unrivaled among existing techniques employed for the examination 

 of macromolecules. 



The second type of ultracentrifuge experiment, mvolving much lower 

 centrifugal fields and known as the sedimentation equilibrium method, 

 impHcates diffusion more directly, so that a balance is achieved ultimately 

 between the transport by sedimentation in a centrifugal direction and the 

 countertransport by diffusion in the centripetal direction. During the first 

 stages of a sedimentation equilibrium experiment, the concentration de- 

 creases at the meniscus and increases at the ceU bottom, owing to sedi- 

 mentation. As a consequence of back diffusion, however, a region totally 

 devoid of solute is not created near the meniscus as in the sedimentation 

 velocity method. Instead, the concentration remains nonzero and finite 

 everywhere as long as the centrifugal field is not too large. In the center of 

 the cell, during the early stages of a sedimentation equilibrium study, the 

 concentration is independent of position and practically the same as the 

 initial concentration. As the experiment progresses, the plateau region 



