Molecular weight distributions by ultracentrifucation

In 1928 Rinde first pointed out the possibility of obtaining molecular weight distributions from sedimentation equilibrium experiments. Although numerous attempts were made to obtain molecular weight distributions from sedimentation equilibrium experiments, it is only recently that Donnelly and Scholte each showed a different and ingenious method for successfully obtaining molecular weight distributions. These methods are restricted to ultracentrifuge cells with sector-shaped centerpieces and to ideal, dilute solutions. The restriction to sector-shaped centerpieces can be removed, and equations analogous to those used by Donnelly or by Scholte that are applicable to nonsector-shaped centerpieces will be presented. In addition, by using some simple assumptions, equations for concentration or concentration-gradient distributions under nonideal conditions can be written; this means that these two methods for molecular weight distributions may be extended to nonideal conditions using either shape centerpiece. A sample of dextran was chosen for the experiments described in this work. The first attempt of searching for the ideal, dilute solution conditions was not successful. The final effort was concentrated on finding a better way to deal with the nonideality of the aqueous dextran solution at 25??. Results by Donnelly's method proved that the correction of nonideality with B (subscript LS), the light scattering second virial coefficient, could give a reasonable answer for the molecular weight distribution for both sector- and nonsectorshaped centerpieces. The effect of rotor speed on the molecular weight distributions may be corrected by adding another factor which depends on the rotor speed to B (subscript LS). Molecular weight distribution of the dextran sample by Donnelly's method gives a very good agreement with that provided by the manufacturer.