Some Modern Aspects of Ultracentrifugation

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Abstract
Shortly after the ultracentrifuge was developed, it was realized that molecular-weight distributions (MWDs) of polymers could be obtained from sedimentation equilibrium experiments. Although numerous attempts have been made to obtain MWDs from sedimentation equilibrium experiments, the results were not very satisfactory, and most MWDs were obtained from sedimentation velocity experiments. Only recently have some satisfactory methods been developed for sedimentation equilibrium experiments. These methods were restricted to ideal, dilute solutions and to ultracentrifuge cells with sector-shaped centerpieces. Both of these restrictions can now be removed. Methods for correcting for nonideal behavior are shown. Procedures for obtaining MWDs from sector—or nonsector—shaped centerpieces are shown. These procedures are illustrated with real examples, and a comparison between MWDs obtained by sedimentation velocity, sedimentation equilibrium, and gel permeation chromatography experiments is shown. Self-associations can be studied by various thermodynamic methods (osmometry, light scattering, or sedimentation equilibrium) that give average or apparent average molecular weights as a function of associating solute concentration. Of the various thermodynamic methods, the sedimentation equilibrium experiment is the best way to study self-associations. Because of the interrelation between average or apparent average molecular weights, the theory developed originally for the sedimentation equilibrium experiment can be extended to other methods. We show methods for analyzing several types of self-associations, using real examples. The advantages of thermodynamic over transport methods for studying self-associations are discussed; furthermore, we show how thermodynamic and transport experiments can be combined to yield more information about the self-associating species.

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