1 Introduction

The study of solutions of macromolecular solutes at sedimentation equilibrium in the analytical ultracentrifuge (AUC) has a long history, and has lead to many insights into the properties and nature of these systems. Historically, the sedimentation equilibrium (SE) method has been of importance in that it has provided a method based upon sound thermodynamic principles for the definition of the molecular weight of solutes. No calibration standards are needed when SE is employed, nor are there any significant theoretical approximations to consider. Equally, the need for knowledge of ancillary parameters is minimal: the absolute solute concentration does not need to be determined, and matters such as refractive index increment need not be considered, even though refractometric optics may be used for data acquisition. In these latter respects SE is superior to an obvious alternative fundamental method (light scattering).

The basis of the SE method derives as do all sedimentation methods from the Lamm equation. A centrifugal field applied to a solution of macromolecular solute(s) results in the development of a sedimentation potential which tends to displace solute towards higher radial positions in the cell: this in turn results in the development of a gradient of concentration (and hence of chemical potential) which opposes the gradient of sedimentation potential. Eventually, an equilibrium state is attained in which the opposing gradients are exactly equal in amplitude at every point in the cell. The general theory has been reviewed by many authors, and is summarised elsewhere...