Quantifying the concentration dependence of sedimentation coefficients for globular macromolecules: a continuing age-old problem

This retrospective investigation has established that the early theoretical attempts to directly incorporate the consequences of radial dilution into expressions for variation of the sedimentation coefficient as a function of the loading concentration in sedimentation velocity experiments require concentration distributions exhibiting far greater precision than that achieved by the optical systems of past and current analytical ultracentrifuges. In terms of current methods of sedimentation coefficient measurement, until such improvement is made, the simplest procedure for quantifying linear s - c dependence (or linear concentration dependence of 1/ s ) for dilute systems therefore entails consideration of the sedimentation coefficient obtained by standard c ( s ), g* ( s ) or G ( s ) analysis) as an average parameter ( s ) that pertains to the corresponding mean plateau concentration (following radial dilution) ( c ) over the range of sedimentation velocity distributions used for the determination of s . The relation of this with current descriptions of the concentration dependence of the sedimentation and translational diffusion coefficients is considered, together with a suggestion for the necessary improvement in the optical system.