van Holde - Weischet Tutorial

Summary

The van Holde - Weischet method is a powerful analysis method, which should be used whenever unknown samples are measured by sedimentation velocity. It excells at identifying heterogeneity, sample composition, aggregation and concentration dependency of S. By comparing G(s) distributions from multiple experiments and concentrations, it is further possible to identify self-association behaviour, stoichiometry of binding, as well as subtle changes due to pH changes or conformational changes. Its greatest strength is that it can correct reliably for diffusional boundary spreading and thus reveal the true heterogeneity present in a sample.

Just like any other method, the van Holde - Weischet analysis is not a panacea. There are several limitations that are better handled by other methods:

• No reliable diffusion coefficients: While it is possible for some mono- and pauci-disperse systems to arrive at diffusion coefficients for these components, the measurement of diffusion coefficients by this method is not very reliable. In such cases it is much better to use a global nonlinear fitting method to a finite element solution. Because of this accurate molecular weights are also difficult to determine with this method.
• Boundary conditions are not considered in the solution: This fact limits the analysis to scans that contain a stable plateau and have cleared the meniscus. Therefore, approach-to-equilibrium data as well as very large S-value ranges are difficult to analyze in a single experiment using this method.
• Unlike the time difference method by W. Stafford, this method does not eliminate time-invariant noise such as window scratches and dn/dc heterogeneity in cell windows when analyzing interference data.