van Holde - Weischet Tutorial

Distribution Plot

The Y-axis intercepts of the van Holde - Weischet extrapolation plot define the diffusion corrected S-values. For each division, a different fraction of the boundary is extrapolated to infinite time, and the sample sedimenting at that position in the boundary is shown with its respective S-value on the Y-axis. If the sample is homogeneous, all extrapolating lines will converge on a single point, the S-value of the (single component) sample. For multiple components, the divisions will extrapolate to multiple intercepts, indicating a range of S-values.

In order to map the S-value distribution, the boundary fractions for each division are plotted against their corresponding S-values. Vertical distributions then refer to homogeneous systems, distributions ranging over multiple S-values indicate sample heterogeneity.

Figure 6: Shown above is a full resolution van Holde - Weischet distribution plot of the 2-component experimental system shown on the previous page. For the construction of this plot, 14 scans have been analyzed with 100 equally spaced boundary fractions. The system clearly shows a 2-component system, with approximately 1.9 S for the slower sample (Chicken Lysozyme) and about 5 S for the faster component (208 bp dsDNA fragment).

Obtaining partial concentrations from distribution plots:
For non-interacting samples and samples displaying ideal, concentration independent sedimentation behavior, the boundary fraction number can be used to identify partial concentrations. In this example, 54% of the total boundary can be attributed to the faster component, while approximately 37% of the boundary are accounted for by the slower component. Depending on the quality of the data, small variations within the data are normal and are caused by instrumental noise.