Limitations on Estimation of Hydration and Asymmetry

One cannot determine accurate, unambiguous values for both the degree of hydration (δ1) and the ellipsoid axial ratios (a/b) from sedimentation experiments alone (Ref. 27 28 29 30). However, maximum values for these parameters may be calculated if s and Mr are available (Teller method). The vbar method may also be used for calculating maximum hydration and asymmetry if vbar is also available. In addition, if one of the parameters (δ1 or a/b) are known independently, estimates of the second parameter can be made. However, at the outset it must be realized that these parameters, especially that of a/b, are merely descriptors for models and may bear little relevance to the actual molecular structure.

There are two sources of error that must be addressed. First, the calculation of a/b and δ1 will rely on an estimate of R0 or Rp and, as discussed above, the equations for these are empirical. Second, this radius is used to calculate f0 using equation 23. However, equation 23 is derived assuming complete "stick" boundary conditions (i.e. a monolayer of solvent moves with the macromolecule). If complete "slip" boundary conditions are assumed (i.e. there is no interaction between the solvent and the macromolecule), equation 23 should be rewritten as equation 24 (Ref. 18).

Equation 24:

Thus, even though all evidence suggest that a purely "stick" boundary condition prevails, this assumption increases the uncertainty in the f0 calculation.