Data from a sedimentation equilibrium experiment

Often it is useful to interpret the results of a sedimentation equilibrium experiment as a molecular weight. Nonlinear least squares fitting will return the exponent σ, which is the slope of the concentration curve on an axis of (r^2)/2. σ is directly available from the data, but to interpret σ as a molecular weight, the following equation is used (from equation 4):

where vbar is the partial specific volume of the solute, ρ is the density of the solvent, ω^2 is the radial velocity of the rotor squared, R is the gas constant, and T is the temperature of the experiment. While R is known, and T and ω^2 are available simply from the experiment, and are physical properties that must be measured or estimated.

Given a certain temperature, rotor speed, and σ, Sednterp will use default approximate values of vbar and ρ automatically to compute a molecular weight. All that is necessary is to enter temperature, rotor speed, and σ in the proper fields. However, for a more accurate computation, better estimates of vbar and ρ are necessary. The default value for vbar is an average value for all proteins. The default value for ρ is the density of pure water (unless you have changed the defaults).

You may know by experiment, or be able to look up in a table, better values for vbar and ρ. If so, enter the values using the partial data button.

Sednterp may already have a particular sample (for vbar calculations) or a particular buffer (for ρ or η) in its database. To use these previously computed values, choose the proper sample or buffer from the drop down lists. Otherwise, Sednterp includes the option to compute interpolations of these measurements for many standard buffers and for known protein compositions.

Do you want to compute:

a. an interpolated vbar from a protein composition?

b. an interpolated ρ from the concentration of common buffer components?