Fig. 4.

Sedimentation of rapidly reversible heterogeneous interactions. a Typical bimodal boundary pattern. Profiles are calculated using Lamm equation solutions, Eq. 6, for the sedimentation of a 40-kDa protein binding a 60-kDa protein with a kinetic off-rate constant of 0.1/s, at loading concentrations at a ratio 2:1:K _D. Sedimentation was simulated at 50,000 rpm, and total absorbance profiles (assuming equal weight-based extinction coefficients of both components) are shown in 10-min intervals. b Cartoon of the effective particle A⋯B (encircled in red). Indicated is the fractional time thatA (green) and B (blue) spend free or in complex (grayed time intervals). A spends a smaller fraction of time free than B, resulting in a match of their time-average velocities. An animated cartoon can be downloaded from https://sedfitsedphat.nibib.nih.gov/tools/Reaction%20Boundary%20Movies/Movies%20S1%20-%20S3.pdf, or created in the software SEDPHAT faithfully reflecting user-defined binding parameters and sedimentation coefficients. c Velocity of the reaction boundary A⋯B as a function of the total loading concentration of a and b, calculated by effective particle theory. For more details, including tools for using these isotherms in the data analysis and experimental design, see Schuck (2010b)