COMPUTER SIMULATION OF SEDIMENTATION IN ULTRACENTRIFUGE .4. VELOCITY SEDIMENTATION OF SELF-ASSOCIATING SOLUTES

Methods are outlined for simulating the sedimentation and diffusion of macromolecular solutes that undergo rapidly reequilibrating association-dissociation reactions of the type nA ⇌ An. The solutes are treated as single species with concentration-dependent sedimentation and diffusion coefficients, and certain limitations are placed on the types of systems that can be so treated. Expressions are given for average diffusion coefficients that are appropriate for the simulation procedure. Diffusion of rapidly polymerizing systems in the absence of a field produces gradient curves that are unimodal and smooth, but are skewed toward the solution side of the boundary. Features of the boundary shape predicted by the theory of Gilbert for sedimentation in the absence of diffusion are often, but not always, qualitatively evident in the presence of diffusion. Monomer-dimer systems produce skewed, unimodal boundaries without shoulders. The tendency of monomer-trimer, monomer-tetramer, and monomer-hexamer systems to generate bimodal gradient curves may persist in the presence of diffusion, or it may be expressed in the development of a single boundary with a shoulder or a trailing foot. In at least one case, the minimum anticipated in the absence of diffusion is entirely obscured, and the resulting gradient curve is broadened but nearly symmetrical. © 1969.