Direct analysis of sedimentation equilibrium distributions reflecting complex formation between dissimilar reactants.

Procedures are developed for the characterization of thermodynamically ideal complex formation between dissimilar macromolecular reactants by direct analysis of sedimentation equilibrium distributions. Studies of an electrostatic interaction between ovalbumin and cytochrome c are used to illustrate the application of analyses pertaining to (i) the situation in which separate sedimentation equilibrium distributions for the two macromolecular constituents are available, (ii) that in which the experimental record reflects the distribution of only one constituent, and (iii) the situation in which a composite distribution for both constituents is the sole experimental record. An association constant of 63 000 (+/- 2000) M-1 is obtained for the 1:1 interaction between ovalbumin and cytochrome c under the conditions examined (pH 6.3, I 0.03). Because of their inherent simplicity, these direct analytical procedures offer potential for accommodating the effects of thermodynamic nonideality in dissimilar reactant systems.