The concentration dependence of the activity coefficient of the human spectrin heterodimer. A quantitative test of the Adams-Fujita approximation.

The heterodimer and tetramer states of human spectrin, in equilibrium at 30 degrees C, have been resolved by means of sedimentation velocity experiments at low temperature. This has allowed quantification of the concentrations of each oligomer at equilibrium, as a function of the total concentration of spectrin. In separate sedimentation equilibrium experiments, the thermodynamic activity of the heterodimer has been estimated as a function of the total spectrin concentration through the use of the Omega function. Combination of the concentration and thermodynamic activity of the spectrin heterodimer allowed estimation of the activity coefficient of the heterodimer as a function of total spectrin concentration. Over the accessible concentration range of 0-16 g/L, the logarithm of the activity coefficient of the heterodimer was linear in the total concentration, and the apparent equilibrium constant for tetramer formation was only weakly dependent on concentration, implying that a single virial coefficient is sufficient to describe the nonideality of this system over this concentration range.