PHASE-SEPARATION IN MULTICOMPONENT AQUEOUS-PROTEIN SOLUTIONS

We present measurements of the phase-separation temperature (T-ph(phi, alpha)) as a function of overall protein volume fraction (phi) and pro tein composition (alpha) for ternary aqueous (W) solutions of calf gam ma(IIIa) (A) and gamma(IIIb) (B) crystallins. Additionally, we have de termined the binodal curve describing coexisting points (phi(I),alpha( I)) and (phi(II),alpha(II)) in the phase diagram at 20 degrees C. We p ropose a mean-field form of the ternary Gibbs free energy G(phi,alpha, T) which contains three interaction energy parameters: E(net)(A,W), E( net)(B,W), and E(net)(A,B), which determine the magnitude of the quadr atic (phi(2)) mixing energy contribution to G. Using a lattice model i t is possible to express each of these interaction parameters in terms of the mean individual protein-water, protein-protein and water-water bond energies. In the limit where the two proteins are not too dissim ilar, as applies in our system, we find quite generally that the terna ry solution can be regarded as a binary solution with an interaction e nergy dependent upon the initial composition (alpha) of the solution. We have used this finding to predict the entire coexistence surface T- ph(phi,alpha) and the positions of coexisting points along the binodal curve. The interaction energy parameters were determined and we show that, within experimental error, this theory accurately describes the full range of our experimental results.