Phase behavior of mixtures of human lens proteins Gamma D and Beta B1.

We have experimentally determined the coexistence surface characterizing the phase behavior of gammaD-betaB1-water ternary solutions. The coexistence surface fully describes the solution conditions, i.e., temperature, protein concentration, and protein composition, at which liquid-liquid phase separation occurs in a ternary solution. We have observed a significant demixing of gammaD and betaB1 i.e., large difference of composition in the two coexisting phases. This demixing suggests that the energy of the gammaD-betaB1 attractive interaction is significantly smaller than the energy of the gammaD-gammaD attractive interaction. We also observed the lowering of the phase separation temperature upon increasing of the fraction of betaB1 in solution. We provide a theoretical analysis of our experimental data, which enables a quantitative description of our principal experimental findings. In this way, we have evaluated the magnitude and temperature dependence of the relevant interprotein interaction energies. Our findings provide insight into the factors essential for maintaining lens proteins in a single homogeneous phase, thereby enabling lens transparency.