Sticky-Sphere Mixture Model for Light Scattering from Concentrated Eye Lens Protein Mixtures

Abstract

We use a sticky-sphere mixture liquid structure model to analyze static light scattering efficiency data obtained from aqueous mixtures of gamma and alpha crystallin eye lens proteins. The model incorporates (i) hard-sphere interactions between alpha crystallins, leading to short-range ordering and transparency at high concentration, (ii) short-range attractive interactions between gamma crystallins, which can produce intense light scattering and liquid-liquid phase separation, and (iii) both excluded-volume interactions and short-range attractions between alpha and gamma crystallins, which also sensitively affect transparency [Dorsaz:2009,Dorsaz:2011]. We derive general expressions for the scattered light intensity within the model, and compare the model with data from aqueous gamma/alpha mixtures. We show that the sticky-sphere mixture model can provide accurate representations of the observed static light scattering cross sections, including their concave-down dependence on alpha/gammaB composition at intermediate concentrations, combined with the concave up and more complicated, temperature-sensitive dependence on relative protein composition at high, more physiological protein concentrations [Thurston:2006]. We use the model to help study possible implications for molecular mechanisms of cataract, by changing gamma-gamma, gamma-alpha, and alpha-alpha interactions. The sticky-sphere mixture model exhibits non-monotonic dependence of light scattering intensity and thermodynamic stability on gamma-alpha attraction strength, consistent with simulation and perturbation theory [Dorsaz:2011]. The model predicts that gamma-alpha mixture light scattering is quite sensitive to gamma-alpha interactions, consistent with previous analysis of a congenital cataract [Banerjee:2011]. The sticky-sphere mixture model is useful for quantitatively analyzing light scattering of concentrated aqueous crystallin mixtures, relevant for understanding lens cytoplasm.