Hard sphere-like glass transition in eye lens α-crystallin solutions

Abstract

We study the equilibrium liquid structure and dynamics of dilute and concentrated bovine eye lens α-crystallin solutions, using small-angle X-ray scattering, static and dynamic light scattering, viscometry, molecular dynamics simulations, and mode-coupling theory. We find that a polydisperse Percus-Yevick hard-sphere liquid-structure model accurately reproduces both static light scattering data and small-angle X-ray scattering liquid structure data from α-crystallin solutions over an extended range of protein concentrations up to 290 mg/mL or 49% vol fraction and up to ca. 330 mg/mL for static light scattering. The measured dynamic light scattering and viscosity properties are also consistent with those of hard-sphere colloids and show power laws characteristic of an approach toward a glass transition at α-crystallin volume fractions near 58%. Dynamic light scattering at a volume fraction beyond the glass transition indicates formation of an arrested state. We further perform event-driven molecular dynamics simulations of polydisperse hard-sphere systems and use mode-coupling theory to compare the measured dynamic power laws with those of hard-sphere models. The static and dynamic data, simulations, and analysis show that aqueous eye lens α-crystallin solutions exhibit a glass transition at high concentrations that is similar to those found in hard-sphere colloidal systems. The α-crystallin glass transition could have implications for the molecular basis of presbyopia and the kinetics of molecular change during cataractogenesis.alpha crystallin | scattering | mode-coupling theory | molecular dynamics | glass transition T he cytoplasm of the tightly packed fiber cells of the eye lens contains concentrated aqueous protein mixtures that have high refractive indexes, while normally remaining clear enough for vision. Lens clarity depends on short-range order between lens proteins (1, 2) and can be disrupted by both protein aggregation and liquid-liquid phase separation in cataract, a leading cause of blindness. At the high protein concentrations of lens cytoplasm, 25-60% by weight in mammals, small changes in interprotein interactions can disrupt transparency. For human lens proteins with cataractogenic point mutations, and for high-concentration lens protein mixtures, protein interaction changes as small as a fraction of thermal energy, k B T, can induce phase separation and thus lead to opacification (3-7).In addition to equilibrium phase transitions, dynamical transitions including glass formation and gelation can also occur at high protein concentrations like those of the eye lens (8, 9). Relatively abrupt viscoelastic changes associated with glass formation or gelation could harden the lens and contribute to presbyopia and could alter cataract formation rates by affecting aggregation and phase separation kinetics.Here we study the equilibrium liquid structure and dynamics of concentrated solutions of eye-lens α-crystallin protein solutions, using small-angle X-ray scattering (SAXS), static light scattering (SLS)...

Non-enzymatic glycation of α-crystallin as an in vitro model for aging, diabetes and degenerative diseases

“…We measured SAXS from a 250 mg/ml sample of alpha crystallin which showed a strong peak around q = 0.42 nm −1 (Fig. 2) consistent with Foffi et al and other research groups (Delaye and Tardieu 1983;Foffi et al 2014). Upon glycation with 10 μM methylglyoxal, the correlation peak diminishes in intensity and moves to smaller q = 0.25 nm −1 .…”

“…SAXS data are very useful in determining the inter-particle spacing and hence help us to understand the packing of α-crystallin in very high concentrations without superaggregation or crystallization. A recent study has detailed how in concentrated suspensions of alpha crystallin that inter-particle correlations are well described by the structure factor for a hard sphere fluid (Foffi et al 2014). At low concentrations, alpha crystallin shows a monotonic falloff of intensity with scattering vector q.…”

Dramatic influence of patchy attractions on short-time protein diffusion under crowded conditions

“…The hard sphere-like protein α-crystallin is a quite polydisperse multisubunit protein with an average molecular weight of about 8 × 10 5 , an average hydrodynamic radius of about R h = 9.6 nm, and a polydispersity of about 20% ( 19 ). It has been shown to behave like ideal hard spheres all the way up to the glass transition occurring at a volume fraction of φ = 0.58.…”

Statistical-thermodynamic model for light scattering from eye lens protein mixtures

“…The present model complements the neutron scattering, molecular dynamics, and perturbation theory work previously applied to this system. [12][13][14] Use of a sticky-sphere mixture light scattering model for γ-α mixtures was suggested by previous findings: (i) A single-component sticky-sphere model successfully represents static light scattering data from concentrated γB-crystallin solutions; 9 (ii) the sticky-sphere model closely represents lowangle neutron scattering 41 from concentrated γB-crystallin solutions; (iii) when sticky interactions are turned off, the sticky-sphere mixture model reproduces the analytic PercusYevick results for hard-sphere mixtures, 22,42,43 which is relevant because a Percus-Yevick hard-sphere liquid structure model can fit light scattering, small-angle X-ray scattering, and neutron scattering from α-crystallin solutions; 44 (iv) sticky spheres provide a convenient representation of the tuned shortrange attraction between α-and γ-crystallins that is needed for stability at high concentrations; [12][13][14] and (v) the stickysphere model can potentially incorporate all 6 types of effective short-range interactions between protein molecule pairs in quaternary mixtures of an aqueous buffer with α-, β-, and γ-crystallins, albeit with the restriction to spherical models of molecular shapes.…”

“…2,3,12,14,44,55 Progressively smaller values of τ ij increase the coefficient of the Dirac delta function in Eq. (2), and thus correspond to increasingly attractive interactions.…”