We have determined the coexistence curves (plots of phase-separation temperature T versus protein concentration C) for aqueous solutions of purified calf lens proteins. The proteins studied, calf y'Ila-, yIlb-, and yIVacrystallin, have very similar amino acid sequences and threedimensional structures. Both ascending and descending limbs of the coexistence curves were measured. We find that the coexistence curves for each of these proteins and for yIcrystallin can be fit, near the critical point, to the function W(Cc -C)/CJI = A[(Tc -T)/TCJ, where fi = 0.325, Cc is the critical protein concentration in mg/ml, T, is the critical temperature for phase separation in K, and A is a parameter that characterizes the width of the coexistence curve. We find that A andCc are approximately the same for all four coexistence curves (A = 2.6 +-0.1, Cc = 289 ± 20 mg/ml), but that Tc is not the same. For yIH-and yIIIb-crystallin, Tc7-5°C, whereas for yIIa-and yIVa-crystallin, Tc 38C. By comparing the published protein sequences for calf, rat, and human y-crystallins, we postulate that a few key amino acid residues account for the division of y-crystallins into low-Tc and high-Tc groups.The y-crystallins constitute a family of highly homologous mammalian lens proteins (1-4). Concentrated aqueous solutions of y-crystallins (5-8) exhibit the phenomena of binaryliquid-phase separation (9-11), also known as coacervation (12). These solutions separate into two coexisting liquid phases of unequal protein concentration at temperatures less than the critical temperature for phase separation Tc. From previous studies (5,7,8), it is known that location of the coexistence curve depends sensitively on the amino acid sequence of the crystallin molecule. Two distinct groups of y-crystallins have been identified in rat (7) and human (8) lenses: high-Tc crystallins and low-Tc crystallins. In these rat and human studies, the precise values of Tc for each crystallin, though inferred from the data, were not determined explicitly. For the high-Tc crystallins, only the ascending limb of the coexistence curves was measured. For the low-Tc crystallins, only an upper bound for the Tc values was established.In this paper, we report on measurements of coexistence curves for three purified calf y-crystallins-yIIIa, yIIIb, and yIVa-[in Table 2 we indicate the current nomenclature for mammalian y-crystallins (2)] and for the native calf y-crystallin mixture yIV. We have determined both the ascending and descending limbs of each coexistence curve. This information enables us to characterize the coexistence curves in detail and to determine explicitly the values of the critical concentration Cc and the critical temperature Tc for each protein. Such detailed analysis of y-crystallin phase separation, which requires gram quantities of purified protein, has been performed only for calf yII (6).We find that the purified calf y-crystallins, in accord with the purified rat and human y-crystallins, fall into two distinct groups: high-Tc (Tc > 350C) proteins, ...

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Geckos owe their remarkable stickiness to millions of dry, hard setae on their toes. In this study, we discovered that gecko setae stick more strongly the faster they slide, and do not wear out after 30 000 cycles. This is surprising because friction between dry, hard, macroscopic materials typically decreases at the onset of sliding, and as velocity increases, friction continues to decrease because of a reduction in the number of interfacial contacts, due in part to wear. Gecko setae did not exhibit the decrease in adhesion or friction characteristic of a transition from static to kinetic contact mechanics. Instead, friction and adhesion forces increased at the onset of sliding and continued to increase with shear speed from 500 nm s 21 to 158 mm s 21. To explain how apparently fluid-like, wear-free dynamic friction and adhesion occur macroscopically in a dry, hard solid, we proposed a model based on a population of nanoscopic stick-slip events. In the model, contact elements are either in static contact or in the process of slipping to a new static contact. If stick -slip events are uncorrelated, the model further predicted that contact forces should increase to a critical velocity (V *) and then decrease at velocities greater than V*. We hypothesized that, like natural gecko setae, but unlike any conventional adhesive, gecko-like synthetic adhesives (GSAs) could adhere while sliding. To test the generality of our results and the validity of our model, we fabricated a GSA using a hard silicone polymer. While sliding, the GSA exhibited steady-state adhesion and velocity dependence similar to that of gecko setae. Observations at the interface indicated that macroscopically smooth sliding of the GSA emerged from randomly occurring stick -slip events in the population of flexible fibrils, confirming our model predictions.

We report measurement of the solid-liquid phase boundary, or liquidus line, for aqueous solutions of three pure calf y6-crystallin proteins: y$I, yHIa, and yIHIb. We also studied the liquidus line for solutions of native yIV-crystallin calf lens protein, which consists of 85% yIVa/15% yIVb. In all four proteins the liquidus phase boundaries lie higher in temperature than the previously determined liquid-lquid coexistence curves. Thus, over the range of concentration and temperature for which liquid-liquid phase separation occurs, the coexistence of a protein crystal phase with a protein liquid solution phase is thermodynamically stable relative to the metastable separated liquid phases. The location ofthe liquidus lines clearly divides these four crystallin proteins into two groups: those in which liquidus lines flatten at temperatures >70rC: yMa and yIV, and those in which liquidus lines flatten at temperatures <500C: yll and -yIlb. We have analyzed the form of the liquidus lines by using specific choices for the structures of the Gibbs free energy in solution and solid phases. By applying the thermodynamic conditions for equilibrium between the two phases to the resulting chemical potentials, we can estimate the temperature-dependent free energy change upon binding of protein and water into the solid phase.Maintenance ofthe lens proteins in a single homogenous fluid phase is an essential condition for transparency of the eye lens (1, 2). Consequently, we previously investigated the location of the coexistence curve (3-5) for liquid-liquid phase separation for four pure calf y-crystallin protein solutions. In those studies, preliminary findings at a few points in the phase diagram suggested that the coexistence curve for solid-liquid phase equilibrium might be higher in temperature than the liquid-liquid coexistence curve (4, 5). We, therefore, undertook the present systematic investigation of the location of the solid-liquid coexistence curve for three pure lens crystallin proteins 'yII, yIIIa, and yIIIb, as well as for native 'yIV protein, which is a mixture of yIVa and yIVb in relative proportion of 85% to 15%, respectively, by number. We report here the measurement for each protein of the ascending limb of the solid-liquid coexistence curve. This limb is called the liquidus line; it is defined as the locus of points in the concentration (c) and temperature (T) plane that corresponds to equilibrium between protein crystals and an aqueous liquid solution of the same protein having concentration c. This locus can be designated by TL(c), or alternatively cL(T). At fixed temperature T, the concentration CL is the solubility of the protein in aqueous solution. The descending limb ofthe solid-liquid coexistence curve is called the solidus line c5(T), and it is the locus of points showing the protein concentration in the solid phase for each temperature T. For cL(T) < c < c,(T), the equilibrium state of the solution consists of a mixture of protein crystals of protein concentration c5(T) and aqueous liquid ...

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