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PubMed1997

Towards a molecular understanding of phase separation in the lens: a comparison of the X-ray structures of two high Tc gamma-crystallins, gammaE and gammaF, with two low Tc gamma-crystallins, gammaB and gammaD.

gamma-Crystallins, although closely related in sequence, show intriguing differences in their temperature-dependent interactions: those that have a high or intermediate Tc for phase separation are cryoproteins whereas low Tc gamma-crystallins are not. To address the molecular basis of phase separation, X-ray crystallography has been used to define the structural differences between high and low Tc gamma-crystallins. A pre-requisite for this study was to clarify the assignment of bovine gene sequences to bovine gamma-crystallin proteins used for biophysical measurements. Based on nucleotide sequence analyses of gamma E and gamma F bovine crystallin genes, gamma F corresponds to the previously crystallised high Tc protein bovine gamma IVa and gamma E corresponds to the high Tc bovine protein fraction previously known as gamma IIIa. The gamma F sequence has enabled the completion of the refinement of the bovine gamma F crystal structure which shows that the molecule has an additional surface tryptophan explaining why gamma F has different spectroscopic properties from gamma B. A high Tc protein from rat lens, gamma E crystallin, has been crystallised and the X-ray structure solved at 2.3 A resolution. Comparison of the X-ray structures of two high Tc proteins, rat gamma E and bovine gamma F, with the structures of two low Tc proteins, bovine gamma B and bovine gamma D, shows that the main conformational change between high and low Tc proteins is in the cd surface loop of motif 3. All four structures have numerous ion pairs on their surfaces leading to a high surface charge density, yet with low overall charge. Comparison of the lattice contacts of the two high Tc proteins with the two low Tc gamma-crystallins indicates that these high Tc proteins utilise more amino-aromatic interactions such as between histidine and arginine. Comparison of the sequences of all the gamma-crystallins which have been characterised for phase separation temperature indicates that only residue Arg/Lys 163 uniquely distinguishes cryo from non-cryo gamma-crystallins and it is close to the altered surface loop. Although this region probably contributes to phase separation, Tc is likely to be a function of an overall global property that is responsive to overall charge distribution. Calculated dipole moments of native gamma-crystallins, low Tc gamma-crystallin sequences threaded into high Tc gamma-crystallin structures, and vice versa, show how both sequence and 3D structure contribute to this overall property. High Tc gamma-crystallins have on average higher Arg/Lys ratios and higher histidine content. It is hypothesised that this increases the proportion of surface static paired charged networks which thus reduces the repulsive hydration force and so increases the attractive interactions of the protein-rich phase in binary liquid phase separation.

Amino Acid Sequence↗

PubMed2005

Multi-scale modeling of phase separation in mixed lipid bilayers.

An approach to bridging the phenomenological field theory description of phase separation in binary mixed lipid bilayers with coarse-grained (CG) molecular dynamics (MD) simulation is presented. CG MD simulation is carried out for a 1:1 dipalmitoylphosphatidylcholine/dipalmitoylphosphatidylethanolamine lipid mixture at the liquid-gel phase coexistence condition. The liquid-gel phase separation can be characterized by the bilayer thickness, area per lipid molecule, and orientation parameter of the lipid tails. After a local order parameter is defined using the lipid tail bond orientation parameter, the CG MD data are bridged to a mesoscopic model based on the phenomenological Landau-Ginzberg free-energy functional. All parameters in this mesoscopic model are defined from the information of the phase boundary structure and the distributions of the order parameter in the liquid and gel phases. It is found that the mesoscopic model reproduces the equilibrium properties of the system very well, including collective fluctuations in both phases, spatial correlation functions of the order parameter, and the line tension. The possibility of using a time-dependent Landau-Ginzberg model to mimic the phase-separation dynamics is also investigated, using the relaxation time constant obtained by fitting the time-dependent correlation functions of the order parameter.

Cell Membrane↗

PubMed1991

Glycerol allows low-temperature phase separation of membrane proteins solubilized in Triton X-114: application to the purification of plant cytochromes P-450 and b5.

The potentiality of the Triton X-114 phase separation technique for the purification of proteins from plant microsomal membranes has been investigated. It was shown that glycerol significantly lowers the cloud point of Triton X-114 solutions in water and of Triton X-114 solubilizates from microsomal membranes. It was also established that solubilized membrane components decrease the temperature of Triton X-114 micellar aggregation. Solubilization of microsomal membrane using detergent to protein ratios lower than 3.5, however, resulted in complete inhibition of detergent phase separation. Phase partitioning of Triton X-114 microsomal solubilizates, performed at low temperature (4 degrees C), in the presence of glycerol, provided a very fast and efficient step for the purification of cytochromes P-450 and b5. Conditions allowing optimal recoveries of these cytochromes have been defined.

Cold Temperature↗

PubMed2001

Kinetics and morphologies of viscoelastic phase separation.

In this paper, the effects of relaxational bulk modulus and the average composition of polymers on the viscoelastic phase separation are investigated in detail. It is found that there are two typical morphologies, i.e., moving droplet phase and phase inversion, and the relaxation of the dynamical asymmetry and the amplification of the concentration fluctuation are responsible for the appearance and evolution of different morphologies of viscoelastic phase separation. It is found that, for the viscoelastic phase separation, the scattering function has two peaks. The growth exponents of the main and the secondary peaks in the late stage are almost the same and approximately 0.6, which also agrees with the experimental observations. On the other hand, the growth exponent of the secondary peak increased from approximately 0.42 to approximately 0.66 with increase of straight phi(0) from 0.275 to 0.4, in the intermediate stage.

Journal Article↗

PubMed2004

Phase diagram of van der Waals-like phase separation in a driven granular gas.

Equations of granular hydrostatics are used to compute the phase diagram of the recently discovered van der Waals-like phase separation in a driven granular gas. The model two-dimensional system consists of smooth hard disks in a rectangular box, colliding inelastically with each other and driven by a "thermal" wall at zero gravity. The spinodal line and the critical point of the phase separation are determined. Close to the critical point, the spinodal and binodal (coexistence) lines are determined analytically. Effects of the finite size of the confining box in the direction parallel to the thermal wall are investigated. These include suppression of the phase separation by heat conduction in the lateral direction and a change from supercritical to subcritical bifurcation.

Journal Article↗

PubMed2006

Modified models of polymer phase separation.

In this paper we discuss continuum models of phase separation in polymer solutions, with emphasis on the thermodynamic foundation of these models. We demand that these models obey a free energy dissipation relation, which in the present context plays the role of the second law of thermodynamics, since the system is isothermal. First, we derive a modified two-fluid model for viscoelastic phase separation from nonequilibrium thermodynamics. Then we study the special case when only diffusion is present, and hydrodynamic effects are neglected. Numerical results demonstrate that our models show better stability properties and at the same time reproduce the expected physical phenomena such as volume shrinking and phase inversion. Our findings suggest that these important phenomena are caused by a diffusional asymmetry of the constituent molecules.

Journal Article↗

PubMedDate not supplied

Characterization of phase separation in film forming biopolymer mixtures.

Enhanced, tailor-made films can be achieved by combining the good gas barrier of the hydrophilic high amylose maize starch (hylon) with the water resistance of the hydrophobic protein zein. Two polymers are not always miscible in solution, and the phase separation behavior of the mixture is therefore important for the final film structure and its properties. Phase separation of a mixture of these two biopolymers was induced either by cooling, which was observed as growing droplets of the hylon phase which in some cases also formed small aggregates, or by solvent evaporation and studied in real-time in a confocal laser scanning microscope. Solvent evaporation had a much stronger effect on phase separation. During the early stage of phase separation, hylon formed large aggregates and subsequently smaller droplets coalesced with other droplets or large hylon aggregates. The later part of the separation seemed to take place through spinodal decomposition.

Amylose↗

PubMedDate not supplied

"Delayed" phase separation in a gelatin/dextran mixture studied by small-angle light scattering, turbidity, confocal laser scanning microscopy, and polarimetry.

Small-angle light scattering, turbidity, and confocal laser scanning microscopy were used to study microstructure formation and evolution in a gelatin/dextran mixture. There was a time-delay of up to tens of minutes between reaching the quench temperature and the onset of phase separation, because demixing only occurred once a certain amount of ordering of the gelatin molecules, measured by polarimetry, was attained. The accompanying phenomenon of gelation retarded the development of the microstructure to different extents, depending on the quench temperature. At low temperatures, the structure was rapidly trapped in a nonequilibrium state with diffuse interfaces, characteristic of the early and intermediate stages of phase separation. At higher temperatures, coarsening continued for a certain amount of time before the structure was trapped. The duration of the coarsening period increased with increasing temperature and the interface between the phases became sharp, characteristic of the late stages of phase separation. Because the ordering process continued after the target quench temperature was reached, the effective quench depth continued to increase after the initial phase separation. At high quench temperatures, the system was able to respond to the thermodynamic requirements of the increasing effective quench depth by undergoing secondary phase separation to form a droplet morphology within the preexisting bicontinuous one.

Dextrans↗

PubMed2001

Direct observation of crystallization and aggregation in a phase-separating colloid-polymer suspension.

The depletion-induced phase separation in a mixture of colloidal particles (PMMA-latex) and nonadsorbing polymers [poly(styrene)] in a solvent (mixture of tetralin, cis-decalin, and carbon tetrachloride) was investigated in real space with confocal scanning laser microscopy in the initial, intermediate, and final stage. It was found that the kinetics and the morphology of the phase separation strongly depend on the polymer concentration, and thus on the strength of the depletion-induced attraction between the colloidal particles. At moderate polymer concentrations, crystallization of the PMMA particles is enhanced. At higher polymer concentrations, only aggregation is observed, resulting in amorphous sediments. The aggregation is diffusion-limited or reaction-limited, depending on the polymer concentration. Digital image processing was used to determine the dependence of the aggregation rate and the size of the clusters on the polymer concentration.

Journal Article↗

PubMed1991

Suppression of phase separation in solutions of bovine gamma IV-crystallin by polar modification of the sulfur-containing amino acids.

The calf lens protein gamma IV-crystallin, a strong determinant of the net phase-separation temperature of the lens, was chemically modified with N-bromoacetylethanolamine phosphate. The phase-separation temperatures of solutions of the modified protein were measured and found to be dramatically reduced with respect to those of the native protein. At neutral pH the reagent alkylates only the cysteine and methionine residues and introduces a doubly charged phosphate anion at a maximum distance of 10-12 A from the sulfur atoms. At a protein concentration of 38 g/liter, and with 30% of the cysteines and 40% of the methionines alkylated, the phase-separation temperature is lowered from approximately 25 +/- 2 degrees C to approximately 12 +/- 2 degrees C. The ascending limbs of the coexistence curves for the native and modified proteins were determined at two different degrees of modification. The coexistence curve of the protein with 35% of the cysteines and 40% of the methionines modified shows that as protein concentration approaches the critical concentration of 289 g/liter, there is a much larger suppression of the critical temperature, from approximately 38 +/- 2 degrees C in the native protein to approximately 16 +/- 2 degrees C. Incubation of intact calf lenses in vitro with the reagent results in the suppression of the phase-separation temperature by 3-9 degrees C. These results are consistent with the view that the observed suppression in the critical temperature is due to an increase in the hydrophilicity of the protein in the vicinity of the sulfur-containing residues.

Animals↗

PubMed2004

Dynamic phase separation: from coarsening to turbulence via structure formation.

We investigate some new two-dimensional evolution models belonging to the class of convective Cahn-Hilliard models: (i) a local model with a scalar order parameter, (ii) a nonlocal model with a scalar order parameter, and (iii) a model with a vector order parameter. These models are applicable to phase-separating system where concentration gradients cause hydrodynamic motion due to buoyancy or Marangoni effect. The numerical study of the models shows transition from coarsening, typical of Cahn-Hilliard systems, to spatiotemporally irregular behavior (turbulence), typical of the Kuramoto-Sivashinsky equation, which is obtained in the limit of very strong driving. The transition occurs not in a straightforward way, but through the formation of spatial patterns that emerge for intermediate values of the driving intensity. As in driven one-dimensional models studied before, the mere presence of the driving force, however small, breaks the symmetry between the two separating phases, as well as increases the coarsening rate. With increasing driving, coarsening stops. The dynamics is generally irregular at strong driving, but exhibits specific structural features.

Algorithms↗

PubMed1982

Phase separation of X-irradiated lenses of rabbit.

The phase separation temperature (Tcat) was studied as a function of time (age) after the administration of a single dose of radiation (2000 rad), which induces cataract in the rabbit lens. In the normal unirradiated lens, Tcat decreases linearly with age at a rate (DTcat/dt) approximately 2.2 degrees/week. In the irradiated lens, Tcat initially decreases with age much less than the normal lens, then rises sharply with age at the time of the appearance of opacity in the living rabbit eye. We suggest that the phase separation temperature may serve as a sensitive and early indicator of cataractogenic processes in the lens.

Animals↗

PubMed1995

Phase separation in cytoplasm, due to macromolecular crowding, is the basis for microcompartmentation.

The macromolecular diversity and concentrations in the fluid phase of cytoplasm constitute conditions necessary and sufficient for aqueous phase separation. Consequences of phase separation in cytoplasm, including its 'compartmentation', are inferred from analogies with the physicochemical properties of aqueous two-phase systems and with the partitioning behavior of biomaterials in them.

Animals↗

PubMed2001

Entropy-driven phase separation and configurational correlations on a lattice: some rigorous results.

We prove that if there is a phase separation in a fully packed (FP) athermal system, it must be between pure components only. We then rigorously demonstrate that no phase separation in an athermal FP state of hard particle mixtures on a lattice is possible merely due to size disparity or nonadditivity, if the configurations are weakly correlated, i.e., are quasirandom. We consider a mixture of linear polymers at all packing fractions and argue that no phase separation is possible in an athermal state. The last result also applies to a mixture of flexible particles and hard dimers. Our results contradict many recent numerical results.

Journal Article↗

PubMed2001

Phase separation in mixtures of hard core bosons.

A theoretical investigation of boson versions of the t-J and t-J(z) models on the square lattice is carried out. In the t-J(z) model, phase separation between a hole-rich and a hole-free phase occurs, at sufficiently low hole doping, for arbitrarily small values of J(z). The boson t-J model, instead, features a uniform ground state at any doping for J/t< or =1.5. No evidence of a striped ground state is found. Relevance of this study to the corresponding fermion models is discussed. Fermi statistics is found to enhance the tendency toward phase separation; in particular, phase separation is predicted, at low doping, in the fermion t-J(z) model, at all values of J(z).

Journal Article↗

PubMed1998

Oligomerization and phase separation in globular protein solutions.

We have chemically crosslinked a globular protein, gamma IIIb-crystallin, to produce a system of well-defined oligomers: monomers, dimers, trimers and a mixture of higher n-mers. Gel electrophoresis, size exclusion chromatography, quasielastic light scattering spectroscopy, and electrospray ionization mass spectrometry were used to characterize the oligomers formed. The liquid-liquid phase separation boundaries of the various oligomers were measured. We find that at a given concentration the phase separation temperature strongly increases with the molecular weight of the oligomers. This phase behavior is very similar to previous findings for gamma II-crystallin, for which oxidation-induced oligomerization is accompanied by an increase in the phase separation temperature. These findings imply that for phase separation, the detailed changes of the surface properties of the proteins are less important than the purely steric effects of oligomerization.

Animals↗

PubMed1997

Effect of trivalent metal ions on phase separation and membrane lipid packing: role in lipid peroxidation.

The capacity of Al3+-related cations (Sc3+, Ga3+, In3+, Be2+, Y3+, and La3+) to promote membrane rigidification and lateral phase separation was evaluated in liposomes containing zwitterionic (phosphatidylcholine, PC) and negatively charged (phosphatidylserine, PS) phospholipids. These effects were correlated with the capacity of the ions to stimulate Fe2+-supported lipid peroxidation. A13+, Sc3+, Ga3+, In3+, Be2+, Y3+, and La3+ (50-200 microM) increased the order parameter of the fluorescent probe 1,3-diphenylhexatriene incorporated in PC:PS membranes. In addition, the electron paramagnetic resonance spectra of spin-labeled fatty acids indicated a reduction in lipid motion induced by Sc3+, Y3+, and La3+. The effect was found to extend down to carbon 16 on the acyl chain. The ions (10-200 microM) were also able to induce lateral phase separation, as evaluated from the increase in fluorescence quenching of the probe 2-(6-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)dodecanoyl-1-hexadec anoyl-sn-glycero-3-phosphocholine. The ability of the ions to alter membrane lipid packing and induce lateral phase separation correlated in a positive manner (r2 = 0.91 and 0.90, respectively) with their capacity to stimulate the production of Fe2+-initiated 2-thiobarbituric-reactive species, a measure of lipid peroxidation. These results show that Al3+-related metal ions cause membrane rigidification and phase separation, which could affect membrane-related processes. The results support the hypothesis that ions without redox capacity can stimulate Fe2+-initiated lipid peroxidation by increasing lipid packing and by promoting the formation of rigid clusters. Both processes will bring phospholipid acyl chains closer together, thus favoring the propagation step of lipid peroxidation.

Aluminum↗

PubMed2003

Phase-separation-induced fractionation in molar mass in aqueous mixtures of gelatin and dextran.

An overview of the effects of phase separation of aqueous mixtures of gelatin and dextran on the fractionation in molar mass of these two components is given. Molar mass distributions in coexisting phases were investigated using size exclusion chromatography with multiangle laser light scattering. The initial molar mass of the native material, concentration, and temperature were varied. The results show a strong fractionation in molar mass for both components. The molar mass of the native material and concentration appeared to be the only factors that affected the final molar mass distributions, temperature having no effect. The results show that in the molar mass range where fractionation is the strongest, i.e., roughly below the maximum in the distribution, fractionation is governed by a Boltzmann factor e(-deltaG/kT), where deltaG denotes the free energy involved in transferring a polymer with a certain length from the enriched to the depleted phase, and in this case turns out to be proportional to the molar mass. Comparison of the results of phase separation with results on dialysis shows that water affinity is not the driving force for the phase separation of gelatin and dextran in aqueous solution. The gelation properties of gelatin in both phases were also determined. The gelation properties of gelatin in the coexisting phases differ from those of native gelatin. In particular, the gelatin in the gelatin-poor phase shows strong differences compared to the native material.

Journal Article↗