135 research outputs found
Interplay between Spinodal Decomposition and Glass Formation in Proteins Exhibiting Short-Range Attractions
We investigate the competition between spinodal decomposition and dynamical
arrest using aqueous solutions of the globular protein lysozyme as a model
system for colloids with short-range attractions. We show that quenches below a
temperature Ta lead to gel formation as a result of a local arrest of the
proteindense phase during spinodal decomposition. The rheological properties of
these gels allow us to use centrifugation experiments to determine the local
densities of both phases and to precisely locate the gel boundary and the
attractive glass line close to and within the unstable region of the phase
diagram
A closer look at arrested spinodal decomposition in protein solutions
Concentrated aqueous solutions of the protein lysozyme undergo a liquid solid
transition upon a temperature quench into the unstable spinodal region below a
characteristic arrest temperature of Tf=15C. We use video microscopy and
ultra-small angle light scattering in order to investigate the arrested
structures as a function of initial concentration, quench temperature and rate
of the temperature quench. We find that the solid-like samples show all the
features of a bicontinuous network that is formed through an arrested spinodal
decomposition process. We determine the correlation length Xi and demonstrate
that Xi exhibits a temperature dependence that closely follows the critical
scaling expected for density fluctuations during the early stages of spinodal
decomposition. These findings are in agreement with an arrest scenario based on
a state diagram where the arrest or gel line extends far into the unstable
region below the spinodal line. Arrest then occurs when during the early stage
of spinodal decomposition the volume fraction phi2 of the dense phase
intersects the dynamical arrest threshold phi2Glass, upon which phase
separation gets pinned into a space-spanning gel network with a characteristic
length Xi
A simple patchy colloid model for the phase behavior of lysozyme dispersions
We propose a minimal model for spherical proteins with aeolotopic pair
interactions to describe the equilibrium phase behavior of lysozyme. The
repulsive screened Coulomb interactions between the particles are taken into
account assuming that the net charges are smeared out homogeneously over the
spherical protein surfaces. We incorporate attractive surface patches, with the
interactions between patches on different spheres modeled by an attractive
Yukawa potential. The parameters entering the attractive Yukawa potential part
are determined using information on the experimentally accessed gas-liquid-like
critical point. The Helmholtz free energy of the fluid and solid phases is
calculated using second-order thermodynamic perturbation theory. Our
predictions for the solubility curve are in fair agreement with experimental
data. In addition, we present new experimental data for the gas-liquid
coexistence curves at various salt concentrations and compare these with our
model calculations. In agreement with earlier findings, we observe that the
strength and the range of the attractive potential part only weakly depend on
the salt content
Diffusing-wave spectroscopy of nonergodic media
We introduce an elegant method which allows the application of diffusing-wave
spectroscopy (DWS) to nonergodic, solid-like samples. The method is based on
the idea that light transmitted through a sandwich of two turbid cells can be
considered ergodic even though only the second cell is ergodic. If absorption
and/or leakage of light take place at the interface between the cells, we
establish a so-called "multiplication rule", which relates the intensity
autocorrelation function of light transmitted through the double-cell sandwich
to the autocorrelation functions of individual cells by a simple
multiplication. To test the proposed method, we perform a series of DWS
experiments using colloidal gels as model nonergodic media. Our experimental
data are consistent with the theoretical predictions, allowing quantitative
characterization of nonergodic media and demonstrating the validity of the
proposed technique.Comment: RevTeX, 12 pages, 6 figures. Accepted for publication in Phys. Rev.
Phase equilibria and glass transition in colloidal systems with short-ranged attractive interactions. Application to protein crystallization
We have studied a model of a complex fluid consisting of particles
interacting through a hard core and a short range attractive potential of both
Yukawa and square-well form. Using a hybrid method, including a self-consistent
and quite accurate approximation for the liquid integral equation in the case
of the Yukawa fluid, perturbation theory to evaluate the crystal free energies,
and mode-coupling theory of the glass transition, we determine both the
equilibrium phase diagram of the system and the lines of equilibrium between
the supercooled fluid and the glass phases. For these potentials, we study the
phase diagrams for different values of the potential range, the ratio of the
range of the interaction to the diameter of the repulsive core being the main
control parameter. Our arguments are relevant to a variety of systems, from
dense colloidal systems with depletion forces, through particle gels,
nano-particle aggregation, and globular protein crystallization.Comment: 20 pages, 10 figure
Internal structure and colloidal behaviour of covalent whey protein microgels obtained by heat treatment
Reduced Stability and Increased Dynamics in the Human Proliferating Cell Nuclear Antigen (PCNA) Relative to the Yeast Homolog
Proliferating Cell Nuclear Antigen (PCNA) is an essential factor for DNA replication and repair. PCNA forms a toroidal, ring shaped structure of 90 kDa by the symmetric association of three identical monomers. The ring encircles the DNA and acts as a platform where polymerases and other proteins dock to carry out different DNA metabolic processes. The amino acid sequence of human PCNA is 35% identical to the yeast homolog, and the two proteins have the same 3D crystal structure. In this report, we give evidence that the budding yeast (sc) and human (h) PCNAs have highly similar structures in solution but differ substantially in their stability and dynamics. hPCNA is less resistant to chemical and thermal denaturation and displays lower cooperativity of unfolding as compared to scPCNA. Solvent exchange rates measurements show that the slowest exchanging backbone amides are at the β-sheet, in the structure core, and not at the helices, which line the central channel. However, all the backbone amides of hPCNA exchange fast, becoming undetectable within hours, while the signals from the core amides of scPCNA persist for longer times. The high dynamics of the α-helices, which face the DNA in the PCNA-loaded form, is likely to have functional implications for the sliding of the PCNA ring on the DNA since a large hole with a flexible wall facilitates the establishment of protein-DNA interactions that are transient and easily broken. The increased dynamics of hPCNA relative to scPCNA may allow it to acquire multiple induced conformations upon binding to its substrates enlarging its binding diversity
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