Phase transitions and coacervates play key roles in natural and synthetic soft matter. In particular, the past few years have seen a rapid expansion in studies of these phenomena in the context of ...dynamic cellular compartmentalization. In this brief review, we mainly focus on a few concepts and selected in vitro and cellular examples of recent developments in the areas of dynamics and multicomponent systems. Topics covered include the flexibility and conformational dynamics of polymeric species involved in phase separation, valence and non-monotonic effects, noise modulation and feedback loops, and multicomponent systems and substructure. The fundamental concepts discussed in this review are widely applicable, including in the context of cellular function and the development of materials with novel properties.
In this work, we study the temperature-induced development of “dynamically arrested” states in dense suspensions of “soft colloids” (multi-arm star polymers and/or block-copolymers micelles) by means ...of molecular dynamics (MD) simulations. Temperature increase in marginal solvents results in “soft sphere” swelling, dynamical arrest, and eventually crystallization. However, two distinct “dynamically arrested” states were found, one almost amorphous (“glassy”) and one with a considerable degree of crystallinity, yet lower than that of the fully equilibrated crystal. It is remarkable that even that latter state permitted self-diffusion in the timescale of the simulations, an effect that underlies the importance of the “ultra-soft” nature of inter-particle potential. The “number of connections” criterion for crystallinity proved to be very successful in identifying the ultimate thermodynamic trend from the very early stages of the α-relaxation.
We consider the effects of a quench to
T=0 on a spin system with axial next–next nearest neighbour Ising interactions, evolving under a conserved 3-spin flip dynamics. Such a model is motivated by ...the kinetics of stacking layers in polytypes near the 3
C–6
H transition. We find that the system generically gets arrested in interesting metastable states which have inhomogeneously distributed quiescent and active regions. In such arrested states, the autocorrelation function decays as a stretched exponential
∼
exp(−(t/τ
o)
1
3
)
. The latter feature can be understood in terms of a mapping of the dynamics within active stretches to the well known simple exclusion process of particles on a line, and bounds can be put on
τ
o
.
In colloidal systems, the interplay between the short range attraction and long-range repulsion can lead to a low density associated state consisting of clusters of individual particles. Recently, ...such an equilibrium cluster phase was also reported for concentrated solutions of lysozyme at low ionic strength and close to the physiological pH. Stradner et al. (2004) Equilibrium cluster formation in concentrated protein solutions and colloids. Nature 432:492-495 found that the position of the low-angle interference peak in small-angle x-ray and neutron scattering (SAXS and SANS) patterns from lysozyme solutions was essentially independent of the protein concentration and attributed these unexpected results to the presence of equilibrium clusters. This work prompted a series of experimental and theoretical investigations, but also revealed some inconsistencies. We have repeated these experiments following the protein preparation protocols of Stradner et al. using several batches of lysozyme and exploring a broad range of concentrations, temperature and other conditions. Our measurements were done in multiple experimental sessions at three different high-resolution SAXS and SANS instruments. The low-ionic-strength lysozyme solutions displayed a clear shift in peak positions with concentration, incompatible with the presence of the cluster phase but consistent with the system of repulsively interacting individual lysozyme molecules. Within the decoupling approximation, the experimental data can be fitted using an effective interparticle interaction potential involving short-range attraction and long-range repulsion.
The activity of the membrane fusion protein Env of Moloney mouse leukaemia virus is controlled by isomerization of the disulphide that couples its transmembrane (TM) and surface (SU) subunits. We ...have arrested Env activation at a stage prior to isomerization by alkylating the active thiol in SU and compared the structure of isomerization‐arrested Env with that of native Env. Env trimers of respective form were isolated from solubilized particles by sedimentation and their structures were reconstructed from electron microscopic images of both vitrified and negatively stained samples. We found that the protomeric unit of both trimers formed three protrusions, a top, middle and a lower one. The atomic structure of the receptor‐binding domain of SU fitted into the upper protrusion. This was formed similar to a bent finger. Significantly, in native Env the tips of the fingers were directed against each other enclosing a cavity below, whereas they had turned outward in isomerization‐arrested Env transforming the cavity into an open well. This might subsequently guide the fusion peptides in extended TM subunits into the target membrane.
Dam break flows of viscoplastic fluids are studied theoretically using a Herschel–Bulkley constitutive law and a lubrication model of the motion. Following initiation these fluids are gravitationally ...driven out of the lock in which they had resided. Their motion is eventually arrested because they exhibit a yield stress and they attain a stationary state in which the gravitational forces are in equilibrium with the yield stress. We study the evolution of these flows from initiation to arrest by integrating the equations of motion numerically. We demonstrate that the final arrested state is approached asymptotically and find analytically that the perturbations to the final state decay algebraically with time as
1
/
t
n
, where
n is the power index of the Herschel–Bulkley model.
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