Liquid-liquid microphase separation leads to formation of membraneless organelles

Proteins and nucleic acids can spontaneously self-assemble into membraneless droplet-like compartments, both in vitro and in vivo. A key component of these droplets are multi-valent proteins that possess several adhesive domains with specific interaction partners (whose number determines total valency of the protein) separated by disordered regions. Here, using multi-scale simulations we show that such proteins self-organize into micro-phase separated droplets of various sizes as opposed to the Flory-like macro-phase separated equilibrium state of homopolymers or equilibrium physical gels. We show that the micro-phase separated state is a dynamic outcome of the interplay between two competing processes: a diffusion-limited encounter between proteins, and the dynamics within small clusters that results in exhaustion of available valencies whereby all specifically interacting domains find their interacting partners within smaller clusters, leading to arrested phase separation. We first model these multi-valent chains as bead-spring polymers with multiple adhesive domains separated by semi-flexible linkers and use Langevin Dynamics (LD) to assess how key timescales depend on the molecular properties of associating polymers. Using the time-scales from LD simulations, we develop a coarse-grained kinetic model to study this phenomenon at longer times. Consistent with LD simulations, the macro-phase separated state was only observed at high concentrations and large interaction valencies. Further, in the regime where cluster sizes approach macro-phase separation, the condensed phase becomes dynamically solid-like, suggesting that it might no longer be biologically functional. Therefore, the micro-phase separated state could be a hallmark of functional droplets formed by proteins with the sticker-spacer architecture.Significance statementMembraneless organells (MO) are ubiquitous in ‘healthy’ living cells, with an altered state in disease. Their formation is likened to liquid-liquid phase separation (LLPS) between MO-forming proteins. However most models of LLPS predict complete macrophase separation while in reality MO’s are small droplets of various sizes, which are malleable to rapid morphological changes. Here we present a microscopic multiscale theoretical study of thermodynamics and kinetics of formation of MO. We show that MO’s are long-living dynamic structures formed as a result of arrested macrophase separation. Our study provides a direct link beween the molecular properies of MO-forming proteins and the morphology and dynamics of MO paving a path to rational design and control of MO.

In situ compatibilisation of alkenyl-terminated polymer blends using cross metathesis

A whiff of diblocks is enough! Straightforward coupling of two vinyl-terminated macromolecules was achieved by cross metathesis (CM) using Grubbs' second-generation catalyst (G2) giving access to a diversity of “self-compatibilised” polymer blends. Solution blends, in the absence of G2, showed clear macrophase separation while metathesized samples no longer did. A small fraction of diblocks only proved to act as an excellent compatibilizer.

Morphology and Structure of Rod/Coil Nanocomposite Thin Film

A rod/coil nanocomposite was prepared by a reactive mixing process of Poly(p-phenylenebenzobisthiazole) (PBZT) and Nylon66 in polyphosphoric acid (PPA). PBZT-Nylon66 block copolymers were synthesized during the reactive mixing process to act as compatibilizers of PBZT and Nylon66. The morphology and structure of the solution-cast thin film of PBZT/Nylon66 nanocomposites were investigated by using Optical Microscopy(OM), Differential Thermal Analysis(DTA), and Transmitted Electron Microscopy(TEM). PBZT were dispersed in Nylon66 matrix as fibrils which had diameters ranging from 100nm to 400nm and lengths ranging from 500nm to 2μm. The crystal behavior of Nylon66 was considerably disturbed by PBZT in the nanocomposite. Even after heating above the melting point of Nylon66, the nanocomposite exhibited no macrophase separation.