Phase separation and superselective binding of multivalent associative polymers

Abstract

Multivalent polymers play pivotal roles in many biological and synthetic systems. Multivalency can often be tuned to provide control over both the enhancement and suppression of binding. In this thesis, Monte Carlo simulations of simplified model chains on a cubic lattice are used to study multivalent polymers in liquid-liquid phase separation, and in superselective binding to a host.

We are particularly interested in liquid-liquid phase separation due to its potential role in the formation of membraneless organelles, which support cellular fitness and exhibit interesting properties and functions. Multivalent polymers are broadly representative of many membraneless organelle components, such as RNA and proteins. We investigate the different ways in which linkers can be represented on a cubic lattice and study how this impacts the individual chain properties and the aggregates they form. We find that the phase separation of multivalent polymers is promoted when the polymers can form dense, highly interconnected structures; this is achieved through increasing the polymer length and valency.


We then study how the binding of individual multivalent polymers can be tuned to achieve highly selective binding. We show that superselective binding of multivalent polymers to a 3D host can be controlled by the presence of crowder species in the 3D host. Superselectivity describes binding that is very sensitive to the density of receptors, resulting in a sharp binding transition at a receptor density threshold. Superselectivity is shown to be a potential mechanism by which membraneless organelles control their composition.


Superselective binding of multivalent polymers at surfaces is also investigated. We find that superselectivity is enhanced on moving from a flat surface to a pitted surface. Furthermore, rough surfaces show potential for polymer sorting or selective recruitment, whereby the binding of polymers to a given surface is dependent on the interaction strengths, polymer properties and pore geometry. The underlying thermodynamics is analysed, allowing us to identify the enthalpic and entropic drivers, and also indicate how this phenomenon can be harnessed in applications.