Interfacial Permeability, Reflectivity and Preferential Internal Mixing of Phase-Separated Condensates

TL;DR

This study develops a combined theoretical and experimental framework to quantify interfacial permeability and reflectivity in phase-separated condensates, revealing how these properties influence internal mixing and depend on salt concentration.

Contribution

It introduces a novel spectral decomposition method to measure interfacial parameters, linking interfacial transport properties to molecular interactions within condensates.

Findings

Interfaces exhibit strong biased reflectivity and resistance.

Interfacial parameters vary with salt concentration.

Transport regimes are governed by interfacial properties.

Abstract

Biomolecular condensates organize biochemical processes by spatially concentrating molecules while allowing for dynamic exchange with their surroundings. However, transport across their interface can be strongly attenuated, leading to enhanced retention and preferential internal mixing. Two key mechanisms have been proposed to describe this behavior: biased interfacial reflectivity, which compares how strongly particles are reflected at the interface when attempting to enter or leave the condensate, and interfacial resistance, which sets the kinetic rate at which particles can cross the interface. Quantifying these parameters experimentally has remained challenging. Here, we present a theoretical and experimental framework to address this issue, extending our previously developed half-FRAP approach. We solve the spherical diffusion problem with a semipermeable interface by spectral decomposition. By evaluating the information content of the integrated recovery curves, we show that they encode sufficient information to recover interfacial parameters over extended regions of parameter space. Applying our framework to tunable coacervates composed of poly-lysine and hyaluronic acid, we find that their interfaces exhibit strongly biased reflectivity and substantial resistance, both driving preferential internal mixing. These parameters depend on salt concentration, linking interfacial transport to intermolecular interaction strength and position in the phase diagram. Our results establish a quantitative connection between interfacial properties and condensate dynamics, revealing how their interplay gives rise to distinct transport regimes.