Dresden 2026 – scientific programme

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CPP: Fachverband Chemische Physik und Polymerphysik

CPP 55: Focus Session: Theoretical Modeling and Simulation of Biomolecular Condensates III (joint session CPP/BP)

CPP 55.3: Talk

Friday, March 13, 2026, 10:15–10:30, ZEU/0260

Local RNA/protein stoichiometry tunes the electrostatic microenvironment inside reconstituted multicomponent condensates — •Patrick M. McCall — Leibniz Institute for Polymer Research Dresden, Dresden, DE

Biomolecular condensates are demixed phases of biopolymers and, in living cells, commonly form through the associative phase separation of strongly-charged nucleic acids together with protein polyampholytes carrying a weak net charge. While condensates are proposed to offer distinct aqueous environments for the organization of cellular biochemistry, it remains unclear which physical aspects of the microenvironment are relevant and how widely they can vary between condensates. Motivated by the large asymmetry in structural charge between typical condensate components such as RNAs and RNA-binding proteins, we explore here the implications of electroneutrality on the electrostatic environment within model multicomponent condensates. Combining classical Donnan theory with recent measurements of the macromolecular composition of condensates reconstituted from full-length FUS protein and a homopolymeric RNA [McCall et al Nat Chem 2025], we compute the partitioning of salt ions as well as the Donnan potential across the phase boundary. We find that RNA/FUS stoichiometry tunes both co-ion exclusions over a wide range and is coupled to a pH jump across droplet interface. We also find that co-ion exclusion is suppressed by counter-ion condensation and enhanced by non-ideality of un-bound ions. These results provide insight into the range of ionic conditions accessible to a prominent class of biomolecular condensate.

Keywords: Biomolecular condensate; RNA-binding protein; Nucleic acid; Donnan equilibrium; Counter-ion condensation