Biomolecular condensates are droplets-like structures originating from the phase-separation of biomolecules. The functions of condensates within living cells span many length scales: from the modulation of chemical reactions at the molecular scale to the compartmentalization of the cell at the mesoscale. We employ single-molecule fluorescence spectroscopy to study the conformations and dynamics of intrinsically disordered proteins within single droplets, combined with all atom simulations, and microrheology approaches to assess mesoscale properties. We find that the nanoscale chain dynamics on the nano- to microsecond timescale can be accurately related to both translational diffusion and mesoscale condensate viscosity by analytical relations from polymer physics. Atomistic simulations reveal that the rapid exchange of inter-residue contacts we observe may be a general mechanism for preventing dynamic arrest in compartments densely packed with polyelectrolytes, such as the cell nucleus.​

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