Choi, S.; Liu, S.; Meyer, M. O.; Bevilacqua, P. C.; Keating, C. D.

Coacervate droplets formed by liquid-liquid phase separation serve as models for intracellular biomolecular condensates and as potential protocellular compartments during the emergence of life. Changes in availability of molecular components can be anticipated for intracellular and prebiotic milieu, and protocells may have also faced fluctuations in salinity and pH. Compartments able to maintain their molecular composition, i.e. homeostasis, under such conditions would be better able to preserve internal functions. Phase separation could in principle provide resistance to local changes in molecular composition. To evaluate this possibility, we investigated the impact of non-stoichiometric charge ratios of coacervate molecules on coacervate formation and RNA compartmentalization in oligoarginine (R10)/ATP coacervates across salinity and pH conditions relatable to plausible prebiotic environments. These R10/ATP coacervate systems resisted changes in oligoarginine concentration in both phases under freshwater and ocean-relevant salt conditions, providing a primitive molecular buffering function. Moreover, RNA accumulation was observed in the coacervates over a range of pH, salinity, and R10/ATP stoichiometry. We also observed salt-dependent differences in molecular buffering and compartmentalization that can be understood in terms of how salinity impacts the relative strengths of intermolecular binding modes that drive coacervation and RNA uptake. By varying relative phase volumes and altering which intermolecular binding modes dominate, LLPS provides general mechanisms for resisting changes in molecular availability and environmental conditions, even without the active homeostasis of living cells. Such primitive molecular buffering could have aided the emergence of life and may find utility in biotechnological or commercial applications based on molecular compartmentalization.