Primitive Molecular Buffering by Low-Multivalency Coacervates

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 the 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 their internal functions. Phase separation could, in principle, provide resistance to local changes in the molecular composition. To evaluate this possibility, we investigated the impact of the nonstoichiometric 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 concentrations in both phases under freshwater- and ocean-relevant salt conditions, providing a primitive molecular buffering function. Moreover, RNA accumulation was observed in coacervates over a range of pH, salinity, and R10/ATP stoichiometry. We also observed salt-dependent differences in molecular buffering and compartmentalization, which can be understood in terms of how salinity impacts the relative strengths of intermolecular binding modes that drive coacervation and RNA uptake. By varying the relative phase volumes and altering which intermolecular binding modes dominate, LLPS provided general mechanisms for resisting changes in molecular availability and environmental conditions, even without the active homeostasis of living cells. Such primitive molecular buffering aids the emergence of life and may have utility in biotechnological or commercial applications based on molecular compartmentalization.