Complex coacervates as functional protocells

Coacervates are condensed liquid-like droplets that can be formed with oppositely charged polyelectrolytes and small molecules through liquid-liquid phase separation. They have been proposed as promising protocell candidates, because of their spontaneous formation and membrane-free nature, which allows for the uptake and concentration of a wide range of nutrients and building blocks required for life. The local physicochemical environment inside coacervates is distinctly different from the surrounding dilute solution and offers an interesting microenvironment for prebiotic reactions. Coacervates can selectively take up reactants and enhance their effective concentration, stabilize products, destabilize reactants and lower transition states, and can therefore play a similar role as micellar catalysts in providing rate enhancement and selectivity in reaction outcome. Here, we will discuss how complex coacervate formation can be controlled by chemical reactions, how reactions, such as amide bond formation, can be accelerated by coacervate droplets, and how the interface can be used as template for the assembly of protein filaments and lipid bilayers, bringing new life to Oparin’s century-old coacervate protocell hypothesis.