A coacervate is a tiny spherical droplet of assorted organic molecules (specifically, lipid molecules) which is held together by hydrophobic interactions with a surrounding liquid. Coacervates measure 1 to 100 micrometers across, possess osmotic properties and form spontaneously from certain dilute organic solutions. The name "coacervate" derives from the Latin coacervare, meaning "to assemble together or cluster".
The process of coacervation was famously proposed by Alexander Oparin as crucial in his early theory of abiogenesis (origin of life). This theory proposes that metabolism predated information replication, although the debate as to whether metabolism or molecules capable of Template replication came first in the origins of life remains open and for decades Oparin's theory was the leading approach to the origin of life question.
These structures were first investigated by the Dutch chemist H.G. Bungenberg de Jong, in 1932. A wide variety of solutions can give rise to them; for example, coacervates form spontaneously when a disordered polypeptide, such as gelatin, reacts with another biologically derived polyelectrolyte, gum arabic. They are interesting not only in that they provide a locally segregated environment, but also in that their boundaries allow the selective absorption of simple organic molecules from the surrounding medium. In Oparin's view this amounts to an elementary form of metabolism. Bernal commented that they are "the nearest we can come to cells without introducing any biological – or, at any rate, any living biological – substance." However, the lack of any mechanism by which coacervates can reproduce leaves them far short of being living systems.
Complex coacervation commonly refers to the liquid-liquid phase separation that results when solutions of two oppositely charged macroions are mixed, resulting in the formation of a dense macroion-rich phase, the precursors of which are soluble complexes.
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