Coacervate

Coacervate

[n. koh-as-er-vit, -veyt, koh-uh-sur-vit; v. koh-as-er-veyt, koh-uh-sur-veyt]
A coacervate is a tiny spherical droplet of assorted organic molecules (specifically, lipid molecules) which is held together by hydrophobic forces from a surrounding liquid.

Coacervates measure 1 to 100 micrometers across, possess osmotic properties and form spontaneously from certain dilute organic solutions. Their name derives from the Latin coacervare, meaning to assemble together or cluster. They were even once suggested to have played a significant role in the evolution of cells and, therefore, of life itself.

Formation

In water, organic chemicals do not necessarily remain uniformly dispersed but may separate out into layers or droplets. If the droplets which form contain a colloid, rich in organic compounds and are surrounded by a tight skin of water molecules, then they are known as coacervates. These structures were first investigated by the Dutch chemist H. G. Bungenburg de Jong, in 1932. A wide variety of solutions can give rise to them; for example, coacervates form spontaneously when a protein, such as gelatin, reacts with 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

Complex coacervation refers to the phase separation of a liquid precipitate, or phase, when solutions of two hydrophilic colloids are mixed under suitable conditions. The general outline of the processes consists of three steps carried under continuous agitation :

Step 1: Formation of three immiscible chemical phases:

The immiscible chemical phases are (i) a liquid manufacturing vehicle phase (ii) a core material phase and (iii) a coating material phase. To form the three phases, the core material is dispersed in a solution of the coating polymer, the solvent for the polymer being the liquid manufacturing vehicle phase. The coating material phase, an immiscible polymer in a liquid state, is formed by utilizing one of the methods of phase separation coacervation, that is,

  • By changing the temperature of the polymer solution
  • By adding a salt
  • By adding a non-solvent
  • By adding incompatible polymer to the polymer solution
  • By inducing a polymer-polymer interaction.

Step 2: Depositing the liquid polymer coating upon the core material:

This is accomplished by controlled, physical mixing of the coating material (while liquid) and the core material in the manufacturing vehicle. Deposition of the liquid polymer coating around the core material occurs if the polymer is adsorbed at the interface formed between the core material and the liquid vehicle phase, and this adsorption phenomenon is a prerequisite to effective coating. The continued deposition of the coating material is promoted by a reduction in the total free interfacial energy of the system, brought about by the decrease of the coating material surface area during coalescence of the liquid polymer droplets.

Step 3: Rigidizing the coating:

This is usually done by thermal, cross linking or desolvation techniques, to form a self sustaining microcapsule.

The Origin of Life

While proposing that modern life came from common ancestors, with the "tree" of life being simpler the farther back one goes, Charles Darwin suggested that, therefore, all living things may come from a single common ancestor, an "ur-organism", presumably something very simple and primitive. Then the question arises: from whence came that first organism?

Coacervates were actually suggested by Aleksandr Oparin, as a means by which that first "ur-organism" could have formed from non-living, organic matter. He noted that organic chemicals could be formed by the exposure of natural substances to sunlight (ultraviolet radiation, more specifically), in an oxygen-free atmosphere and then would sometimes recombine into larger molecules, until sufficient to form colloids and, therefore, coacervates. Since these coacervates do superficially resemble living cells, Oparin suggested that they eventually became complex enough to be simple life. While this is vaguely similar to modern theories, regarding the formation of first life, coacervates are no longer thought actually to have become the first cells directly; life is thought to have gone through many intermediate steps before becoming cellular.

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