Microspheres coacervation

A second historical model for protocells is provided by the microspheres (Fox, 1980 Nakashima, 1987 Lehninger, 1975). These are formed when hot saturated proteinoid solutions are allowed to cool (see Sect. 5.4.2). In recent years, the microspheres were also consigned to the limbo of unimportant scientific models. Perhaps there will come a time when coacervates or microspheres (in their original or in modified forms) find their way back into the scientific discussion. 

Today, Oparin s coacervates are not as favoured as Fox s microspheres or Wachtershausers s vesicles, and RNA replicators are preferred to Haldane s viroids, but these differences have not changed the substance of the original opposition. Between the two fundamental functions of life - metabolism and replication - Oparin gave an evolutionary priority to metabolism, while Haldane gave it to replication, and the choice between these two alternatives is still the key point that divides the origin-of-life theories in two contrasting camps. 

In favour of the metabolism paradigm there are, first of all, the results of the simulation experiments, and in particular the fact that the abiotic production of amino acids is so much easier than that of nucleic acids. Chemistry tells us that the primitive Earth could indeed generate enormous amounts of organic molecules that were potentially capable of having some type of metabolism, and of producing structures as complex as Oparin s coacervates, Fox s microspheres, or Wachtershauser s vesicles. 

The advantage of complex coacervates is that high payloads can be obtained. Chitosan/alginate coacervates have been reported to encapsulate up to 87% shark liver oil, which is rich in omega-3 fatty acids (Peniche et al. 2004). Microspheres of carboxymethyl chitosan/alginate hardened with calcium chloride have been used for encapsulation of up to 80% bovine serum albumin (Zhang et al. 2004). 

Proteinoids produced in laboratories can cluster together into droplets that separate, and that may protect their components from degrading influences of the surrounding environment. These droplets are like extremely simple cells, although they can not reproduce. Such droplets are called microspheres. When fats (i.e., lipids) are present, the microspheres that form are even more cell-like. If a mixture of linked amino acids called polypeptides, sugars called polysaccharides, and nucleic acids is shaken, droplets called coacervates will form. All of these kinds of droplets are called protobionts, and they may represent a stage in the genesis of cellular life. 

Thomasin, C. Merkle, H.P. Gander, B. Drug micro-encapsulation by PLA/PLGA coacervation in the light of thermodynamics. 2. Parameters determining microsphere formation. J. Pharm. Sci. 1998, 87 (3), 269-275. 

The concept of packaging microscopic quantities of materials within microspheres dates back to the 1930s and the work of Bungenberg de Jong and coworkers on the entrapment of substances within coacervates. The first commercial application of encapsulation was by the National Cash Register Company for the manufacture of carbonless copying paper. The technology... 

The polymer employed to prepare microspheres must be characterized in terms of molecular weight and purity,however this topic is beyond the scope of this article. Characterization of the materials may have implications for the formation of the microspheres. The viscosity and film-forming properties of the polymers used should be known. Viscosity can affect the tendency to form microspheres, their size, and even their shape. Burgess and coworkers have shown that albumin-acacia coacervates do not form microcapsules under certain conditions of pH and ionic strength, if the viscosity of the coacervate phase is too high. Burgess and Carless developed a method to predict the optimum conditions for complex coacer-vation based on the charge carried by the two polymers involved. 

In the area of controlled release, the preparation of indomethacin sustained-release microparticles from alginic acid (alginate)-gelatin hydrocolloid coacervate systems has been investigated. In addition, as controlled-release systems for liposome-associated macromolecules, microspheres have been produced encapsulating liposomes coated with alginic... 

Bayomi MA, al-Suwayeh SA, el-Helw AM, Mesnad AF. Preparation of casein-chitosan microspheres containing diltiazem hydrochloride by an aqueous coacervation technique. Pharma Acta Helv 1998 73 187-192. 

Kalyanasundaram S, Feinstein S, Nicholson JP, Leong KW, Garver RI Jr. Coacervate microspheres as carriers of recombinant adenoviruses. Cancer Gene Ther 1999 6 107-112. 

YIN Yin, X. and Stoever, H.D.H., Hydrogel microspheres by thermally induced coacervation of poly(A/,A/ -dimethylaciylamide-co-glycidyl methaciylate) aqueous soXviAoas, Macromolecules, 36, 9817, 2003. 

Solvent evaporation, coacervation and emulsification are the main reported methods of preparation of Chitosan beads. Blended beads of chitosan have been widely researched. The applicability of chitosan/alginate microparticles, chitosan/xanthan microspheres [8] and chitosan/gelatin microspheres have also been reported [16]. 

Alternatively, coacervation or solvent extraction is often used to produce microspheres (Fig. 3). The protein and polymer emulsion is stirred with a nonsolvent for the polymer such as silicone oil, resulting in the formation of embryonic microspheres (for a review, see Lewis, 1990). The nonsolvent extracts the methylene chloride or ethyl acetate firom the polymer phase, causing precipitation of the polymer and entrapment of the protein in the polymer matrix. To remove the nonsolvent, a volatile second nonsolvent (e.g., heptane) is added, and the microspheres are allowed to harden in the nonsolvent. After repeated extraction with the volatile nonsolvent, the final microspheres are then dried. While this method offers the advantage of avoiding contact between the protein phase and an aqueous phase as in the solvent evaporation method, the additional solvents utilized in this process are often difBcult to completely remove and are a safety and toxicity concern. 

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