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Proteinoids, or thermal proteins, are protein-like, often cross-linked molecules formed inorganically from amino acids. Definition of proteinoids, slightly altered from Hayakawa et al. (1967): "macromolecular preparations of mean molecular weights in the thousands containing most of the twenty amino acids found in protein hydrolyzates. Although these polymers have other properties of contemporary protein as well, identity with the latter is not a necessary inference". Some theories of abiogenesis propose that proteinoids were a precursor to the first living cells.
The inorganic polymerization of amino acids into proteins through the formation of peptide bonds was thought to occur only at temperatures over 140°C. However, the biochemist Sidney Walter Fox and his co-workers discovered that phosphoric acid acted as a catalyst for this reaction. They were able to form protein-like chains from a mixture of 18 common amino acids at only 70°C in the presence of phosphoric acid, and dubbed these protein-like chains proteinoids. Fox later found proteinoids similar to those he had created in his laboratory in lava and cinders from Hawaiian volcanic vents and determined that the amino acids present polymerized due to the heat of escaping gases and lava. Other catalysts have since been found; one of them, amidinium carbodiimide, is formed in primitive Earth experiments and is effective in dilute aqueous solutions.
When present in certain concentrations in aqueous solutions, proteinoids form small structures called microspheres, protobionts, or protocells. This is because some of the amino acids incorporated into proteinoid chains are more hydrophobic than others, and so proteinoids cluster together like droplets of oil in water. Many argue that these are not themselves alive in the traditional sense, but these structures exhibit many of the characteristics of cells accepted as living cells:
- a film-like outer wall.
- osmotic swelling and shrinking.
- binary fission (dividing into two daughter microspheres).
- streaming movement of internal particles.
It is thought that the microspheres may have provided a cell compartment within which biochemistry could have become concentrated and protected from the outside environment during the process of chemical evolution.
Proteinoid microspheres are today being considered for use in pharmaceuticals, providing microscopic biodegradable capsules in which to package drugs for delivery to a specific region in the patient's body.
- Fox, Sidney W.; Kaoru Harada (14). "Thermal Copolymerization of Amino Acids to a Product Resembling Protein". Science. New Series 128 (3333): 1214.
- Fox, S. W. and K. Dose (1977). Molecular Evolution and the Origin of Life, Marcel Dekker, Inc.