Thermal Condensations
Thermal Condensations
Most biological polymerizations are condensation (dehydration) reactions, in which monomers are linked together by the removal of water. In living systems, condensation reactions always occur in an aqueous (cellular) environment containing appropriate enzymes. Without enzymes and energy supplied by ATP, macromolecules (proteins and nucleic acids) of living systems soon decompose into their constituent monomers.
One way in which dehydration reactions could have occurred without enzymes in primitive earth conditions is by thermal condensation. The simplest dehydration is accomplished by driving water from solids by direct heating. For example, heating a mixture of all 20 amino acids to 180° C produces a good yield of polypeptides.
The thermal synthesis of polypeptides to form “proteinoids” has been studied extensively by the American scientist Sidney Fox. He showed that heating dry mixtures of amino acids and then mixing the resulting polymers with water forms small spherical bodies. These proteinoid microspheres (Figure 2-13) possess certain characteristics of living systems. Each is not more than 2 µm in diameter and is comparable in size and shape to spherical bacteria. The outer walls of the microspheres appear to have a double layer, and they show osmotic properties and selective diffusion. They may grow by accretion or proliferate by budding like bacteria. We do not know whether proteinoids may have been the ancestors of the first cells, or whether they are just interesting creations of a chemist’s laboratory. Their formation requires conditions likely to have occurred only in volcanoes. Organic polymers might have condensed on or in volcanoes and then, wetted by rain or dew, reacted further in solution to form polypeptides or polynucleotides.
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| Figure 2-13 Electron micrograph of proteinoid microspheres. These proteinlike bodies can be produced in the laboratory from polyamino acids and may represent precellular forms. They have definite internal ultrastructure. (×1700) |
Most biological polymerizations are condensation (dehydration) reactions, in which monomers are linked together by the removal of water. In living systems, condensation reactions always occur in an aqueous (cellular) environment containing appropriate enzymes. Without enzymes and energy supplied by ATP, macromolecules (proteins and nucleic acids) of living systems soon decompose into their constituent monomers.
One way in which dehydration reactions could have occurred without enzymes in primitive earth conditions is by thermal condensation. The simplest dehydration is accomplished by driving water from solids by direct heating. For example, heating a mixture of all 20 amino acids to 180° C produces a good yield of polypeptides.
The thermal synthesis of polypeptides to form “proteinoids” has been studied extensively by the American scientist Sidney Fox. He showed that heating dry mixtures of amino acids and then mixing the resulting polymers with water forms small spherical bodies. These proteinoid microspheres (Figure 2-13) possess certain characteristics of living systems. Each is not more than 2 µm in diameter and is comparable in size and shape to spherical bacteria. The outer walls of the microspheres appear to have a double layer, and they show osmotic properties and selective diffusion. They may grow by accretion or proliferate by budding like bacteria. We do not know whether proteinoids may have been the ancestors of the first cells, or whether they are just interesting creations of a chemist’s laboratory. Their formation requires conditions likely to have occurred only in volcanoes. Organic polymers might have condensed on or in volcanoes and then, wetted by rain or dew, reacted further in solution to form polypeptides or polynucleotides.
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