Water prebiotic role

As discussed in the previous chapters, water plays a pivotal role in the structure, dynamies, and function of biomolecules. They cannot lunction without water. The role of water in the evolutionary process is another intriguing aspect that has drawn some attention, particularly in the context of the prebiotic soup, which refers to the aqueous solution of various constituents that ultimately form proteins, DNA, etc. 

The authors chose pyruvic acid as their model compound this C3 molecule plays a central role in the metabolism of living cells. It was recently synthesized for the first time under hydrothermal conditions (Cody et al., 2000). Hazen and Deamer carried out their experiments at pressures and temperatures similar to those in hydrothermal systems (but not chosen to simulate such systems). The non-enzymatic reactions, which took place in relatively concentrated aqueous solutions, were intended to identify the subsequent self-selection and self-organisation potential of prebiotic molecular species. A considerable series of complex organic molecules was tentatively identified, such as methoxy- or methyl-substituted methyl benzoates or 2, 3, 4-trimethyl-2-cyclopenten-l-one, to name only a few. In particular, polymerisation products of pyruvic acid, and products of consecutive reactions such as decarboxylation and cycloaddition, were observed the expected tar fraction was not found, but water-soluble components were found as well as a chloroform-soluble fraction. The latter showed similarities to chloroform-soluble compounds from the Murchison carbonaceous chondrite (Hazen and Deamer, 2007). 

To investigate the role of water photolysis in ozone production in a prebiotic atmosphere, the following mechanism can be explored ... 

Water ice in the form of interstellar grains may have played an important role in prebiotic chemistry. These tiny particles permeate the interstellar medium, where they gather carbonaceous and silicate material and absorb ultraviolet radiation. Complex chemical processes on their surfaces produce a large variety of organic substances for possible later delivery to planets. 

Proline is a stable, nontoxic, cyclic, secondary pyrrolidine-based amino acid with an increased pK value. Thus, proline is a chiral bidentate compound that can form catalytically active metal complexes (Melchiorre et al. 2008). Bidentate means that proline has not only one tooth but also a second one to bite and react. The greatest difference to other amino acids is a Lewis-base type catalysis that facilitates iminium and enamine-based reactions. It is especially noteworthy that cross-aldol condensations of unprotected glycoladehyde and racemic glyceralde-hyde in the presence of catalytic amounts of the Zn-(proline)2 gave a mixture of pentoses and hexoses (Kofoed et al. 2004). Again, proline seems to play the decisive role. The conditions are prebiotic the reaction proceeded in water for seven days at room temperature. It is remarkable that the pentose products contained ribose (34%), lyxose (32%), arabinose (21%), and xylose (12%) and that all are stable under the conditions. Thus, the diastereomeric and enantiomeric selection observed support the idea that amino acids have been the source of chirality for prebiotic sugar synthesis. 

The reaction is basically the citric acid cycle run in reverse. Where the Krebs cycle takes complex carbon molecules in the form of sugars and oxidizes them to C02 and water, the reverse cycle takes C02 and water to make carbon compounds. This process is used by some bacteria to synthesize carbon compounds, sometimes using hydrogen or sulfates as electron donors. The reaction is a possible candidate for prebiotic early Earth conditions and so it is of interest in the origin of life. On the early Earth, a primitive form of acetyl-CoA-like thioacetate played the role of the essential coenzyme. It has been found that some of the steps can be catalyzed by minerals. Thus, the FeS world proposes that the reverse citric acid cycle operated nonenzymatically on the primitive Earth. The question is whether it is possible to retrace other ancient metabolic pathways. Combination of the recently found plugged... 

In nature, protein adsorption manifests itself in various areas. For instance, even in the prebiotic stage of the genesis of terrestrial life, accumulation of proteins and proteinaceous components at soil/water interfaces may have played an essential role. 

That insoluble porous oxides may have had a fundamental role in prebiotic conditions was first hypothesized by Bernal [3]. He proposed that the organic compounds dissolved in the primordial soup had been concentrated by adsorption on the surfaces of clay mineral particles suspended in the early oceans, and that the subsequent prebiotic reactions synthesizing the organic substrates were catalyzed at active sites on the clay surfaces. It is interesting to note that water adsorbed in restricted geometries (in which the organic compounds had concentrated) has properties different from those of liquid water. For instance, in particular situations (water adsorbed in porous silica) K is preferentially... 

The role of comets and primitive asteroids in supplying water and prebiotic organics to early Earth was reviewed by Mumma, 2008 [240]. 

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