Prebiotic reaction

However, the question must always be asked as to whether these processes could have taken place on the primordial Earth in its archaic state. The answer requires considerable fundamental consideration. Strictly speaking, most of the experiments carried out on prebiotic chemistry cannot be carried out under prebiotic conditions , since we do not know exactly what these were. In spite of the large amount of work done, physical parameters such as temperature, composition and pressure of the primeval atmosphere, extent and results of asteroid impacts, the nature of the Earth s surface, the state of the primeval ocean etc. have not so far been established or even extrapolated. It is not even sure that this will be possible in the future. In spite of these difficulties, attempts are being made to define and study the synthetic possibilities, on the basis of the assumed scenario on the primeval Earth. Thus, for example, in the case of the SPREAD process, we can assume that the surface at which the reactions occur could not have been an SH-containing thiosepharose, but a mineral structure of similar activity which could have carried out the necessary functions just as well. The separation of the copy of the matrix could have been driven by a periodic temperature change (e.g., diurnal variation). For his models, H. Kuhn has assumed that similar periodic processes are the driving force for some prebiotic reactions (see Sect. 8.3). 

Four billion years ago, the Earth s thin crust consisted of geochemicals (i.e., compounds containing the elements Si, O, Al, Fe, Mg, Ca, K and Na, as well as traces of other elements). Thus, some biogenesis researchers believed that the first replicating material consisted of geochemical material rather than substances containing carbon and other bioelements. Clay minerals in particular were included in experimental and theoretical studies. The most important are kaolinite and montmorillonite the latter was, and still is, used in many experiments carried out to simulate prebiotic reactions. 

Hazen and Deamer looked at the chemical and physical properties of the end products of hypothetical prebiotic reactions carried out under extreme conditions of pressure and temperature, for example in CCh-rich regions of hydrothermal vents. The results of laboratory experiments indicate that prebiotic syntheses leading to a variety of products could have occurred in hydrothermal systems some of these have amphiphilic properties, and would have been capable of self-organisation processes. 

The question of the elimination of water in polycondensation reactions still provides an unsolved problem. Solutions are being searched for in many laboratories, for example in Italy Paly6 and Zucchi from the University of Modena consider it possible that limited regions where liquid or supercritical CO2 phases were present could have existed on the young Earth. Such regions, with non-aqueous media, could have been particularly favourable for some prebiotic reactions, such as those involving the elimination of water. Experiments to study this hypothesis are planned (Paly6 and Zucchi, 2002 Holm and Andersson, 1998). 

These successful experiments suggest possible prebiotic reaction pathways for the formation, growth and multiplication of the first cells (Hanczyc et al., 2003). 

Solving the quadratic equation in gives the extent of the reaction = 0.00155, from which the mole fractions of the components can be calculated. This highlights a real problem with prebiotic reactions even if the equilibrium constants are known and the kinetics are favourable, the reaction may only produce small quantities of the required material simply because the concentrations of the reactants are too low. 

The objective of the preceding equilibrium calculation has been to determine the state of a molecule such as an amino acid in the conditions that prevailed on the early Earth. The pH, degree of dissociation and the extent of the reaction all have a direct effect on the population of the species present. Temperature and cooperative effects have not been considered but serve to complicate the problem. Any prebiotic reaction scheme must take account of that troublesome restriction to chemistry - the second law of thermodynamics. 

Formose reaction A prebiotic reaction producing sugars but little ribose. 

From these examples, it becomes clear that molecular symmetry can spontaneously break into chiral domains in the absence of any external force or seed. However, in every known case, the net symmetry remains intact, and the overall chirality sums to zero in the environment. Nevertheless, the spontaneous formation of macroscopic chiral regions in systems of associating achiral molecules is of interest to those who contemplate induction of molecular chirality in the context of prebiotic reaction chemistry. 

The metal-catalyzed amplification of e.e. in small molecules, demonstrated by Soai and coworkers, along with the chiral enrichment of amino arid polymers by sequential polymerization/depolymerization steps, have shown that small enantiomeric excesses in nearly racemic mixtures can be reactively amplified to produce chiral dominance. These real chemical systems, which include plausible prebiotic reactions, experimentally demonstrate the principle of the chiral amplification of a spontaneously broken chiral symmetry in a dynamic and authentic chemical milieu. Therefore amplification to dominance of a small chiral excess of both small and polymeric molecules can be credibly incorporated into an origin-of-life model. 

Prior sequestration of the prebiotic reactions within the micropores of weathered feldspars or other porous rock matrices also avoids many of the other problems of catalysis and dilution encountered by models of chemical biogenesis. That is, this mechanism attains viable evolutionary chemical selection among spatially discrete systems without the need to assume an unlikely capture-and-enclosure event involving a pre-existing lipid membrane. [192] Thus autocatalysis of chiral molecules could evolve before the actual appearance of free-floating lipid vesicles. 

To this short review of prebiotic reactions the work on prebiotic membraneforming compounds should be added. This will be considered later on in the chapter in the section on surfactant self-organization. 

Shen, C., Lazcano, A., and Or6, J. (1990a). The enhancement activities of histidyl-histidine in some prebiotic reactions. J. Mol. Evol, 31, 445-52. 

If components of the primitive atmosphere to be integrated into ligands do not act as ligands at all, step 1 will be realized in any case conversely, however, substantial amounts of CO (E CL) = -fO.99 V) in that atmosphere would have impeded step 1 exactly since it is readily used in prebiotic reactions (e.g. Miller and... 

Bean HD, Sheng Y, Collins IP, Anet FA, Leszczynski J, Hud NV. Formation of a beta-pyrimidine nucleoside by a free pyrimidine base and ribose in a plausible prebiotic reaction. J. Am. Chem. Soc. 2007 129 9556-9557. 

Many Components of Biochemical Macromolecules Can Be Produced in Simple, Prebiotic Reactions... 

How did this particular set of amino acids become the building I blocks of proteins First, as a set. they are diverse their structural and chemical properties span a wide range, endowing proteins with the ver-.satility to assume many functional roles. Second, many of these amino acids were probably available from prebiotic reactions, that is. from reactions that occurred before the origin of life. Finally, other possible amino acids may... 

In the early RNA world, the increasing populations of replicating RNA mol -ecules would have consumed the building blocks of RNA that had been generated over long periods of time by prebiotic reactions. A shortage of these compounds would have favored the evolution of alternative mechanisms... 

An answer could come from experiments which demonstrate that under chemical or prebiotic reaction conditions the cyclotetramer-ization of four PBG units gives a mixture of the four possible... 

Joshi PC, Pitsch S, Ferris JP Selectivity of montmorillonite catalyzed prebiotic reactions of D, L-nucleotides. Orig Life Evol Biosph 2007, 37(l) 3-26. 

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