Prebiotic molecules

Sidney Fox and Kaoru Harada, in particular, used simulation experiments to show how volcanism may have been involved in the synthesis of prebiotic molecules. They heated a stream of gas (CH4, NH3 and H2O) to about 1,123 K (using a silicate contact) after cooling, they could detect glycine, alanine, p-alanine and aspartic acid (among others). This experiment was intended to simulate exhalation from the earth s crust, as in volcanoes (Fox and Harada, 1961 Harada and Fox, 1964). 

Changing the initial conditions in the Urey-Miller experiment to favour a primordial atmosphere with CO2 as the primary carbon source does not produce such a rich mixture of prebiotic molecules and there is a significantly lower yield of amino acids an argument against an endogenous source of organic molecules. 

Endogenous organic synthesis Urey-Miller experiments as a source of prebiotic molecules via the Strecker synthesis for amino acids, HCN polymerisation for purines and pyrimidines and the formose reaction for sugars... 

Calculation of the internal cell potential is a very complicated matter because the electrochemistry of all of the species within the protocell would have to be balanced subject to their composition quotient Q, after which the standard free energy would have to be established from tabulations. The transport of Na+ would also change this balance, along with the ionic strength of the solution and the stability of the proteins or prebiotic molecules within the protocell. Such non-equilibrium thermodynamics forms the basis of the protocell metabolism. The construction... 

Polymerisation of HCN species is also possible once the initial monomers have been formed by the reactions with nitrogen HCN polymers have been postulated in many places in the solar system, from the clouds of Jupiter and Saturn to the dark colour of the surface of comet Halley, not to mention its possible role in the formation of the prebiotic molecule adenine. Photolysis of HCN produces CN and then the formation of nitrile polymers ... 

Was Nature s selection of these stereochemical structures a random event, or was it the consequence of determinate processes Although the presence of amino adds and other organic molecules on primitive Earth has been attributed to both endogenous and exogenous sources, [5,6] the origin of the chiral bias of prebiotic molecules remains a fundamental mystery. A mystery perhaps even more central to the origin of life is the mechanism that brought into dominance and survival any putative small chiral molecular excess on early Earth. 

Greenberg, J. M. Mendoza-Gomez, C. X. Interstellar dust evolution a reservoir of prebiotic molecules in Chemistry of Life s Origins Greenberg, J. M., Mendoza-Gomez, C. X. and Pirronello, V., Ed. Kluwer. Dordrecht, Netherlands, 1993 Vol. 1, pp 1-32. 

In particular, the aminoacyl phosphate derivatives of 5 -AMP could condense to a polypeptide containing up to 56 residues. This is certainly a very elegant and impressive reaction, hut its relevance from the point of view of prebiotic chemistry is doubtful since, as already mentioned, it is not obvious whether and to what extent this reaction can be extended to co-oligopeptides. Also, of course, AMP cannot be considered (thus far) a prebiotic molecule. Here again we encounter the old problem of the RNA world, the fact that the prebiotic synthesis of mononucleotides is not understood. There is no doubt that if a robust and credible prebiotic route... 

In the past 10 years a large number of organic molecules have been found in interstellar dust clouds mostly by emission lines in the microwave region of the spectrum (for a summary see Ref. 38). The concentration of these molecules is very low (a few molecules per cm3 at the most) but the total amount in a dust cloud is large. The molecules found include formaldehyde, hydrogen cyanide, acetaldehyde, and cyanoacetylene. These are important prebiotic molecules, and this immediately raises the question of whether the interstellar molecules played a role in the origin of life on the earth. In order for this to have taken place it would have been necessary for the molecules to have been greatly concentrated in... 

Amino acids were the first prebiotic molecules to be identified more than 50 years ago in experiments reproducing hypothetic prebiotic environ-... 

Sugars play a dual role in all living organisms they are the principal source of energy for cellular processes and are indispensable components of DNA and RNA information carriers. The possibility of prebiotic synthesis of various sugars was demonstrated by Butlerow in 1861 [28], who showed that a simple prebiotic molecule - formaldehyde - can be used for the synthesis of a mixture of various sugars in a Ca(OH)2-catalyzed series of aldol condensations. The reaction starts... 

The photocatalytic processes that could have been involved in the synthesis of prebiotic molecules are not limited to iron sulphides. Photocatalytic processes occurring on the surface of other sulphide and oxide materials should be considered as well. 

Simplicity of structure and ease of formation makes it reasonable to infer that polyP and PHB were components of early cells, possibly preceding RNA. PolyPs are prebiotic molecules, formed by condensation of phosphates in volcanic condensates and thermal vents at the bottom of the ocean.2,34,146 The synthesis of PHB is more demanding but requires only acetate and a reducing agent (see Figure 2B), both available in the primordial milieu. The conservation of these rudimentary homopoly-... 

Purine derivatives are considered to be prebiotic molecules. Purine as well as adenine (HCN) can be formed from hydrogen cyanide in the presence of ammonia in a prebiotic atmosphere. Similar reaction in the presence of water can lead to other purines. Purine chemistry goes back to the roots of organic chemistry. Uric acid was the first purine isolated from bladder stones by Scheelc in 1776. The correct empirical formula of uric acid was established much later by Liebig and Mitscherlich. The correct structure was suggested by L. Medicus and was shown to be correct by the syntheses ofHorbaezewski. Behrend and Roosen, and unambiguously by E. Fischer. The development of purine chemistry by E. Fischer is a milestone in heterocyclic chemistry. 

Delano J. W. (2001) Redox history of the earth s interior since 3900 Ma implications for prebiotic molecules. Origins Life Evol Biosphere 31, 311-341. 

One can conceive interesting developments of this scheme. For example, assume that one of these three surfactant aggregates, say C , forms a stable complex with a prebiotic molecule in the soup, say Z. Then, there would be a tendency to form this complex which would shift the chemical equilibrium towards the formation of C so as to form (CJ Z, which is thermodynamically more stable. In this case, C would be formed preferentially to the other aggregates. This would simulate again a kind of biological competition and selection mechanism. 

An alternative source of prebiotic molecules on the early Earth is from meteorites. Carbonaceous chondrite meteorites contain abundant carbon, present in a variety of molecular forms, including amino acids, polyols such as sugars and sugar-alcohols. These molecules are all important to life and form key components of nucleic acids and cell membranes (Cooper et al., 2001). Similar molecular evidence for amino acids is found in even more ancient fragments of the early Universe -IDPs. These molecules formed either in the solar nebula or in an interstellar environment (Flynn et al., 2003). Comets have also been considered a viable potential source of prebiotic molecules although currently the view is that most organic material was delivered by asteroids rather than by comets (Dauphas Marty, 2002). 

In addition, alternative hypotheses to Oparin s place the synthesis of prebiotic molecules in the hot, dry atmosphere of early volcanoes rather than the oceans (there are some laboratory xperiments showing that peptides and small proteins can be ibiotically synthesized under these conditions). Some still main- ain, like the astronomer Fred Hoyle, that the earth was seeded ... 

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