Amino acids prebiotic synthesis

Considerable attention has been focussed upon possible amino acid prebiotic syntheses in this section, because of their biological importance. It should be pointed out that many other compounds of biological significance have also been made under simulated primitive conditions and it is proposed to conclude this review with a brief examination of such reactions, particularly with respect to the possible role of coordination compounds in them. 

Weber AL. Prebiotic amino acid thioester synthesis thiol-dependent amino acid synthesis from formose substrates (formaldehyde and glycolaldehyde) and ammonia. Orig. Life Evol. Biosph. 1998 28 259-270. 

Palchetti I, Mascini M (2005) Electrochemical Adsorption Technique for Immobilization of Single-Stranded Oligonucleotides onto Carbon Screen-Printed Electrodes. 261 27-43 Pascal R, Boiteau L, Commeyras A (2005) From the Prebiotic Synthesis of a-Amino Acids Towards a Primitive Translation Apparatus for the Synthesis of Peptides. 259 69-122 Paulo A, see Santos I (2005) 252 45-84 Perez EM, see Leigh DA (2006) 265 185-208 Perret F, see Coleman AW (2007) 277 31-88 Perron H, see Coleman AW (2007) 277 31-88 Pianowski Z, see Winssinger N (2007) 278 311-342 Piestert F, see Gansauer A (2007) 279 25-52... 

The term chemical evolution" was introduced by the Nobel Prize winner Melvin Calvin and refers to the process of the synthesis of biochemically important molecules from small molecules and certain chemical elements under the (hypothetical) conditions present on prebiotic Earth. It is assumed that the smaller building block molecules such as amino acids, fatty acids or nucleobases were formed initially, and that these underwent polycondensation to give macromolecules in later stages of development. 

The variety of prebiotic organic reactions seems to be almost unlimited. Strasdeit et al. (2002) from the University of Hohenheim (Germany) reported the synthesis of zinc and calcium complexes of the amino acids valine and isovaline. They assume that these could have had a certain significance on the mineral-rich primeval Earth on heating to 593 K under nitrogen, valine was converted to the corresponding cyclic dipeptide. 

Arthur L. Weber (1998), now working at the Seti Institute of the Ames Research Center at Moffett Field, reports the successful synthesis of amino acid thioesters from formose substrates (formaldehyde and glycolaldehyde) and ammonia synthesis of alanine and homoserine was possible when thiol catalysts were added to the reaction mixture. On the basis of his experimental results, Weber (1998) suggests the process shown in Fig. 7.10 to be a general prebiotic route to amino acid thioesters. 

The pH of the oceans forming the primordial soup is important in controlling the charged nature, or otherwise, of the amino and carboxylic acid species and hence their chemistry. Generating reaction schemes for the prebiotic synthesis of molecules requiring basic conditions will not be relevant if the oceans are acidic. Consider dissolving CO2 into water, simply written as ... 

Amino acid formation in the Urey-Miller experiment and almost certainly in the prebiotic environment is via the Stecker synthesis shown in Figure 8.3. This reaction mechanism shows that the amino acids were not formed in the discharge itself but by reactions in the condensed water reservoir. Both HCN and HCO are formed from the bond-breaking reactions of N2 and H2O in a plasma, which then react with NH3 in solution. The C=0 group in formaldehyde or other aldehydes is replaced by to form NH and this undergoes a reaction with HCN to form the cyano amino compound that hydrates to the acid. The Strecker synthesis does not provide stereo-control over the carbon centre and must result in racemic mixtures of amino acids. There is no room for homochirality in this pathway. 

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... 

Strecker synthesis The prebiotic synthesis of amino acids. 

Several factors indicate that the amino acids detected in all of these carbonaceous chondrites are indigenous and that they must have originated abiotically. First, the presence of protein and non-protein amino acids, with approximately equal quantities of D and L enantiomers points to a nonbiological origin and precludes terrestrial contamination. In addition, the non-extractable fraction of the Murchison is significantly heavier in 13C than terrestrial samples. Finally, the relative abundances of some compounds detected resemble those of products formed in prebiotic synthesis experiments. The aliphatic hydrocarbons are randomly distributed in chain length, and the C2, C3, and C4 amino acids have the highest concentrations (i.e., the most easily synthesized amino acids with the least number of possible structures are most abundant) [4]. 

The synthesis in many extant organisms of these two amide residues from their respective precursors glutamate and aspartate esterified to tRNA (the indirect aminoacylation pathways described in Sections 5.14.3 and 5.14.4) and that of other amino acid residues, such as selenocysteine (which is also synthesized from a precursor esterified on a tRNA °) support the model of prebiotic metabolism taking place at the surface of solid particles, " analogous to ancestral RNAs. 

Pascal R, Boiteau L, Commeyras A (2005) From the Prebiotic Synthesis of a-Amino Acids Towards a Primitive Translation Apparatus for the Synthesis of Peptides. 259 69-122 Paulo A, see Santos I (2005) 252 45-84... 

It is of course surprising that amino acids can be obtained via the Strecker synthesis, purines from the condensation of HCN, pyrimidines from the reaction of cyanoacetilene with urea, and sugars from the autocatalytic condensation of formaldehyde. The synthesis of chemical constiments of contemporary organisms by non-enzymatic processes under laboratory conditions does not necessarily imply that they were either essential for the origin of life or available in the primitive environment. However, the significance of prebiotic simulation experiments is... 

Going back to Miller s synthesis in the flask, one question is why a-amino acids have been obtained and not, for example, p-amino acids, cyclic diketopiperazines, or some other isomers. The answer is important a-amino acids form because they are the most stable products under the selected initial conditions. In other words the formation of those a-amino acids is under thermodynamic control. The same can be said for Oro s synthesis of adenine and other prebiotically low-molecular-weight substances formed in hypothermal vents, or found in space certain molecules and not others form because they are thermodynamically more stable. 

Hydrogen cyanide and methanal are especially reasonable starting materials for the prebiotic synthesis of amino acids, purine and pyrimidine bases, ribose and other sugars. Formation of glycine, for example, could have occurred by a Strecker synthesis (Section 25-6), whereby ammonia adds to methanal in the... 

Electric discharges acting on a mixture of CO, N2, and H2 are not effective in amino acid synthesis unless the ratio of H2 to CO is greater than about 1.0. Glycine is produced in fair yield, but only small amounts of any higher amino acids are produced. Large amounts of formaldehyde are obtained, however, and formaldehyde is important in the prebiotic synthesis of sugars. 

The above review shows the progress that has been made in the last 30 years. The prebiotic synthesis of amino acids, purines, pyrimidines, and sugars is understood at a basic level, although more details of the reactions are needed. The polymerization processes are less well understood, and while some of them are plausible it is necessary to work them out in greater detail. The template polymerization reactions are an exciting beginning and may show how genetic information started to accumulate. So far the problem of nucleic acid directed enzyme synthesis has not been dealt with on an experimental level. The problems in this area, which are very difficult, are considered by other speakers in this symposium. 

Four mechanisms have been advanced for the prebiotic formation of amino acids. The first involves a cyanohydrin (reaction 2) and a related route (reaction 3) can be invoked to account for the presence of hydroxy acids. These particular reactions have been studied in considerable detail both kinetically and in terms of thermodynamic quantities.347 An alternative route (4) involves the hydrolysis of a-aminonitriles, which are themselves formed directly in anhydrous CH4/NH3 mixtures.344 Cyanoacetylene, formed in CH4/N2 irradiations,349 yields significant amounts of asparagine and aspartic acids (reaction 5). Finally, a number of workers336,350"354 have proposed that HCN oligomers, especially the trimer aminoacetonitrile and the tetramer diaminomaleonitrile, could have been important precursors for amino acid synthesis. Reaction mixtures involving such species have yielded up to 12 amino acids. Table 11 indicates the range of amino acids produced in these kinds of sparking syntheses. Of some interest is the fact that close parallels between these kinds of experiments and amino acid contents of carbonaceous chondrite meteorites exist.331,355,356... 

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