Amino acids prebiotic systems

The predominance of L-amino acids in biological systems is one of life s most intriguing features. Prebiotic syntheses of amino acids would be expected to produce equal amounts of L- and D-enantiomers. Some kind of enantiomeric selection process must have intervened to select L-amino acids over their D-connterparts as the constituents of proteins. Was it random chance that chose L- over D-isomers ... 

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Soon after the first successful prebiotic syntheses of amino acids by Miller and Urey, the next step, polycondensation of these monomers, was attempted. But how could the activation of the monomers have occurred on the primeval Earth without the help of special enzymes In order to try and solve this question (in fact, there is a whole series of questions), some research groups began to work on the question using systems which were as simple as possible, in the hope of either solving it or at least coming close to an answer. 

The processes occurring at hydrothermal systems in prebiotic periods were without doubt highly complex, as was the chemistry of such systems this is due to the different gradients, for example, of pH or temperature, present near hydrothermal vents. Studies of the behaviour of amino acids under simulated hydrothermal conditions showed that d- and L-alanine molecules were racemised at different rates the process was clearly concentration-dependent. L-Alanine showed a low enantiomeric excess (ee) over D-alanine at increasing alanine concentrations. The same effect was observed with metal ions such as Zn2+ in the amino acid solution. Thus, homochi-ral enrichment of biomolecules in the primeval ocean could have resulted under the conditions present in hydrothermal systems (Nemoto et al., 2005). 

The accumulation of the primordial genome is a chance event by any hypothesis whereby one must grant the possibility that there may have been a tendency to form a certain sequence faster than another, but that problem is left waiting until one knows how primordial condensation occurred. In any case, the genomist claims that all variations observed today are due to prebiotic events1 in contrast to the old model.2 Proteins would change continuously in the Darwinian system with survival as the only selective force. Proteins in this type of study come, for obvious reasons, exclusively from survivors. The testable aspect of the hypothesis is the proposal that functionally important amino acids remain constant in a protein and that functionally unimportant ones are subject to mutational replacement. 

NaC103 [ 158,159]. The asymmetric induction in these reactions is most probably due to specific interactions between the Zn atom and the chiral surface of the crystals. This amplification reaction could be also performed on racemic mixtures of amino acids or helicenes irradiated first with circularly polarized light to yield non-racemic mixtures of very low ee that transformed the solution into a non-racemic mixture, which was then further amplified to an ee of beyond 95% [160]. Although the reaction system is prebiotically unrealistic, since it was performed in non-aqueous solutions, it still demonstrates the feasibility of spontaneously driving non-chiral systems towards single handedness by auto catalysis. 

Various comprehensive studies on the polymerization of enantiopure and racemic esters of a-amino acids performed at the air/water interface to yield peptides have been reported over the years [189,190]. Recent reinvestigations of the products of these reactions by MALDI-TOF MS have demonstrated, however, that they are not longer than dipeptides [191]. For this reason, such esters cannot be regarded as realistic prebiotic model systems for the formation of long oligopeptides. On the other hand, amphiphilic Na-carboxyanhydrides [192] and thio-esters [193] of a-amino acids yield longer oligopeptides. 

Amino acids have probably formed in different places of the Solar system through processes independent of living organisms as shown by their occurrence in carbonaceous chondrites, a particular class of meteorites. Among these processes, only a particular subset of them can be considered to be truly prebiotic. It is the processes that make amino acids available in environments capable of developing a complex chemistry compatible, or at least presumed to be compatible, with the emergence of life. 

Particularly important here is the role of transition metal sulfides. In 1988 Wachtershauser proposed that pyrite, abundant in hydrothermal vent systems, provided an energy source for the first life. He suggested that pyrite provided the catalyst necessary to drive a number of essential chemical reactions which are important precursors to life. More recent studies have confirmed this view and have shown that the sulfides of Fe, Ni, Co, and Zn can play an important role in the fixation of carbon in a prebiotic world (Cody et al., 2004). Transition metal sulfides also play a role in more advance organic synthesis, and Huber and Wachtershauser (1998) showed how amino acids were converted into their peptides using a (NiFe)S catalyst. 

In virtually all experiments that simulate the synthesis of a primordial soup, enantiomers of amino acids and sugars do not occur instead only racemates have been produced (Miller 1953). It is difficult to imagine how only one of the enantiomers formed under the conditions of a primordial broth. Instead, import and identification of biologically relevant molecules in meteorites and comets appears as a more straightforward path. The compounds found in a meteorite provide a natural record of prebiotic chemistry in the early solar system and is closest to the onset of life. 

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