Amino acids hydrothermal vents

Simulation experiments of a different type were carried out by two Japanese researchers (Matsunu, 2000 Imai et al., 1999a, b). They used a simulation reactor to study processes which may occur at hydrothermal vents (see Sect. 7.2). In this case, the activation energy for the polycondensation reaction of amino acids has its origin in the Earth s interior. In the high pressure hot water reactor used, the reaction... 

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). 

Amino Acid Formation in Submarine Hydrothermal Vents. 78... 

This requirement is fulfilled for electric discharges in a reduced atmosphere containing methane, ammonia, and water, as in the original Miller experiment. It has also been observed for atmospheres based on N2 and CO or CO2 on the condition that H2 or methane is also present in snfflcient amonnts (19). A neutral atmosphere (based on N2, CO2, and water) wonld produce much lower yields of organics (by several orders of magnitude). In the absence of other species to be oxidized, the rednction of CO2 reqnires the concomitant thermodynamically nnfavorable conversion of water into O2 (as in photosynthesis). However, even if the atmosphere was nentral when life arose, as nsnaUy believed, the Earth was not nniform with respect to redox state simply becanse the rednced state of the mantle and the high volcanic activity favored the occnrrence of locally rednced environments (for instance, in hydrothermal vents in the oceans). Then, a preservation of the hydrogen content of the early atmosphere or the diversity of environments on the early Earth is likely to have made amino acid formation possible, at least at specific places. 

Unlike two previous theories of life origin, only a few pieces of experimental evidence exist at present to prove the theoretical speculations. However, we have to notice the verification of the basic mechanism of molecular hydrogen generation as a reducing power, furthermore, the amide bond synthesis has been also demonstrated, both at temperatures within the range of hydrothermal vents (100 °C). In addition, the evidence for at least sulfide-based amino acid synthesis and polymerization from simple precursors has been shown. The formation of acetic acid and an activated thioester from carbon monoxide, methanethiol and various combinations of ferrous and nickel sulfides has been experimentally proved as well. However, further verification is necessary for the modes and rates of organic synthesis. 

However, amino acids are unlikely to form themselves into polymers without the help of some form of catalyst (Bada, 2004). Possible natural catalysts are mineral surfaces such as in the regular, repeating structure of clays although once bound to a clay the polymer has to be released. This is achieved in the laboratory with salt solutions and may, in nature, reflect an evaporative marine environment. An alternative venue is at hydrothermal vents where peptide bond formation is favored, and where catalysis may take place on sulfide mineral surfaces (Bada, 2004). Such a process has been described by Holm and Charlou (2001) who found linear saturated hydrocarbons with chain lengths of 16 to 29 carbon atoms in high-temperature hydrothermal fluids from a vent in the Mid-Atlantic Ridge. 

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 the late 1980 s, Gunter Wachtershauser, a chemist at the University of Regensberg in Germany, described in more detail how amino acids and peptides could be made near hydrothermal vents. The general steps he proposed were as follows ... 

One of the perceived weaknesses of Wachtershauser s theory was the need for ammonia. Some scientists questioned whether it would be present in high enough concentrations for the conversion of pyruvic acid to amino acids, but Braudes has done experiments that show that ammonia is formed when a mixture of water and nitrogen oxides known to be found in the vents is heated to 500 °C and compressed to 500 atm. Experiments done by Brandes duplicating the conditions at hydrothermal vents also show a 40% conversion of pyruvic acid and... 

Prebiotic peptides, peptides formed before the origin of life. Most likely, amino acids were already present on primitive Earth. They are supposed to have been produced in the primitive atmosphere, in hydrothermal vents, or to have been imported in meterorites. a-Amino acids can undergo peptide formation by activation with carbon monoxide under hot aqueous conditions in the presence of freshly co-precipitated colloidal (Fe,Ni)S. Peptides may have been formed via —>-N-carboxy anhydrides. A replicative synthesis involving aminoacyl-RNA intermediates has also been suggested. The question of whether a peptide/protein world preceded the RNA-driven template synthesis, or whether RNA and proteins should not be viewed as eti-ologically discrete entities in the origin of life, is still under debate [V. Borsenberger et al., Chem. Biodivers. 2004, 1, 203 C. Huber et al., Science 2003, 301, 938 A. Brack, Chem. Biodivers. 2007, 4, 665]. 

Gustafson LB, Williams N (1981) Sediment-hosted stratiform deposits of copper, lead, and zinc. In Skinner BJ (ed) Economic geology. 75th Anniversary volume. Society of Economic Geologists, El Paso, TX, pp 139-178 Haberstroh PR, Karl DM (1989) Dissolved free amino acids in hydrothermal vent habitats of the Guaymas basin. Geochim Cosmochim Acta 53 2937-2945 Hall GA (1949) The kinetics of the decomposition of malonic acid in aqueous solution. J Am Chem Soc 71 2691-2693... 

In the 1970s black smokers (Fig. 2.2) were detected. These are chimney-like structures above hydrothermal vents. In these smoker chimneys sulfides of iron, copper and zinc are found. At the mixture of the hot mineral rich water with cold water, these sulfides are precipitated and the vent water therefore appears black in color. The most striking discovery was that these warm chemical rich environments are the living space for many species. Huber and Wachtershauser, 1998 [166], modelled volcanic or hydrothermal settings. They showed that amino acids were converted into their peptides by use of precipitated (Ni, Fe)S and CO in conjunction with H2S (or CH3SH) as a catalyst and condensation agent at 100°C and pH 7 to 10 under anaerobic, aqueous conditions. Thus a thermophilic origin of life seems plausible. 

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