Meteorites sugars

The question also arises as to where the chiral molecules came from. Were the L-amino acids or the D-sugars selected on the primeval Earth, or are exuaterresuial sources responsible for the homochirality This second possibility is dealt with by hypotheses on the effect of circularly polarised light, of extraterrestrial origin, on chiral molecules in the molecular clouds from which the solar system was formed. One such hypothesis was proposed by Rubenstein et al. (1983) and developed further by others, particularly A. W. Bonner (Bonner and Rubenstein, 1987) both scientists worked at Stanford University. The authors believe that the actual radiation source was synchrotron radiation from supernovae. The excess of one enantiomeric form generated by this irradiation process would have needed to be transported to Earth by comets and meteorites, probably during the bombardment phase around 4.2-3.8 billion years ago. 

Besides being desiccated and irradiated, microorganisms traveling in space will be exposed to space vacuum that can reach 10-14 pascal (a unit of pressure—100 Pa = 1 mbar).57 The result is extreme dehydration, and naked spores can survive for only days if exposed to space vacuum. Survival of spores is increased if they are associated with various chemicals such as sugars, or are embedded in salt crystals. Nicholson et al. (2000) discuss the various stresses that a microbial cell or spore would have to endure to survive interplanetary travel.58 They include the process that transports them out of Earth s atmosphere, such as volcanic eruptions and bolide impacts, long periods of transit in the cold of space, and atmospheric entry into a new planetary home. Spores have been shown to survive the shock conditions of a meteorite impact and the ultraviolet radiation and low temperature of space.59 It is clear that panspermia is possible and even probable if bacterial spores become embedded in rocks that are ejected from one planet and eventually enter the atmosphere of another. Bacterial... 

Phosphorus is abundant on Earth, both as an element (the llth-most abundant atom in Earth s crust) and as phosphate. Meteorites hold a variety of phosphate-containing minerals and some phosphide minerals.10 Scientists at the University of Arizona have recently suggested that Fe3P, the mineral schreibersite, leads to the formation of phosphate and phosphite when corroded in water. Although phosphorylation of alcohols was not demonstrated, mechanistic considerations suggest that it should be possible. It is noteworthy that a clear prebiotic pathway for the chemical incorporation of phosphate into RNA or DNA has not been found. No nucleosides (nucleobases joined to sugars) have been reported from meteorites. Nor has evidence been found in any meteorite of the presence of nucleosides or nucleotides (nucleosides attached to phosphates). That suggests that nucleic acids were first formed as products of metabolism. 

Cooper, G., Novelle, K., Belisle, W., Sarinana, J., Brabham, K., and Garrel, L. 2001. Carbonaceous meteorites as a source of sugar-related organic compounds for the early Earth. Nature 414 879-884. 

The question of the (nearly) exclusive use of l amino acids and d sugars in living organisms is one of the unsolved enigmas of the chemical sciences. As there are nonracemic amino acids found in meteorites [143,144] and as cpl occurs in outer space [145] and can be produced under natural terrestrial environmental conditions, cpl photolysis is discussed as a source of homochirality (up to scenarios like the synchronization of terrestrial cpl day cycles with the periodicity of tides [146]). The question of amino acid homochirality being due to asymmetric photolysis in space is discussed by Cerf and Jorisson [147], who summarize, In conclusion, we hope to have convinced the reader that the role of extraterrestrial asymmetric photolysis in the origin of the homochirality of natural amino acids on Earth, if at all involved, is far more complicated than is usually apprehended in the astronomical literature. The same authors more recently gave a critical review of the ample literature on asymmetric photochemistry as a possible source of homochirality [148]. 

Degens E. T. and Bajor M. (1962) Amino acids and sugars in the Brudeheim and Murray meteorites. Naturwiss. 49, 605-606. 

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 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. 

When L-amino acids (with the exception of proline) catalyzed the formose reaction, an excess of D-glyceraldehyde formed. In contrast, without any special catalyst, an equal mixture of the d- and L-glyceraldehyde formed. The reaction conditions were prebiotic. Furthermore, addition of small amounts of water increased the enantiomeric excess to more than 90% (Breslow et al. 2010). An intriguing mechanism of chiral induction takes place when chiral L-amino acids (such as the ones found in the Murchison meteorite) were used as basic catalysts in the formose reaction induced about 10% ee of D-threose (Pizzarello and Weber 2004). Stereo-selective syntheses of pentose sugars occur under realistic prebiotic conditions when LL-dipeptides catalyzed the formose reaction (Pizzarello and Weber 2010). 

The formose reaction formed chiral sugar molecules when catalyzed by an appropriate chiral amino acid under credible prebiotic conditions. Alternatively, chiral amino acids likely formed via import from meteorites with subsequent transfer of chirality to other molecules. The solid chiral nucleobase cytosine arose by mechanical stirring from solution, Evaporation of ribonucleosides and subsequent hydrolysis led to chiral ribonucleosides. 

Cooper G, Kimmich N, Belisle W et al (2001) Carbonaceous meteorites as a source of sugar-related organic compounds for the early earth. Nature 414 879-883 Cordova A, Engqvist M, Ibrahem I et al (2005) Plausible origins of homochirality in the amino acid catalyzed neogenesis of carbohydrates. Chem Comm 2047-2049 Cronin JR, Pizzarello S (1997) Enantiomeric excesses in meteoritic amino acids. Science 275 951-955 Di Guilio M (1997) On the RNA world evidence in favour of an early ribonucleopeptide. J Mol Evol 45 571-574... 

The possibility that small organic molecules could catalyze organic transformations in a stereoselective way can even be considered as a key element in the origins of life, as it is widely accepted that the source of homochirality in biological systems should be attributed to the presence of enantiomerically enriched a-amino acids in meteorites, and especially once it was demonstrated that these a-amino acids were able to catalyze aldol reactions in a stereoselective way under prebiotic conditions conducing to sugar-type products. ... 

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