Murchison meteorite compounds

Analysis of carbon compounds—even amino acids—from extraterrestrial sources might provide deeper insights into this mystery. John Cronin and Sandra Pizzarello have examined the enantiomeric distribution of unusual amino acids obtained from the Murchison meteorite, which struck the earth on September 28, 1969, near Murchison, Australia. (By selecting unusual amino... 

Aromatic hydrocarbons were found in more recent analyses pyrene, fluoran-threne, phenanthrene and naphthalene in the ratio of 10 10 5 1 (Cronin, 1998). The majority (around 70%) of the hydrocarbons extracted from the Murchison meteorite are polar compounds such as ... 

The analysis of extraterrestrial matter is concentrated on the detection of nucleic acid and protein building blocks, i.e., N-heterocycles and amino acids. The search for such compounds began immediately after the fall of the Murchison meteorite. Twenty-two amino acids were detected in it as early as 1974 eight of them pro-teinogenic, ten which hardly ever occurred in biological material, and four which were unknown in the biosphere. Up to now, about 70 amino acids have been identified (Cronin, 1998), the most common being glycine and a-aminoisobutyric acid. The latter is a branched-chain amino acid with the smallest possible number of carbon atoms. The most frequently found amino acids occur in concentrations of... 

Analysis of the Murchison meteorite led to a completely different type of phosphorus compound the only phosphorus-containing compounds found were alkanephos-phonic acids. Spurred on by these results, de Graaf et al. (1995) irradiated mixtures of o-phosphorous acid in the presence of formaldehyde, primary alcohols or acetone with UV light (low pressure Hg lamp, 254 nm with a 185-nm component) and obtained phosphonic acids, including hydroxymethyl and hydroxyethyl phosphonic acids, which had been found in the Murchison meteorite. Alkanephosphonic acids can be derived from phosphorous acid, with a P-H bond being replaced by a P-C bond. 

The formation of relatively stable vesicles did not require the presence of pure compounds mixtures of components could also have done the job. However, whether the concentrations of the compounds isolated from the Murchison meteorite would have been sufficient for the formation of prebiotic protocells or vesicles is unclear, even if concentration effects are assumed. Sequences in which the technical Fischer-Tropsch synthesis is the role model have been proposed as possible sources of amphiphilic building blocks. 

D. W. Deamer and J. P. Dworkin have reported in detail on the contribution of chemistry and physics to the formation of the first primitive membranes during the emergence of precursors to life the authors discussion ranges from sources of amphiphilic compounds, growth processes in protocells, self-organisation mechanisms in mixtures of prebiotic organic compounds (e.g., from extracts of the Murchison meteorite) all the way to model systems for primitive cells (Deamer and Dworkin, 2005). 

The Murchison meteorite shown in Figure 6.7, like all meteorites, is named after the place from which it was recovered and in this case it is the town of Murchison, Victoria in Australia about 100 km north of Melbourne. The fall occurred in 1969 and was followed by an analysis of the chemical composition in some considerable detail. The Murchison meteorite is a carbonaceous chondrite containing about 2 per cent carbon, some as inorganic carbonates, and some as soluble compounds such as amino acids but the bulk as a macromolecular heterogeneous material referred to as kerogen. 

The kerogen-like material from the Murchison meteorite is shown in Figure 6.8 and consists of a rich macromolecular carbonaceous material made from aromatic and aliphatic compounds observed in fluorescence following excitation at 280 nm. 

Figure 9.29 Membrane formation by meteoritic amphiphilic compounds (courtesy of David Deamer). A sample of the Murchison meteorite was extracted with the chloroform-methanol-water solvent described by Deamer and Pashley, 1989. Amphiphilic compounds were isolated chromatographically on thin-layer chromatography plates (fraction 1), and a small aliquot ( 1 p,g) was dried on a glass microscope slide. Alkaline carbonate buffer (15 p,l, 10 mM, pH 9.0) was added to the dried sample, followed by a cover slip, and the interaction of the aqueous phase with the sample was followed by phase-contrast and fluorescence microscopy, (a) The sample-buffer interface was 1 min. The aqueous phase penetrated the viscous sample, causing spherical structures to appear at the interface and fall away into the medium, (b) After 30 min, large numbers of vesicular structures are produced as the buffer further penetrates the sample, (c) The vesicular nature of the structures in (b) is clearly demonstrated by fluorescence microscopy. Original magnification in (a) is x 160 in (b) and (c) x 400.

Stocks, P. G. and Schwarz, A. W. (1982). Basic nitrogen-heterocyclic compounds in the Murchison meteorite. Geochim. Cosmochim. Acta, 46, 309-15. 

Table 3. Amounts of low molecular weight compounds found in extracts of the Murchison meteorite. Data are expressed in nanomoles per gram, (the list is not exhaustive)... 

Is this a plausible premise In order to approach this question, we can assume that the mixture of organic compounds in carbonaceous meteorites such as the Murchison meteorite resembles components available on the early Earth through extraterrestrial infall. A series of organic acids represents the most abundant water-soluble fraction in carbonaceous meteorites [ 15,67,68]. Samples of the Murchison meteorite were extracted in an organic solvent commonly used to extract membrane lipids from biological sources [69,70]. When this material was allowed to interact with aqueous phases, one class of compounds with acidic properties was clearly capable of forming membrane-bounded vesicles (Fig. 7). 

Fig. 7 Membranes can be formed by components of carbonaceous meteorites [69,70]. a The Murchison meteorite contains approximately 2% organic carbon by weight, b Organic compounds can be extracted from the meteorite by a lipid solvent system (chloroform-methanol), then separated by two-dimensional chromatography. Polycyclic compounds in the mixture produce fluorescent spots, c The organic acid fraction from the TLC plate readily assembles into membranous vesicles when exposed to dilute aqueous solutions buffered at pH 8-9. The vesicles were photographed by their autofluorescence. Scale bar shows 20 im...

The enantiomeric excess of several a-alkyl amino acids from the Murchison meteorite has been measured by Cronin and Pizzarello [69-71], who found significant values (1 -15% ee l). Conversely to previous estimates often biased by terrestrial contamination, these results can be considered as reliable since target AA are both non-biogenic and non-terrestrial in origin (the latter confirmed by D/H and 13C/12C isotopic ratios). The quaternary a-carbon prevents the racemization of the compounds, thus allowing the long-term preservation of the enantiomeric excess. 

An alternative to the terrestrial synthesis of the nucleobases is to invoke interstellar chemistry. Martins has shown, using an analysis of the isotopic abundance of 13C, that a sample of the 4.6 billion year old Murchison meteorite which fell in Australia in 1969 contains traces of uracil and a pyrimidine derivative, xanthine. Samples of soil that surrounded the meteor when it was retrieved were also analyzed. They gave completely different results for uracil, consistent with its expected terrestrial origin, and xanthine was undetectable [48], The isotopic distributions of carbon clearly ruled out terrestrial contamination as a source of the organic compounds present in the meteorite. At 0°C and neutral pH cytosine slowly decomposes to uracil and guanine decomposes to xanthine so both compounds could be the decomposition products of DNA or RNA nucleobases. They must have either travelled with the meteorite from its extraterrestrial origin or been formed from components present in the meteorite and others encountered on its journey to Earth. Either way, delivery of nucleobases to a prebiotic Earth could plausibly have been undertaken by meteors. The conditions that formed the bases need not have been those of an early Earth at all but of a far more hostile environment elsewhere in the Solar System. That environment may have been conducive to the production of individual bases but they may never have been able to form stable DNA or RNA polymers this development may have required the less extreme conditions prevalent on Earth. 

Figure 2 shows that the C1-C5 aliphatic hydrocarbons, amino acids, carboxylic acids, and sulfonic acids from the Murchison meteorite appear to follow a common trend when their values are plotted against carbon number, values generally decrease as the amount of carbon in the molecules increases. This trend has been interpreted as the result of a kinetic isotope effect during the sequential formation of higher-molecular-weight compounds from simpler precursors (Yuen et al., 1984). The more reactive is preferentially added during the synthesis of the carbon skeleton of these compounds. 

Jungclaus G. A., Yuen G. U., Moore C. B., and Lawless J. G. (1976b) Evidence for the presence of low molecular weight alcohols and carbonyl compounds in the Murchison meteorite. Meteoritics 11, 231-237. 

Pizzarello S., Feng X., Epstein S., and Cronin J. R. (1994) Isotopic analyses of nitrogenous compounds from the Murchison meteorite ammonia, amines, amino acids, and polar hydrocarbons. Geochim. Cosmochim. Acta 58, 5579-5587. 

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