Murchison meteorite amino acids

Wolman, Y., Haverland, W.J. and Miller, S.L., 1972. Nonprotein amino acids from spark discharges and their comparison with the Murchison meteorite amino acids. Proc. Acad. Sci. U.S., 69 809 -811. 

Several of them have been recently identified by Meierhenrich et al. in the Murchison meteorite, at the 5-50 ppb level [67]. This discovery extends the meteoritic amino acid suite to alkaline ones, of which the proteogenic representatives, namely Arg and Lys, remain both undetected in meteorites however. 

Carhonaceous chondrites are also of interest because of the fascinating mix of relatively complex organic molecules they contain. Scientists have now discovered both amino acids and nitrogen bases in meteorites. Amino acids are the compounds of which proteins are made, and nitrogen bases are one of the building blocks of nucleic acids such as DNA and RNA. Researchers have found 92 amino acids in just one meteorite, the Murchison meteorite that fell about 60 miles (100 km) north of Melbourne, Australia, in 1969. Of these 92 amino acids, only 19 are found on Earth. Studies of other meteorites have shown that the Murchison results are not unique. In fact, amino acids occur in a number of carhonaceous chondrites. 

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... 

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... 

We might think that the Murchison meteorite would have been studied thoroughly enough in the years since its arrival on Earth. But the results obtained always depend on the performance of the technical resources available in this case, the analytical methods and the apparatus. Thus, it is not really surprising that a new class of amino acid has been discovered in Murchison material diamino acids, such as DL-2,3-diaminopropionic acid, DL-2,4-diaminobutanoic acid etc. These were identified using a new enantioselective GLPC/MS method, which is also being used in analysis of material from the Rosetta mission. 

The question of the stability of the biomolecules is a vital one. Could they really have survived the tremendous energies which would have been set free (in the form of shock waves and/or heat) on the impact of a meteorite Blank et al. (2000) developed a special technique to try and answer this question. They used an 80-mm cannon to produce the shock waves the shocked solution contained the two amino acids lysine and norvaline, which had been found in the Murchison meteorite. Small amounts of the amino acids survived the bombardment , lysine seeming to be a little more robust. In other experiments, the amino acids aminobutyric acid, proline and phenylalanine were subjected to shock waves the first of the three was most stable, the last the most reactive. The products included amino acid dimers as well as cyclic diketopiperazine. The kinetic behaviour of the amino acids differs pressure seems to have a greater effect on the reaction pathway than temperature. As had been recognized earlier, the effect of pressure would have slowed down certain decomposition reactions, such as pyrolysis and decarboxylation (Blank et al., 2001). 

UV radiation hypothetical, but so is the transport of molecules from outer space to Earth. Recent analyses of the Murchison meteorite by two scientists from the University of Arizona, Tucson (Cronin and Pizzarello, 1997 Cronin, 1998) have shown it to contain the four stereoisomeric amino acids DL-a-methylisoleucine and DL-a-methylalloisoleucine. In both cases, the L-enantiomer is present in a clear excess (7.0 and 9.1%). Similar results were obtained for two other a-methyl amino acids, isovaline and a-methylvaline. Contamination by terrestrial proteins can be ruled out, since these amino acids are either not found in nature or are present in only very small amounts. Since the carbonaceous chondrites are thought to have been formed around 4.5 billion years ago (see Sect. 3.3.2), the amino acids referred to above must have been subject to one or more asymmetric effects prior to biogenesis. 

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. 

Essentially the same amino acids, and nearly equal quantities of D and L enantiomers, were detected in the Murray meteorite, another type II carbonaceous chondrite [6]. Recent expeditions to Antarctica have returned with a large number of meteorites, many of which are carbonaceous chondrites. These may have been protected from terrestrial contamination by the pristine Antarctic ice. Careful analysis of two of these, the Yamato (74662) and the Allan Hills (77306), both type II carbonaceous chondrites, by ion exchange chromatography, gas chromatography, and GC/MS, have detected a wide variety of both protein and non-protein amino acids in approximately equal D and L abundances [9,10]. Fifteen amino acids were detected in the Yamato meteorite and twenty in the Allan Hills, the most abundant being glycine and alanine. The amino acid content of the Yamato meteorite is comparable with that of the Murchison and Murray, but the Allan Hills contains 1/5 to 1/10 that quantity. Unlike earlier meteorites from other locations, the quantities of amino acids in the exterior and interior portions of the Yamato and Allan Hills meteorites are almost identical [9,10]. Thus, these samples may have been preserved without contamination since their fall in the blue ice of Antarctica, which js 250,000 years old in the region of collection. 

Hydroxypyrimidines have been detected in the Murchison, Murray, and Orgueil carbonaceous chondrites in abundances similar to those of amino acids [7]. Earlier analyses of the Orgueil meteorite By thin layer chromatography of organic extracts indicated the presence of melamine, ammeline, adenine, and guanine [8]. Although these could not be confirmed by Folsome, et al., [7] using GC/MS, recent studies by Schwartz [11] and by Hayatsu, et al., [12] have shown that these constituents of the nucleic acids may indeed exist in the carbonaceous chondrites. 

EmUiani C (1966) Paleotemperature analysis of Caribbean core P6304-8 and P6304-9 and a generalized temperature curve for the past 425000 years. J Geol 74 109-126 Emiich K, Ehhalt DH, Vogel JC (1970) Carbon isotope fractionation during the precipitation of calcium carbonate. Earth Planet Sci Lett 8 363-371 Engel MH, Macko SA, SUfer JA (1990) Carbon isotope composition of individual amino acids in the Murchison meteorite. Nature 348 47-49... 

Epstein S, Krishnamurthy RV, Cronin JR, Pizzarello S, Yuen GU (1987) Unusual stable isotope ratios in amino acid and carboxylic acid extracts from the Murchison meteorite. Nature 326 477 79... 

It was mentioned earher that many molecules can be formed in cosmic space before arriving on Earth. What about chiral compounds We know that amino acids are present in meteorites (Epstein et al., 1987 Pizzarello etal., 1994 Pizzarello and Cronin, 2000 Pizzarello and Weber, 2004). In this regard, of particular interest is the report on a-methyl amino acids, which have been found in L-enantiomeric excess in Murchison and Murray meteorites (Cronin and Pizzarello, 1997). These compounds are particularly resistant to racemization, and it is perhaps because of this that chirality has been preserved. It is not simple to assess whether these chiral exogenous compounds were the seeds for homochirahty of life on Earth (Bada, 1997). 

Knenvolden, K., Lawless, J. G., Pering, K., et al. (1970). Evidence for extraterrestrial amino acids and hydrocarbons in the Murchison meteorite. Nature, 228, 923-6. 

Pizzarello, S. and Cronin, J. R. (2000). Non-racemic amino acids in the Murray and Murchison meteorites. Geochim. Cosmochim. Acta, 64, 329-38. 

TABLE IV. Relative Abundances of Amino Acids in the Murchison Meteorite and in an Electric Discharge Synthesis"... 

Amino acid Murchison meteorite Electric discharge... 

Before leaving the aminoacids problem, it is interesting to note that aminoacids have been detected in carbonaceous chondrites found in Antarctica. The risk of contamination is much less important in Antarctica than in Australia and this is one of the reasons why these studies were undertaken. They fully confirm the results obtained on Murchison 54,55), even if in one CM carbonaceous chondrite the amino acid content was only 10% of what was observed in Murchison 56,57). The contamination is in fact lower than in Murchison the aminoacid content was very similar for samples taken near the surface of the Antarctica chondrites or from their bulk. On the other hand, all the significant analyses on Murchison were performed on samples from the interior of the meteoritic fragments due to the high degree of surface contamination. In the case of the Allende meteorite, which has the same terrestrial age as Murchison, contamination was found to extend to a depth of more than 5 mm below the surface 52). 

Table 4.1. Relative abundance of amino acids in the Murchison meteorite and in an electric discharge synthesis...

We analyzed several organic and inorganic phases identified from the Murchison and Murray meteorites [100] in order to determine whether they contain any possible chirality besides the enantiomeric amino acids [98]. Asymmetric autocatalysis is a method capable of revealing chiral imbalances in various media, therefore, we employed this method as a possible chiral sensor for the meteorites. 

Interestingly, the proportions of the mostly racemic amino acids produced in this mixture are approximately the same as those found in fragments of the Murchison meteorite, which fell near the village of Murchison, Victoria, Australia in 1969.82,83... 

Except recent radioastronomic observations, exogenous amino acids have for the moment only been known through the analysis of meteorite content, especially of the Murchison meteorite (the most organic-rich meteorite known to have fallen on Earth). 

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. 

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