Amino acids meteorites

The four stereoisomers of this amino acid include the D- and L-forms of a-methylisoleucine and a-methylalloisoleucine. Cronin, J.R., and Pizzarello, S., 1997. Enantiomeric excesses in meteoritic amino acids.. Science 275 951—955. 

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. 

The close match of the amino acid distributions in meteorites and the Miller-Urey synthesis suggests that the meteoritic amino acids were produced by the essential steps of the Miller-Urey synthesis. It is not yet clear what these essential steps would be in a nebular setting. Synthesis of intermediates obviously does not require an electric discharge, but can also be achieved from CO, NHj, and H2. The required... 

Cronin J. R. and PizzareUo S. (1997) Enantiomeric excesses in meteoritic amino acids. Science 275, 951-955. 

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... 

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. 

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. 

Kawasaki T, Shimizu M, Nishiyama D, Ito M, Ozawa H, Soai K (2009) Asymmetric autocatalysis induced by meteoritic amino acids with hydrogen isotope chirality. Chem Commun 4396 398. doi 10.1039/b908754k... 

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... 

Miniaturized columns have provided a decisive advantage in speed. Uracil, phenol, and benzyl alcohol were separated in 20 seconds by CEC in an 18 mm column with a propyl reversed phase.29 A19 cm electrophoretic channel was etched into a glass wafer, filled with a y-cyclodextrin buffer, and used to resolve chiral amino acids from a meteorite in 4 minutes.30 A 6 cm channel equipped with a syringe pump to automate sample derivatization was used to separate amino acids modified with fluorescein isothiocyanate.31 Nanovials have been used to perform tryptic digests on the 15 nL scale for subsequent separation on capillary Electrophoresis.32 A microcolumn has also been used to generate fractions representing time-points of digestion from a 40 pL sample.33 A disposable nanoelectrospray emitter has been... 

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... 

New analyses of material from the interior of the Orgeuil and Ivona meteorites show the presence of P-alanine, glycine and y-amino-n-butyric acid as the main components (0.6-2.0ppm) traces of other amino acids were also detected. The amino acids were present as racemic mixtures, i.e., d/l = 1, so that an extraterrestrial origin can be assumed (Ehrenfreund et al., 2001). 

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 number of scientific articles published on meteorites has increased dramatically in the last few years few of these, however, concern themselves with small meteorites, the size of which lies between that of the normal meteorites (from centimetres to metres in size) and that of interplanetary dust particles. In the course of an Antarctic expedition, scientists (mainly from French institutions) collected micrometeorites from 100 tons of Antarctic blue ice (Maurette et al 1991). These micrometeorites were only 100 400 pm in size five samples, each consisting of 30-35 particles, were studied to determine the amount of the extraterrestrial amino acids a-aminoisobutyric acid (AIBS) and isovaline—both of which are extremely rare on Earth—which they contained. The analysis was carried out using a well-tested and extremely sensitive HPLC system at the Scripps Institute, La Jolla. Although the micrometeorites came from an extremely clean environment, the samples must have been contaminated, as they all showed traces of L-amino acids. Only one sample showed a significantly higher concentration of AIBS (about 280 ppm). The AIBS/isovaline ratio in the samples also lay considerably above that previously found in CM-chondrites. 

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). 

The still hypothetical RNA world must fill the gap. Orgel suspects that, in this still dark phase of chemical and molecular evolution, a class of molecules was present which could have been relatively easily formed under the conditions present on the young Earth, and which was found in rocks from meteorites perhaps amino acids. These need not to have been the 20 proteinogenic amino acids which we know today, and which are in part quite complex, such as phenylalanine, histidine and tryptophane simple examples would have sufficed. 

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. 

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. 

Amino acid measurements in ALH84001 are almost certainly the result of Antarctic ice contamination. Amino acids are readily soluble in water but PAHs are practically insoluble. Isotopic measurements of 14C show that terrestrial carbon is incorporated into the meteorite during extended stays in the Antarctic ice fields. In addition, microbial activity on the exposed surfaces provides an additional source of biogenic organic material that may be incorporated over time. 

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. 

---