Carbonaceous amino acids

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. 

Several factors indicate that the amino acids detected in all of these carbonaceous chondrites are indigenous and that they must have originated abiotically. First, the presence of protein and non-protein amino acids, with approximately equal quantities of D and L enantiomers points to a nonbiological origin and precludes terrestrial contamination. In addition, the non-extractable fraction of the Murchison is significantly heavier in 13C than terrestrial samples. Finally, the relative abundances of some compounds detected resemble those of products formed in prebiotic synthesis experiments. The aliphatic hydrocarbons are randomly distributed in chain length, and the C2, C3, and C4 amino acids have the highest concentrations (i.e., the most easily synthesized amino acids with the least number of possible structures are most abundant) [4]. 

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. 

Four mechanisms have been advanced for the prebiotic formation of amino acids. The first involves a cyanohydrin (reaction 2) and a related route (reaction 3) can be invoked to account for the presence of hydroxy acids. These particular reactions have been studied in considerable detail both kinetically and in terms of thermodynamic quantities.347 An alternative route (4) involves the hydrolysis of a-aminonitriles, which are themselves formed directly in anhydrous CH4/NH3 mixtures.344 Cyanoacetylene, formed in CH4/N2 irradiations,349 yields significant amounts of asparagine and aspartic acids (reaction 5). Finally, a number of workers336,350"354 have proposed that HCN oligomers, especially the trimer aminoacetonitrile and the tetramer diaminomaleonitrile, could have been important precursors for amino acid synthesis. Reaction mixtures involving such species have yielded up to 12 amino acids. Table 11 indicates the range of amino acids produced in these kinds of sparking syntheses. Of some interest is the fact that close parallels between these kinds of experiments and amino acid contents of carbonaceous chondrite meteorites exist.331,355,356... 

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). 

Amino acids - and more generally organic substances - are mainly detected in carbonaceous chondrites, a minor, carbon-rich class of meteorites. These are believed to have originated from parent bodies having underwent alteration by liquid water at some stages of their existence, as attested by geochemical studies [63]. 

Exobiological research offers reasons to expect that amino acids might be universal. Amino acids appear to be products of prebiotic synthesis. They are famously the products of the Miller process by which methane, ammonia, and water were subjected to electrical discharge. And amino acids are found in carbonaceous meteorites both as alpha amino acids that are standard in terran life and as the nonstandard alpha-methylamino acids found by Cronin and Pizzarello.24... 

FIGURE 4.5 Amino acids reported from carbonaceous chondrite meteorites. 

We do not know the extent to which the Murchison organics reflect what was available on early Earth before life emerged. The rich inventory of amino acids does not appear to be universal in carbonaceous chondrites (although the number that have been examined in detail is very small). For example, only a few amino acids (glycine, alanine, a-aminoisobutyric acid, a-amino-n-butyric acid, y-aminobutyric acid) are found in the meteorite that fell in 2000 on Tagish Lake, Canada (Table 5.3).7 The near absence of complex amino acids is significant, inasmuch as the meteorite was captured in a pristine condition soon after it fell. 

Carbonaceous chondrites are rare stony meteorites that contain complex carbon compounds from which they get their name. Some contain water and amino acids, the building blocks of life. Carbonaceous chondrites are believed to be samples of our solar system s earliest rocks, unchanged after nearly 4.6 billion years. 

Although the first demonstration that amino acid racemization took place in fossils used mollusc shells (23), the application of this reaction in dating these materials has been extensively investigated only recently (19,20). Work on Mercenaria (19), Chione (20), and other species (24) has tested the application of racemization dating to fossil mollusc shells from geological contexts and Indian shell middens. These and other studies have shown that there are problems with amino acid racemization dating of carbonaceous fossils which are not encountered with bone. Reversible first-order racemization kinetics which are observed in bone... 

Amino acids have probably formed in different places of the Solar system through processes independent of living organisms as shown by their occurrence in carbonaceous chondrites, a particular class of meteorites. Among these processes, only a particular subset of them can be considered to be truly prebiotic. It is the processes that make amino acids available in environments capable of developing a complex chemistry compatible, or at least presumed to be compatible, with the emergence of life. 

Cronin J. R. and Moore C. B. (1976) Amino acids of the Nogoya and Mokoia carbonaceous chondrites. Geochim. Cosmochim. Acta 40, 853 —857. 

Ehrenfreund P., Glavin D. P., Botta O., Cooper G., and Bada J. L. (2001) Extraterrestrial amino acids in Orgueil and Ivuna tracing the parent body of Cl type carbonaceous chondrites. Proc. Natl. Acad. Set 98, 2138-2141. 

Shimoyama A. and Harada K. (1984) Amino acid depleted carbonaceous chondrites (C2) from Antarctica. Geochem. J. 18, 281-286. 

Shimoyama A., Ponnamperuma C., and Yanai K. (1979) Amino acids in the Yamato carbonaceous chondrite from Antarctica. Nature 282, 394-396. 

Shock E. L. and Schulte M. D. (1990) Amino acid synthesis in carbonaceous meteorites by aqueous alteration of polycyclic aromatic hydrocarbons. Nature 343, 728-731. 

PAHs are believed to be a major class of carbon-bearing molecules in the interstellar medium 1138], They are found in carbonaceous chondrites tliat have fallen to Earth (see section 4.2.1) and in interplanetary dust particles [28]. Shock and Schulte [139] suggested that amino acids could be syntliesized by aqueous alteration of precursor PAHs in carbonaceous chondrites. We directed attention to shock reaction of PAHs [135,140,141], and conducted shock reactions using benzene, tire simplest aromatic hydrocarbon, as a starting material to simulate possible reactions occurring in interstellar space. Furtliermore, we examined the mechanism of shock reaction on the basis of quantum chemistry and discussed the implication for cosmocheniistiy. 

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