Pizzarello and Cronin

The question was whether impurities were present in the samples analysed (Bada et al., 1983). In a more recent publication, Cronin and Pizzarello (1997) reported amino acid analyses using Murchison material in which an excess of L-enantiomers was present. Contamination with terrestrial biological material can be ruled out, as the amino acids in question are not proteinogenic a-methylamino acids, which occur either extremely seldom or not at all in terrestrial life forms, were detected. GLPC/mass spectrometry (MS) analysis gave the following enantiomeric excess (ee) values ... 

Such enantiomeric excesses have not been observed in analyses of the corresponding a-H-a-aminoalkanoic acids. According to the authors, the excess of the L-forms could be due to a partial photocleavage of the racemic amino acid mixture as a result of the influence of circularly polarized UV light in a presolar cloud (Cronin and Pizzarello, 2000). 

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. 

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). 

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. 

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... 

The natiue of the higlier-molecular-weight aliphatic hydrocarbons from the Murchison has been controversial. Kvenvolden et al. [33] detected a wide variety of coeluting isomers dominated by polycyclic aliphatic structures. Oro el al. [41] reported Cg to Cis metliyl and dimetliyl alkanes, alkenes, and cycloalkanes. Studier et al. [42] detected straight-chain alkanes with some isoprenoidal hydrocarbons. It is now widely believed that straight-chain alkanes were the dominant components of the ahphatic fraction in the solar nebula. However, Cronin and Pizzarello [43] analyzed organic materials from the Murchison under less environmental contamination and concluded lliat tire n-alkanes, methyl alkanes, and isoprenoid alkanes reported in the Murchison were terrestrial contaminants. Tliese authors identified Cis to C30 cyclic alkanes as the major indigenous alipliatic components. 

Sandra Pizzarello and her team were the first to publish that carbonaceous chon-dritic meteorites contain distinct a-methylated amino acids in low ee (Cronin and Pizzarello 1997 Pizzarello 2006 Pizzarello et al. 2008). As confirmed independently, the Murchison meteorite contained the following nonproteinogenic amino acids in ee 15.2% L-isovaline, 2.8% L-a-methyl-norvaline, 2.8% L-a-methyl-valine, and 9.1% L-a-methyl-isoleucine (Cronin and Pizzarello 1997). As an example, Fig. 3.2 shows the structure of L-a-methyl-norvaline. 

The events that led to this ee may predate the origin of life on Earth. The Murchison meteorite contains almost all natural amino acids as racemates and, importantly, some additional anomalous amino acids that do not occur on Earth (Cronin and Pizzarello 1997 Pizzarello 2006). Among them are four amino acids that do not carry a proton on the asymmetric a-carbon atom but a methyl group. Among the four unnatural amino acids that carry a-methyl substituents is L-a-methyl-aUo-isoleucine. The systematic chemical name of the latter is 2-amino-2.3.-dimethyl-penta-carbonic acid, and it has two chiral centers. Therefore, it should... 

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 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. 

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... 

Cronin JR, Pizzarello S, Cruikshank DP (1988) In Kerridge JF, Matthews MS (eds) Meteorites and the Early Solar System. University of Arizona Press, Tucson, AZ, p 819-857... 

SOURCE Data from Cronin, J.R., and Pizzarello, S. 1986. Amino acids of the Murchison meteorite. III. Seven carbon acyclic primary alpha-amino alkanoic acids. Geochim. Cosmochim. Acta 50 2419-2427. 

Cronin JR, Pizzarello S, Epstein S, Krishnamurthy RV (1993) Molecitlar and Isotopic Analyses of the Hydroxy Acids, Dicarboxylic Acids, and Hydroxydicarboxyhc Acids of the Murchison Meteorite. Geochim Cosmochim Acta 57 4745-4752... 

Kenso Soai at the University of Tokyo discovers an auto-catalytic reaction that yields an absolute asymmetric synthesis K. C. Nicolaou, Scripps Research Institute, completes the total synthesis of brevetoxin B John R. Cronin and Sandra Pizzarello, Arizona State University, discover significant excesses of L-enantiomers of uncommon amino acids on the Murchison meteorite... 

Collins AG (1964) Eh and pH of oilfield waters. Prod Monthly 28 11-12 Collins AG, Pearson DH, Attaway DH, Watkins JW (1961) Methods of analyzing oil field waters. US Bur Mines Rep Invest 5819 11-17 Cronin JR, Pizzarello S, Cruikshank DP (1988) Organic matter in carbonaceous chondrites, planetary satellites, asteroids and comets. In Kerridge IF, Matthews MS (eds) Meteorites and the early solar system. University of Arizona Press, Tucson, pp 819-857... 

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