Recently published data gave fresh support for the assumption that interstellar processes had originally generated the chiral asymmetry of life.1 These data were obtained in Earth-based laboratories giving access to the observation of chemical processes under simulated interstellar conditions. The results indicated that chiral amino acid structures form in laboratory-analogues of dense interstellar clouds and that these chiral structures were subjected to interstellar asymmetric radiation with the potential to generate a small but decisive enantiomeric excess tipping the balance in one direction towards life's homochirality. To investigate this theory for an interstellar origin of biomolecular asymmetry we will now widen our view and describe the stereochemistry of organic molecules identified in authentic samples of extraterrestrial origin. We will focus on samples of meteorites, comets, and Mars. The knowledge of enantiomeric ratios on an extraterrestrial body can be assumed to provide significant information to verify the above assumption based on simulated interstellar data and thus to better understand the origin of biomolecular asymmetry on Earth.
8.1 Chiral Organic Molecules in Meteorites?
In the morning of September 28th, 1969, a meteorite exploded over the small town of Murchison in Australia, spreading a total of about 100 kg extraterrestrial material in different fragments on Earth. A meteorite is by definition a solid-state object that originated in extraterrestrial space, passed Earth atmosphere, and fell down on Earth's surface without being destroyed. This particular meteorite was of a type called carbonaceous chondrite, extremely rich in carbon compounds. Chondritic meteorites, in particular the CM type carbonaceous chondrites such as Murchison, make up a unique subset of primitive meteorites, which is of particular interest in
1 According to this assumption - just imagine its far-reaching consequences e.g. on the probability on the existence of life elsewhere in the Universe - not only the molecular building blocks of living organisms such as amino acids and other organic molecules had been generated in interstellar space ("we are made of stardust" - Cliff Matthews), but also the preselection of life's asymmetrical use of mirror image isomers had been performed by extraterrestrial processes.
U. Meierhenrich, Amino Acids and the Asymmetry of Life. Advances in Astrobiology 145
and Biogeophysics, ©c Springer-Verlag Berlin Heidelberg 2008
the context of origin-of-life research. Not only because of their relatively high carbon content but also since most of this carbon is present as organic matter. This organic matter is a diverse mixture of compounds that in particular includes car-boxylic acids, dicarboxylic acids, hydroxy acids, sulfonic acids, phosphonic acids, and amino acids in the form of monoamino alkanoic acids and monoamino alkan-dioic acids (Cronin and Chang 1993) but also in the form of diamino acids. Some of these species are chiral.
Among these classes of compounds, a small fraction of the isomers is indeed believed to support prebiotic evolutionary processes. The molecular building blocks of both protein enzymes and genetic material might have been originated in interstellar and circumstellar environments and subsequently been delivered by meteorites, interplanetary dust particles, and/or by comets (Oro 1961; Kissel et al. 1986a, 1986b; Anders 1989; Jessberger 1999) to the early Earth where they have triggered the appearance of life (Chyba and Sagan 1992; Ehrenfreund 1999). Based on persisting advances in chemical analysis techniques, amino acids (Cronin 1976; Cronin and Chang 1993; Cronin and Pizzarello 1997; Cronin 1998; Engel and Macko 2001; Botta et al. 2002; Meierhenrich et al. 2004), and molecular DNA constituents such as purine- (Van der Velden and Schwartz 1977) and pyrimidine bases (Stoks and Schwartz 1979), as well as sugar-related organic compounds (Cooper et al. 2001) have already been identified in various carbonaceous meteorites.
Which conclusions can be drawn today from the meteoritic occurrence of different amino acids? Is any chiral configuration preferred by meteoritic amino acids and if "yes", is there any chiral fingerprint written in these amino acids informing us on their eventual contribution to the evolution of life on Earth?
Until today, 89 different amino acids have been detected in the Murchison meteorite; a list of which can be found in the Annex. Other carbonaceous chondrites also include a high content of a variety of amino acids. Most of them show slightly different constitutions of the side chains located at the a-carbon atom. Often, the presence of amino acids in carbonaceous meteorites was confirmed by their isotopic composition (Pizzarello et al. 2003), clearly indicating their extraterrestrial origin.
What about their enantiomeric enhancements? Experimental investigations on meteoritic amino acids resulted in the detection of a rather small, yet significant, enantiomeric excess of a special family of amino acids called a-methyl-a-amino acids in the Murray and Murchison meteorite. The well-known research team of John Cronin and Sandra Pizzarello at Arizona State University detected the highest enantiomeric excesses of an amino acid in a meteorite. For isovaline, a non-proteinaceous amino acid, an enantiomeric excess of 8.4% was determined for its L-configuration, and the 2S, 3^-enantiomer of 2-amino-2,3-dimethylpentanoic acid DMPA was detected in the Murchison meteorite with an e.e. = 9.1% (Cronin and Pizzarello 1997, 1999; Pizzarello and Cronin 2000).
More recent measurements of the Cronin-Pizzarello research team on several Murchison meteorite fragments revealed that L-isovaline displays enantiomeric excesses ranging up to 15.2% (Pizzarello et al. 2003). Some proteinaceous amino acids were also found to be non-racemic, but with smaller enantiomeric excesses: An excess of 2.2% was detected for L-valine, and 1.2% for L-alanine (Cronin and Pizzarello 1999), which is actually a value closely matching the above outlined experimental data obtained by photolytic irradiation of amino acids with circularly polarized light under simulated interstellar/circumstellar conditions.
In general, meteoritic a-methyl (a-Me) amino acids, i.e., amino acids with a methyl group bound to the a-carbon atom just next to the carboxyl group, display higher enantiomeric excesses than their a-hydrogenated (a-H) analogues. Actually, a-hydrogen amino acids are known to racemize more efficiently than a-Me amino acids (Pollock et al. 1975), because a hydrogen atom bound to the a-carbon close to a carbonyl can be removed by deprotonation. When an a-hydrogen amino acid lose its a-hydrogen, the asymmetric carbon loses its chirality. The resulting plane geometry around this atom allows protonation from either side with roughly the same probability, leading to a racemization. This racemization process cannot occur by this mechanism with a-Me amino acids. In this context it is noteworthy to remark that other different racemization mechanisms are known, for instance, the tunnelling racemization of enantiomers (Cattani and Bassalo 1998).
A racemization procedure could explain - at least in part - why, after interstellar thermal, photochemical, and radiolytic adulteration of meteoritic samples, proteina-ceous amino acids, which are all a-hydrogen compounds, were found to display excesses with factors up to five times smaller than a-Me amino acids (Cronin and Pizzarello 1999). The study of a-Me amino acids produced by vacuum ultraviolet circularly polarized light irradiation of simulated interstellar ices would thus be really interesting in the future. In this kind of studies, particular attention should be paid to the asymmetric photolytic behaviour of a-Me amino acids, checking if such compounds display high enantiomeric excesses.
a-Methyl amino acids such as isovaline were recently shown to be more effective asymmetric catalysts than a-H amino acids, i.e., they can efficiently influence chemical enantioselective reactions to the production of one type of enantiomer (Pizzarello and Weber 2004), and thus they appear to be key compounds to investigate in order to elucidate their role in the origin of homochirality. However, until today a-Me amino acids have remained difficult to study in simulated interstellar ices since they have been produced in much lower abundances than a-H compounds in the residues. It was very recently that isovaline2 was identified in an organic residue formed by unpolarized ultraviolet irradiation of a CH3OH : NH3 = 1 : 1 ice mixture (Nuevo et al. 2007). In simulating experiments of our laboratories involving circularly polarized light, no a-Me amino acids were detected yet, confirming their relatively low production yields (Nuevo et al. 2006).
2 The total amount or relative quantity of isovaline in simulated interstellar ices is not precisely given.
We have seen that proteinaceous amino acids such as a-alanine and a-methyl amino acids such as isovaline have been unambiguously identified in the Murchison meteorite. What about the identification of other families of chiral organic molecules in this meteorite and their possible contribution to the origin and evolution of life on Earth? Very recently, diamino acids moved into the center of scientific interest. Why is this 'new' class of particular interest?
8.1.2 The Untold Story of Diamino Acids in Meteorites
Ever since the identification of deoxyribonucleic acid DNA by James Watson, Francis Crick, and Rosalind Franklin as the genetic material and the elucidation of its unique information bearing double-helical structure there have been speculations as to the evolutionary origin of this material and to life as we know it. Did DNA evolve in evolutionary timescales before proteins or, alternatively, did proteins precede the evolution of DNA double helices? Until the 1980s this question remained difficult to answer, since DNA requires proteins for molecular self-replication and the information how to construct proteins is written in the genetic code of the information-bearing DNA. Fig. 8.1 tries to illustrate this evolutionary paradox by referring to the old hen-or-egg enigma.
So far, based on abundant experimental data we have de facto information on the early development of the DNA genome. Several independent lines of evidence suggest that DNA- and protein-based life was preceded by a simpler form of life based
Fig. 8.1 The picture refers to the older question on the origin of chicken and egg. What was first? Genes like DNA (illustrated by the chicken/hen) or proteins (symbolized by the egg)? In order to answer this longstanding question, data of new meteorite analysis include hints that - in this analogy - "self-replicating eggs" may exist that do not necessarily require a hen for originating (Bredehoft 2007). Image of [SH]2 Kommunikations-Design, Bremen
primarily on ribonucleic acid (RNA). This earlier era is referred to as the 'RNA-world' (Gilbert 1986; Gesteland et al. 1999; Joyce 2002), during which the genetic information resided in the sequence of RNA molecules and the phenotype derived from the diverse catalytic properties of RNA (Joyce 2002). In general, the approach of the RNA-world for the understanding of the origin of life seems to be widely accepted, even if some variations exist.3
The instability of the ribose molecule and considerable difficulties in finding a selective prebiotic reaction yielding ribose (Cooper et al. 2001) remain unresolved features of the RNA-world scenario. Therefore, at present there is a growing consensus that the RNA-world could not have evolved directly from monomer molecules formed by prebiotic processes (Ferris 1993). Pre-RNA genetic material is assumed to be required and this fundamental question provoked that numerous world-leading research teams today try to elucidate its molecular structure.
The main obstacle to understanding the origin of pre-RNA oligonucleotides is identifying a plausible mechanism for overcoming the gap between prebiotic chemistry and primitive biology. It is generally assumed that chemical processes have led to a substantial level of complexity before the pre-RNA world came into play. The precise knowledge of the chemical constitution of the potential molecular building blocks of pre-RNA oligonucleotides can be assumed to fill the substantial gap in the scientific understanding concerning how the RNA world arose. The data given below might contribute to fill this gap:
Due to astrophysical research today, we have numerous and undebated data, where exactly in the universe organic molecules can be synthesized spontaneously: In samples coming from interplanetary, interstellar, and circumstellar regions potential molecular building blocks of biopolymers were detected. This is particularly true for a wide variety of amino acids identified in carbonaceous meteorites (Cronin, 1976; Cronin and Chang, 1993; Cronin and Pizzarello, 1997; Cronin, 1998; Engel and Macko, 2001; Botta et al., 2002) and interstellar ice analogues simulated in the laboratory (Bernstein et al., 2002; Munoz Caro et al., 2002). However, the molecular constituents of pre-RNA oligonucleotides have never been identified, - neither in simulated interstellar and circumstellar ices nor in situ.
In 2004, chemical analyses of the Murchison meteorite led to the identification of diamino acids, a 'new' class of organic compounds: Analytical studies were performed with a sample of the carbonaceous Murchison chondrite in the Department of Physical Chemistry at the University of Bremen. The sample itself was obtained from a collection of the Max-Planck-Society in Mainz, where it was specially taken from the protected interior of the Murchison meteorite. 1 g of the Murchison meteorite was powdered by a planetary micro mill, imaged by a raster electron microscope (Fig. 8.2), and extracted with water in a positive pressure "Class 100"
3 Christian de Duve, for example, favors a specific "weak version" of the RNA world, in which most RNA acting as catalyst was relatively short. The catalytic activity of this RNA is supposed to be supported by short peptides in combination with other compounds that de Duve calls multimers (de Duve 2006).
Fig. 8.2 A meteorite is an object originating from outer space impacting on Earth's surface. (Top) As an example, a 110 kg fragment of a typical iron meteorite found in the Canyon Diablo, Arizona, is given. (Bottom) Raster electron microscope image (CS44, CamScan, Cranberry Township, PA) of the powdered sample of the Murchison meteorite showing a rather homogenous particle size distribution in the low micrometer range. An aliquot of this sample was subjected to diamino acid analysis
Fig. 8.2 A meteorite is an object originating from outer space impacting on Earth's surface. (Top) As an example, a 110 kg fragment of a typical iron meteorite found in the Canyon Diablo, Arizona, is given. (Bottom) Raster electron microscope image (CS44, CamScan, Cranberry Township, PA) of the powdered sample of the Murchison meteorite showing a rather homogenous particle size distribution in the low micrometer range. An aliquot of this sample was subjected to diamino acid analysis clean room. The obtained extracts were hydrolysed,4 derivatized,5 and analysed subsequently by enantioselective GC-MS technique using a Chirasil-L-Val stationary phase of 12 m length. With a new efficient derivatization method and an improved detection technique we focused on chiral compounds with three and more functional groups.
4 Hydrolysis for 24 h at 110 °C in 6 molar HCl for amino acid analysis. Such conditions are typical for the hydrolysis of proteins to release "free" amino acids.
5 Due to a method described by Abe (1996), diamino acids were transformed into N,N'-diethoxycarbonyl diamino acid ethyl ester (ECEE) derivatives.
Surprisingly, the application of this technique enabled the identification of di-amino acids in the meteorite, never seen before (Meierhenrich et al. 2004).6 Diamino acids are amino acids with an additional amino group.
The presence of diamino acids in Murchison was confirmed by comparison with external standards showing identical mass spectra and retention times. Figure 8.3 combines chromatograms of the Murchison carbonaceous chondrite sample and a simulated interstellar ice sample in which the diamino acids 4,4'-diaminoisopentanoic
Fig. 8.3 Treasure trove of amino acids: Gas chromatogram of a sample of the Murchison meteorite (Mur) showing the diamino acids 4,4'-diaminoisopentanoic acid 3, d-2,3-diaminopropanoic acid d-2, l-2,3-diaminopropanoic acid l-2, d-2,4-diaminobutanoic acid d-l, and l-2,4-diaminobutanoic acid L-l. Detection in the single-ion monitoring mode (SIM) of m/z = 175 amu, typical for amino acid ethoxy carbonyl ethyl ester derivatives (see Table 8.1). The insets show the external standard of the enantiomer separated diamino acids d,l-2,3-diaminopropanoic acid and d,l-2,4-diaminobutanoic acid (Ref) purchased from Fluka and detected in the total ion current (TIC); a laboratory sample produced by ultraviolet irradiation of circumstellar/interstellar ice analogues (ISM) detected in SIM of mass trace m/z = 175 amu and a serpentine blank (Blk) at mass trace m/z = 175 amu that had been heated before extraction for 4h at 500°C and passed through the analytical protocol
6 Until 2004, diamino acids had not been identified in samples of meteorites. The reason for this is probably to be found in the applied analytical procedures. Until 2004, for the analyses of meteorites mostly capillary columns of 30-50 m length were used to obtain high resolution for potential enantiomers. Capillary columns with such a performance are generally too long to elute diamino acids. Here, capillary columns of 12 m length were applied (as they are installed in the COSAC-experiment onboard the cometary ROSETTA mission), which were suitable for the elution and enantioselective resolution of diamino acids.
Rt of analyte [min]
203, 157, 85
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