Chirogenesis in Outer Space

This section will briefly introduce the context in which interstellar chirogenesis is of interest. Chap. 6 will then summarize modern concepts and advances in the field of asymmetric photochemistry in interstellar space, and discuss our current understanding of the underlying mechanisms involved.

1.3.3.1 Interstellar Formation of Chiral Molecules

The chemical generation of simple prebiotic organic molecules from inorganic precursors has important implications for the understanding of the crucial first steps of Chemical Evolution. The topical discussion focuses on the question of whether these processes occurred in the atmosphere or hydrosphere of the early Earth or whether the required prebiotic organic molecules were spontaneously generated under interstellar conditions including their subsequent delivery to the early Earth via (micro-) meteorites and/or comets. In spite of the fact that neither any single process, nor any single source of energy is likely to account for all the organic compounds on the primitive Earth, this book will present arguments for an interstellar origin of prebiotic amino acid structures and critically review the main alternatives.

Until very recently, prebiotic amino acids were believed to have been mainly generated in the atmosphere of the early Earth, as successfully simulated by the Urey-Miller experiments (Miller 1953). More recently, two independent groups, one by Max Bernstein (Bernstein et al. 2002) of NASA Ames in the US and the other by our team from European universities and institutes (Mufioz Caro et al. 2002), put forward another scenario (Shock 2002). This involves chemical reactions on small ice grains that develop in the interstellar medium as it is illustrated in Fig. 1.2. In the laboratory, ultraviolet irradiation of ice mixtures containing well-known interstellar molecules (such as H2O, CO2, CO, CH3OH, and NH3) in conditions of vacuum and low temperature found in the interstellar medium generated amino acid structures including glycine, alanine, serine, valine, proline, and aspartic acid. After warm-up, hydrolysis, and derivatization, 16 amino acids as well as furans and pyrroles were identified. With the exception of glycine, all of these amino acids were chiral, but not homochiral. A more detailed presentation of these amino acid structures including comments on constitutional isomers, their chirality and the identification of a "new" family of amino acids namely diamino carboxylic acids will be given in Chaps. 7 and 8. We will discuss the mechanism of their formation and comment on the hydrolysis step required to release "free" chiral amino acids.

Fig. 1.2 Defined developing phases of interstellar dust particles in dense and diffuse interstellar clouds. First, in the dense interstellar medium tiny silicate grains of sub-micrometer diameter accrete ice layers that contain simple organic molecules such as H2O, CO2, CO, CH3OH, and NH3 (left). During growth process the ice layers are irradiated with energetic UV-photons in the diffuse interstellar medium (center) resulting in photoreactions that form radicals and polymers of yellowish color including new organic molecules like amino acids (right). These amino acids are chiral. The aggregation of such dust particles occurs as a result first of grain-grain collisions and then by collisions between aggregates leading to intermediate cometesimals and ultimately to comets. Illustration: Stephane Le-Saint, Universite de Nice-Sophia Antipolis

Fig. 1.2 Defined developing phases of interstellar dust particles in dense and diffuse interstellar clouds. First, in the dense interstellar medium tiny silicate grains of sub-micrometer diameter accrete ice layers that contain simple organic molecules such as H2O, CO2, CO, CH3OH, and NH3 (left). During growth process the ice layers are irradiated with energetic UV-photons in the diffuse interstellar medium (center) resulting in photoreactions that form radicals and polymers of yellowish color including new organic molecules like amino acids (right). These amino acids are chiral. The aggregation of such dust particles occurs as a result first of grain-grain collisions and then by collisions between aggregates leading to intermediate cometesimals and ultimately to comets. Illustration: Stephane Le-Saint, Universite de Nice-Sophia Antipolis

Today, one assumes that dust particles, such as those depicted in Fig. 1.2, serve as containers for chiral amino acids. These particles coagulate forming eventually larger bodies like comets. The chiral products of interstellar photochemical reactions could be preserved in these bodies, and in time be delivered to the Earth during the heavy bombardment geological period, which ended about 3.8 billion years ago. The comet components were heated during the atmospheric entry and impact with the planet's surface. The delivered chiral organic molecules might have played an important role in the appearance of primitive life and the emergence of biomolecular asymmetry on Earth. A prebiotic interstellar origin of amino acid structures is now accepted to be a plausible alternative to the Urey-Miller mechanism.7

Today, we assume that chiral organic molecules do form in interstellar environments. This formation has successfully been simulated in the laboratory in the case of amino acids. Furthermore, different meteorites have been analyzed and - supporting the above model - chiral organic molecules have been detected therein as well. A detailed description of meteoritic amino acids and diamino acids will be given in Chap. 8; here, the occurrence of chiral molecules in meteorites will be overviewed briefly.

7 In personal discussions during a conference on Chemical Evolution and the Origin of Life in the Abdus Salam International Center for Theoretical Physics in Triest, Italy, 2003, the late Stanley Miller conceded that based on scientific argumentation today one should not assume that all prebiotic amino acids were formed either in the Early Earth's atmosphere or in interstellar environments. One should rather ask for the quantities of amino acids formed on Early Earth versus in interstellar regions. According to S. Miller, they were formed here and there.

1.3.3.2 Chiral Molecules in Meteorites

The detection of chiral molecules and the identification of eventual enantioenrich-ments in extraterrestrial samples such as meteorites have always been considered crucial for the understanding of the origin of biomolecular homochirality. Are chiral molecules present in meteorites and can one resolve and quantify single enantiomers therein?

Early reports on the analysis of carbonaceous chondrites - a particular family of meteorites showing high amounts of organic compounds - claimed the identification of chiral amino acids. However, these studies often examined terrestrial contaminations only (e.g. Oro et al. 1971; Engel and Nagy 1982). Nowadays, they have been superseded by experimental work employing better samples and more powerful analytical techniques (Cronin and Chang 1993). Most of this work has focused on the Murchison meteorite, which fell spectacularly in several pieces close to Murchison village in Australia in 1969. This meteorite belongs to the class of the carbonaceous chondrites. The high carbon content of carbonaceous chondrites is largely macromolecular but also contains a complex mixture of organic compounds. Some of these contents can be chiral such as carboxylic acids, dicarboxylic acids, amino acids, diamino acids, hydroxy acids, amines, alcohols, and others.

The breakthrough in the successful enantioselective detection of chiral amino acids in the Murchison meteorite was in 1997. It was performed simultaneously by independent research teams in the US and reported twice (Engel and Macko 1997; Cronin and Pizzarello 1997; see also Chyba 1997). Particularly for a-methyl amino acids such as isovaline enantiomeric excesses of up to 15% were reported. Enan-tiomeric excesses of classical (a-H) amino acids such as alanine, valine, leucine etc. were examined up to 3%. In all these enantioselective analyses of carbonaceous chondrites L-amino acids were found in excess towards their D-counterparts.8 This finding is in intriguing coherence with the fact that biological organisms such as plants, animals, and human beings exclusively use L-amino acid enantiomers for the construction of their proteins.

As mentioned above, a-methyl amino acids were identified in the Murchison carbonaceous chondrite showing higher enantiomeric excesses compared to a-H amino acids. A higher resistance of a-methyl amino acids to racemization could explain this observation. Here, the observed enantiomeric excesses likely represent the original state of these a-methyl amino acids, whereas the more easily racemized a-H amino acids could have initially existed in non-racemic proportions too, but racemized during alteration of the carbonaceous chondrite parent body (Cronin and Pizzarello 1997).

Later on in this book we will discuss any asymmetric influence of meteoritic L-amino acid excesses on organic chemical evolution and the origin of life itself.

8 During a personal discussion at the 10th ISSOL conference in Oaxaca, Mexico in 2002, Sandra Pizzarello, professor at Arizona State University and one of the world's leading experts in meteorite analysis confirmed that the enantioselective analyses in her institute of chiral amino acids in carbonaceous chondrites never revealed an excess of the d-enantiomers. All samples - even those from different pieces of the inhomogeneous Murchison meteorite - showed exclusively excesses of the l-amino acids.

So far, however, no direct evidence verifies that both the generation of amino acids themselves and the formation of an enantiomeric excess therein have initially occurred in an interplanetary or interstellar environment. So, alternatively both processes might have taken place during the entry of achiral and reactive amino acid forming molecules on Earth only. Some experimental results may not exclude this latter assumption because significantly higher amounts of amino acids were detected in samples of interstellar ice analogues and/or parent bodies of meteorites after sample warm-up and acid-mediated hydrolysis in the laboratory (see Chaps. 7 and 8). During the warm-up step - simulating the arrival on Earth - reactive molecules that had been generated under hard solar irradiation in space might have produced complex organic precursors that form free amino acids after contact with liquid water -the required hydrolysis step.

On the other hand, the occurrence of an excess of L-enantiomers in six a-methyl-a-amino acids, more stable against racemization reactions than their a-H homologues, detected by Sandra Pizzarello and John Cronin (2000) in both the Murchison and Murray meteorites might be a hint that robust non-racemic chiral molecules were originated in the interplanetary or interstellar medium. A mechanism that describes the early delivery of organic compounds incorporated in comets or meteorites to early Earth was published by J. Mayo Greenberg et al. (1994) and Pascale Ehrenfreund (1999). Based on these observations, non-racemic chiral molecules might have been delivered to early Earth where crucial processes of chemical evolution started in the required dissymmetric environment.

The essential question, whether considerable enantiomer excesses - such as was reported in 1997 by Engel & Macko and Cronin & Pizzarello - were formed due to the interaction of interstellar circularly polarized light or any other mechanism prior to the entry of the extraterrestrial material on Earth, or during the entry process itself as a remote possibility, too, could only be answered finally by in situ measurements on meteoritic parent bodies, cometary nuclei, or interstellar ice analogues at interstellar temperature. These experiments have not yet been performed. But interestingly, we are very close to realizing them in the frame of the ROSETTA Mission. Enantioselective analyses of chiral cometary species are foreseen in the frame of the ROSETTA-COSAC project and will be presented and illustrated in Chap. 9. Here is an appetising introduction:

1.3.3.3 Chiral Molecules in Comets

It is generally assumed that organic molecules must have existed in solar nebular grains before comets formed. These molecules were incorporated into the comets' nuclei during their formation (Huebner and Boice 1992; Greenberg et al. 1994; Mufioz Caro et al. 2002). In addition to this, there is increasing evidence that these organic molecules showed enantiomeric excesses since they had been subjected to chiral interstellar fields (Bonner 1995a; Meierhenrich et al. 2005b). According to a further idea, comets were assumed to have delivered some of their volatile inventory (Chyba 1990) and essential "biogenic" compounds (Oro 1961; Chyba and Sagan 1992) in a non-racemic ratio to the terrestrial (and lunar (Oro et al. 1970))

surface during the final impact stages of the early inner Solar System (Oberbeck and Aggarwal 1992). This delivery of cometary asymmetric organic compounds was suggested to have triggered not only the appearance of life on Earth but also the selection of left-handed amino acids for incorporation into early organisms. This is a rather spectacular hypothesis, but could it really be true?

Which role did comets play in spreading out non-racemic components over large interplanetary distances? Could comets indeed be used as vehicles bringing asymmetric molecular building blocks of life to Earth? And would there be - against any pessimistic point of view (Siegel 1998) - an experimental option to elucidate this kind of active participation of comets in chemical evolution processes e.g. by measuring the occurrence of enantiomeric excesses on a comet? Based on the above, comets are probably the most exciting place to search for enantioenrichments in our Solar System (MacDermott 1997). Comets represent unprocessed relics of the pre-solar nebula.

With the ambitious aim of investigating the above hypothesis the European Space Agency ESA is, for the very first time, going to perform an experiment with cometary matter, in which enantiomeric excesses of a number of cometary organic molecules will be determined in-situ. More than 20 years ago, in 1986, the European GIOTTO mission took the first photo of a cometary nucleus. After this highly successful mission, ESA focussed on cometary research and started to design, plan, and construct the spacecraft ROSETTA, an official cornerstone-mission, which was launched with an Ariane 5 rocket in March 2004. Since then, the ROSETTA probe has been in space and already performed several fly-by manoeuvres at Earth and Mars in order to accelerate towards the target comet that is called 67P/Churyumov-Gerasimenko. ROSETTA will reach the orbit of comet "Chury" in 2014 and subsequently detach its robotical Lander named Philae, which is designed to land softly on the comet's surface (Ulamec et al. 1997; Bibring et al. 2007). An artist's image of the Lander Philae on the comet's nucleus is given in Fig. 1.3.

In the context of this book, the "Chirality-Module" of the ROSETTA mission is of particular interest. This module is an integral part of the Cometary Sampling and Composition Experiment (COSAC) which is a sophisticated gas chromatograph coupled to a mass spectrometer (GC-MS) system capable of separating, identifying, and quantifying enantiomers (Rosenbauer et al. 1999; Goesmann et al. 2007b). COSAC's multicolumn gas chromatograph GC is equipped with both a thermo-conductivity TCD and a mass spectrometric MS detector. It is constructed to analyse cometary matter in situ, i.e., after landing on the surface of the cometary nucleus from where it will telecommunicate the recorded data to Earth. COSAC's GC/TCD-MS equipment was mainly designed and fabricated at the Max Planck Institute for Solar System Research in Katlenburg-Lindau, Germany.

In supplement to COSAC's achiral stationary phases whose selection, development, and combination are described by Szopa et al. (1999, 2000, 2007), three capillary columns that are doted with a chiral liquid film on its inner surface were designed to separate a variety of chiral organic molecules into their enantiomers in order to determine their enantiomeric excess (Meierhenrich et al. 1999, 2001b). Cometary non-complex alcohols, amines, diols, hydrocarbons, carboxylic acids, and

Fig. 1.3 The ROSETTA Orbiter swoops low over the ROSETTA Lander Philae soon after touchdown on the nucleus of comet 67P/Churyumov-Gerasimenko. This landing is scheduled for November 2014. Graphical simulation, photo courtesy of EADS Astrium

amino acids are intended to be resolved into their enantiomers in order to quantify individual enantiomers and thus to verify the above hypothesis.

After introducing the required stereochemical terms, nomenclature, and enantio-selective techniques in Chap. 2 and up-to-date theories on the origin of biomolecu-lar asymmetry in Chaps. 3-6, this book will expose the formation of chiral amino acid structures in interstellar space (Chap. 7). We will discuss the analysis of amino acids and diamino acids in extraterrestrial samples such as meteorites in Chap. 8, before we outline in Chap. 9 the basic concept of the COSAC instrument onboard the ROSETTA Lander. I will describe ROSETTA's "Chirality-Module" and summarize preliminary experiments in order to investigate the operation of the chiral capillary columns with material that is expected to show similar physico-chemical properties compared with original cometary matter. I will report on our first investigations of chiral stationary phases like Chirasil-Val coated with chiral valine molecules and cyclodextrin coated phases applicable for gas chromatographic resolution of enan-tiomers of underivatized alcohols and diols.

Rather far-reaching conclusions are expected from ROSETTA's "Chirality-Module". In the case of a successful mission, we would be able to better specify the molecular precursors of life based on chiral carbon chemistry in its prebiotic forms. ROSETTA's "Chirality-Module" will moreover try to shed some light on all the theories on the origin of biomolecular asymmetry. Doing so, it might contribute to the elucidation of the ultimate cause of the spectacular biochirality phenomena on Earth.

Chapter 2

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