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For glycine, the quantum yield is (£(Gly) = 3.6 • 10~5. Numbers designated in boldface are the main mass fragments; a.m.u. atomic mass unit, A first eluting enantiomer, B second eluting enantiomer, c.i.i. cyclic immonium ion, d.l. detection limit, McLaff McLafferty rearrangement. * Exact molecular structure not yet known. Ornithine or its constitutional isomer, t Exact molecular structure not yet known. Lysine or its constitutional isomer.

be noted that in the Urey-Miller model, the intermediate products like amino and hydroxy nitriles also require a similar hydrolysis step to form amino acids and hy-droxy acids.

Most of the amino acids reported in the experiment above were previously identified in meteorites called carbonaceous chondrites (Kvenvolden et al. 1979; Cronin and Pizzarello 1983, 1997; Engel and Macko 2001; Ehrenfreund et al. 2001b; Botta et al. 2002). However, no amino acids with more than one amino group were detected in these samples. The above experiment suggests that these species might also be present in meteorites. Subsequent enantioselective ultra-trace analyses of the Murchison meteorite verified indeed the presence of a variety of diamino acids (Meierhenrich et al. 2004).

7.3.3.4 Diamino Acids in the Simulated Interstellar Medium

The amino acids identified in interstellar and circumstellar ice analogues were not a random mixture of organic molecules. From the 16 generated amino acids, the diamino carboxylic acids1 were of particular interest, since they belong to a new family of amino acids, also called diamino acids. These molecules have now been identified in residues of UV-photoprocessing of interstellar ice analogs and in the Murchison meteorite. They were proposed to provide hints for elucidating important reactions in prebiotic chemistry with respect to the further development of early genetic material.

The RNA-world scenario (Gilbert 1986), in which informational and catalytic properties of the first self-replicating system were provided by single RNA molecules, gives rise to some unanswered questions concerning the central ribose molecule. First, the ribose molecule is difficult to synthesize under prebiotic conditions. The formose reaction generates about 40 sugar compounds with a low yield of ribose. This was recently demonstrated by George Cooper and his colleagues who identified a great number of different polyols in meteoritic samples (Cooper et al. 2001). The evolutionary selection of the ribose molecule out of this random mixture of polyols remains problematic and until now unresolved. Second, ribose molecules are unstable, particularly in liquid solution.

Therefore, at present there is a growing consensus that the RNA-world did not evolve directly from monomeric molecules formed by prebiotic processes (Ferris 1993). It is assumed that RNA itself was preceded in evolution by a pre-RNA material of oligomer or polymer structure. In addition to numerous proposed alternative structures (mainly based on substituted ribose analogues), an attractive substituent of the ribose phosphate backbone in the RNA molecule is that of a peptide nucleic acid (PNA) (Nielsen et al. 1991; Nielsen 1993). The PNA molecules serve as DNA analogues and consist of peptide backbones made of diamino carboxylic acids to which nucleotide bases are attached via carbonyl methylene linkers (see

1 Diamino acids identified in interstellar ice analogues are 2,3-diamino propanoic acid, 2,3-diamino butanoic acid, diamino isobutanoic acid, diamino isopentanoic acid, diamino pentanoic acid, and diamino hexanoic acid.

Chap. 8). PNA molecules were found to bind to oligo(deoxy)ribonucleotides and obeyed Watson-Crick base pairing rules, i.e., A-T and G-C base pairs are highly preferred (Egholm et al. 1993a). In template-directed reactions, information can be transferred from PNA to RNA, and vice-versa (Brack and Ferris 2002).

PNA can be easily produced from diamino carboxylic acids. The first identification of 6 molecular structures of diamino carboxylic acids in the residues made from UV-photoprocessing of interstellar ice analogues reported here might stimulate future research through the study of the reaction pathways from diamino carboxylic acids that result in PNA structures. Chapter 8 will present more details on this 'new' amino acid family including their eventual role in chemical evolution.

7.3.3.5 Topical Discussions on Amino Acids in Residues made by UV-Processing of Simulated Interstellar Ice Analogues

Based on the previous observations, how can we obtain additional experimental evidence that the identified amino acid structures in interstellar ice analogues really triggered the appearance of life on Earth? Jordan et al. (2005) compared proteins from organisms representing all three domains of life (Bacteria, Archaea, and Eukaryota) with regard to their amino acid content. Amino acids with declining amounts in proteins during biological evolution were identified, and some have increased in amounts. The amino acids with declining frequencies were proposed by Jordan et al. (2005) to be among the first incorporated into the genetic code, and most of those with increasing frequencies were probably recruited later. Based on these results, a consensus order of amino acid recruitment into the genetic code was established starting with glycine, alanine, aspartic acid, valine, then skipping over proline to serine. These six amino acids are consistent with the six proteinaceous amino acids identified in residues made from UV-photoprocessing of interstellar ice analogues.

But it is still a question of topical debate how amino acids got into the meteorites in the first place (Elsila et al. 2007). There appear to be two reaction-pathways. One suggestion is that Strecker-type chemical reactions involving liquid water took place on or in the rocky bodies that formed when the Solar System was young. The herewith-related presence of liquid water in solar system objects will be discussed in Chap. 9. The alternative model, discussed here, assumes that interstellar and circum-stellar photoreactions produce radicals in interstellar ices that recombine at higher temperatures to form a complex polymer material. After hydrolysis this polymer material releases amino acids and diamino acids. At present, we are not in a position to explain the mechanism of interstellar amino acid formation in detail - neither the intermediate precursors nor the function of water in the form of solid or liquid. Since analyses of amino acids in meteoritic samples and in residues made from UV-photoprocessing of interstellar ice analogues both require a hydrolysis step, we assume for these matrices a chemical composition of peptide-like molecules that contain amino acid structures. Yet this assumptions needs to be verified in the future.

Several research groups tried to shed some light on the interstellar amino acid formation pathway after the simultaneous experimental identification of amino acid structures in residues made from UV-photoprocessing of interstellar ice analogues by NASA Ames researchers and our European group (Bernstein et al. 2002; Mufioz Caro et al. 2002). These teams focused on (a) the theoretic ab initio calculation of spontaneous interstellar and circumstellar amino acid synthesis by quantum chemical modelling studies (e.g. Woon 2002), (b) the application of different energy sources for irradiation of interstellar ices and its effect on the synthesized organic molecules (Klanova et al. 2003; Takano et al. 2004b, 2007), or (c) the deposition of different types of gas mixtures (Takano et al. 2003b; Takahashi et al. 2005; Nuevo 2005; Nuevo et al. 2007). The precise mechanism, however, remains unknown.

The simulated interstellar formation of amino acid structures has been pointed out by Nobel Prize laureate Christian de Duve (2003) who said that there is now clear evidence that large quantities of organic molecules continually arise at many sites in outer space by mechanisms that are beginning to be understood and even reproduced in the laboratory. However, - and this is the central conflict - amino acids have not been identified unambiguously by spectroscopic methods in interstellar environments. Why not? One argument is based on the so-called partition function problem, which is valid for spectroscopic measurements of spectroscopi-cally "complex" molecules like amino acids, making specific electronic transitions and vibrations spectroscopically invisible at a given detection limit among a variety of other electronic transitions and vibrations.

Another argument states that amino acids in interstellar environments mainly occur embedded in polymers, which release free amino acids after acid hydrolysis only. This refractory polymer material remains in the solid state making it invisible to radio astronomy techniques. Thus, amino acids cannot be identified spectroscopi-cally in different interstellar environments with classical methods. Recently, Martin Schwell from Paris University Denis Diderot presented experimental evidence for a third argument in this conflict: Schwell's experimental set-up was constructed in order to study photolysis reactions (i.e., photodegradation reactions) of amino acids in well-defined vacuum ultraviolet radiation fields. In the chosen irradiation fields, organic molecules and amino acids in particular exhibited limited stability against electromagnetic radiation. Schwell et al. irradiated amino acids in the gaseous phase, characterized their photochemical properties, and deduced that amino acids should undergo photodecomposition reactions if they are subjected to energetic interstellar vacuum ultraviolet irradiation (Schwell et al. 2006; Schwell 2007). Regarding pho-todecomposition, the same can be said about methanol. Methanol photodecomposes "easier" than amino acids, yet methanol is observed abundantly both in the solid and gas phases. Almost any molecule can photodecompose, but one has to take into account that there is an equilibrium between formation and destruction of interstellar molecules by ultraviolet irradiation.

Ehrenfreund et al. (2001a) studied the photostability of amino acids in an argon matrix at 12 K, irradiated by energetic radiation of 135 to 165 nm wavelengths. These experiments also indicated that amino acids are degraded photochemically. Nevertheless, ultraviolet destruction cross-sections were determined in an argon matrix and the extrapolation of the here determined values to other solid phases, for example to interstellar grains, or the gas phase should be handled with caution.

The tentative spectroscopic identification of the amino acid glycine in the interstellar clouds Sagittarius B2(N-LMH), Orion KL, and W51 e1/e2 was published recently (Kuan et al. 2003), but is waiting for confirmation by other research teams. At present, radio astronomers do not rely on the results of Kuan and coworkers. In summary, amino acids have not yet been identified by direct measurements in interstellar environments. This difficulty may be explained by various reasons, as outlined above.

Nevertheless, the obtained experimental data support the assumption that tiny ice grain mantles can play host to important reactions when irradiated by ultraviolet light. It is possible that such ice mantles could have become incorporated into the cloud that formed our Solar System and ended up on Earth, assisting the start of life. Several lines of evidence suggest that some of the building blocks of life were delivered to the primitive Earth via (micro-) meteorites and/or comets from space. These results suggest that interstellar chemistry may have played a significant part in supplying Earth with some of the organic materials needed to trigger life. Furthermore, since new stars and planets are formed within the same clouds in which new amino acids are being created, this probably increases the chance that life has evolved elsewhere.

Coming back to our original question on the origin of biomolecular homochi-rality, the chiral amino acids produced by simulated interstellar photochemical reactions showed racemic occurrence, contrary to that found in all proteins of living organisms. Nevertheless, the above experiment is assumed to be of crucial importance for the understanding of biomolecular asymmetry. It provides evidence that chiral organic molecules like amino acids spontaneously form under interstellar and circumstellar conditions (Fig. 7.5).

In the experiment outlined above, non-polarized light was used to induce photochemical reactions of the simulated interstellar/circumstellar ice mixture. However, in interstellar and circumstellar regions, electromagnetic radiation has asymmetric components and is partly circularly polarized. The transfer of asymmetry from 'chiral photons' (in the form of circularly polarized electromagnetic radiation) to prochiral or racemic organic molecules can provoke molecular enantiomer enrichments and has experimentally been proven to be feasible as described for enantio-selective photolysis reactions in Chap. 6. Here, I will present recent experimental approaches on the enantioselective synthesis of amino acids under simulated interstellar conditions.

Fig. 7.5 Photochemical formation of amino acids with unpolarized Light from Ci and Ni components, resulting in the formation of racemic mixtures mimicking the interstellar medium. Alanine and 15 other amino acids were produced under specific low temperature/low pressure conditions. The first publication of the illustration by Griesbeck and Meierhenrich (2002) is acknowledged

Fig. 7.5 Photochemical formation of amino acids with unpolarized Light from Ci and Ni components, resulting in the formation of racemic mixtures mimicking the interstellar medium. Alanine and 15 other amino acids were produced under specific low temperature/low pressure conditions. The first publication of the illustration by Griesbeck and Meierhenrich (2002) is acknowledged

7.3.4 Illumination with Circularly Polarized Light

Interstellar electromagnetic radiation is known for its energy and its intensity. What about its polarization? Circularly polarized electromagnetic radiation was identified in the Orion star formation region by Bailey and his group (1998). Maps of circularly polarized infrared radiation from the OMC-1 region of Orion were recorded by Buschermohle et al. (2005). The authors concluded that the observed circular polarization in the near infrared can also produce circular polarization in the range of ultraviolet wavelengths (see Chap. 6).

Consequently, in advanced experimental series, the light source described for the above experiments on the interstellar amino acid formation - emitting unpo-larized light - was substituted by a circularly polarized synchrotron beam in the French synchrotron facility LURE, Paris. Interstellar ice analogues were produced in the closely located team of Louis d'Hendecourt at the Institut d'Astrophysique Spatiale (IAS) following a proposition made by J. Mayo Greenberg. With these experiments, the aim was to mimic interstellar and circumstellar conditions and to study whether one could introduce an enantiomeric excess into the produced chiral amino acid structures by absolute asymmetric photochemical synthesis (Griesbeck and Meierhenrich 2002; Meierhenrich and Thiemann 2004). Our common aim was to study hitherto unknown enantioselective photosynthesis reactions of amino acids in contrast to well-known enantioselective photodecomposition reactions already described by Balavoine et al. (1974), Flores et al. (1977), and Meierhenrich et al. (2005b).

Experiments were conducted with right- and left-circularly polarized light from the synchrotron in two separate runs. Each irradiation took two days, the wavelength of the synchrotron radiation was set to X = 167 nm. After irradiation at liquid nitrogen temperature (T = 77 K), the ice samples were warmed to room temperature and the organic residue products remaining at room temperature were analyzed by infrared spectroscopy, residue hydrolysis (in 6 molar hydrochloric acid), and deriva-tization, followed by GC-MS analysis using an enantioselective column. Carbon-13 isotope labelled molecules were used in the initial gas mixture in order to carefully exclude biological contaminants. As a first result, we were able to identify eight amino acids, all of them composed of 13C isotopes, in samples produced with circularly polarized light (Table 7.2) (Nuevo et al. 2006).

The amino acids have been analyzed with special emphasis for any evidence of an enantiomeric excess in either L or D. The results and discussion focused on two of the chiral amino acid products: a-alanine, the most abundant chiral pro-teinaceous amino acid, and 2,3-diaminopropanoic acid (DAP), a non-proteinaceous chiral diamino acid recently identified in the Murchison meteorite (Meierhenrich et al. 2004). After photochemical synthesis and hydrolysis of the interstellar ice analogues, these amino acids were detected in form of the 13C-ECEE derivatives. The obtained enantiomeric excesses were compared to those measured for the same amino acids produced by unpolarized ultraviolet irradiation of the same ice mixtures (expected to be zero) in order to determine the contribution of circularly polarized

Table 7.2 Amino acids and diamino acids identified in simulated interstellar ices irradiated with circularly polarized light (Nuevo et al. 2006)

Amino acid

MS 13C sample [a.m.u.]

Rt of analyte [min]

Sarcosine

103,118

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