Interstellar ices

Laboratory data from two groups (see Sect. 3.2.4) indicate that chiral amino acid structures can be formed in simulations of the conditions present in interstellar space. The experimental results support the assumption that important asymmetrical reactions could have taken place on interstellar ice particles irradiated with circularly polarised UV light. The question as to whether such material was ever transported to the young Earth remains open. But the Rosetta mission may provide important answers on the problem of asymmetric syntheses of biomolecules under cosmic conditions (Meierhenrich and Thiemann, 2004). 

Interstellar ice The formation of ice layers or mantles on the surface of interstellar dust grains formed by the adsorption of O and H separately, forming water ice on the surface. 

Gibb E. F. et al. (2000). An Inventory of Interstellar Ices toward the Protostar W33A1, Astrophysical Journal, 536 347-356. 

Artificial hydrothermal vents might be constructed and supplied with plausible concentrations of simple reactants such as CO, H2, NH3, and H2S. Appropriate levels of amino adds induding a small chiral excess, along with the sorts of amphiphilic molecules described above, can be rationalized by the findings from the Murchison meteorite. Organic molecules such as found in irradiated interstellar ice models, including HMT, can also be induded. The system should indude weathered feldspars, which can be modified to indude the reduced transition-metal minerals that they are known to contain. [134] Such minerals as Fe,Ni sulfides are likely to have been both present and stable in the environment of early Earth and are known [153, 155] to catalyze formation of organic molecules from simpler precursors. 

Significantly, the photoproducts of interstellar ice simulations also include amphiphilic compounds having self-assembly properties [31]. Figure 8 shows micrographs of Murchison vesicles, as well as vesicles formed by products of interstellar ice simulations and known fatty acid-fatty alcohol mixtures. It is clear that the vesicle-forming behavior of all of these amphiphiles is... 

Fig. 8 Phase and fluorescence micrographs of membranous vesicular structures formed from a Murchison meteorite extract (left) compared to vesicles formed by a 20 mM de-canoic acid-decanol mixture [72] (center) and a vesicular structure produced by the photoproduct of an interstellar-ice analog [31]. The vesicles produced by the photochemical ice analog product were allowed to capture pyranine, a fluorescent anionic dye, to demonstrate that a true membrane was present. Scale bars show 20, 10, and 5 pm, from left to right...

As for meteorite analysis, amino acids from such interstellar ice analogues are usually detected after (often acidic) hydrolysis of crude reaction products, and subject to the same interpretation restrictions. Pointing out the formation of peptides [85] (without further arguments) to explain the detection of amino acids in such acid-hydrolyzed irradiated ices is at least questionable since many other precursors can be involved. 

Given that interstellar ices are the building blocks of comets and comets are thought to be an important source of the species that fell on primitive Earth, the structures of molecules in comets may be related to the origin of life. It is possible that organic materials formed in the solid ice phase of interstellar materials provided raw materials used for life originating solely on Earth. If so, the deep freeze of ice in the Oort cloud would have been an excellent place to store these, especially the unstable ones, awaiting delivery to a planet. 

Although information on the chemical composition and reactivity of interstellar ices can be obtained only from remote observations and laboratory simulations, cometary ices and dusts are subject to direct studies, eg within Vega and Giotto (comet Halley), Stardust (comet Wild 2) and Rosetta (comet Churyumov-Gerasimenko) missions (Figure 8.12). 

Ehrenfreund P, Schutte WA. ISO observations of interstellar ices implications for the pristinity of comets. Adv Space Res 2000 25 2177-88. 

Schutte WA. Production of organic molecules in interstellar ices. Adv Space Res 2002 3 1409-17. 

Photochemical transformation of organic compounds and in particular PAHs on ice, as a medium, has not received much attention from the photochemical community. As a result, information on such transformations is limited. Astrophysical research on water ice, on the other hand, has evolved at a rapid pace in recent years after its discovery on outer solar system bodies and in interstellar space [21-23]. A recent review article by Klan and Holoubek [24] on ice photochemistry provides the current knowledge on the distribution, accumulation, and chemical/photo chemical transformation of persistent bio accumulative and toxic compounds in water ice. Since PAHs constitute a substantial portion of the interstellar carbon inventory [25,26], their photochemical behavior is of paramount importance in the radiative processing of interstellar ices. 

Bernstein MP, Dworkin JP, Sandford SA et al (2002) Racemic amino acids from the ultraviolet photolysis of interstellar ice analogues. Nature 416 401-403... 

Bennett CJ, Jamieson CS, Osamura Y, Kaiser RI. (2005) A combined experimental and computational investigation on the synthesis of acetaldehyde CHjCHO (XW) in interstellar ice. Astrophys J 624 1097-1115. 

Munoz Caro GM, Meierhenrich UJ, Schutte WA, Barbier B, Arcones Segovia A, Rosenbauer H, Thiemann WHP, Brack A, Greenberg JM. Amino acids from ultraviolet irradiation of interstellar ice analogues. Nature 2002 416 403-406. 

Sandford S. A., Bernstein M. P., and Swindle T. D. (1998) The trapping of noble gases by by the irradiation and warming of interstellar ice analogs. Meteorit. Planet. Sci. 33, A135. 

The recent ai roach of large comets such as IP/Halley, C71996 B2 Hyakutake, and C/1995 Ol Hale-Bopp to the Earth provided a good opportunity to investigate the detailed composition of cometary ices by various methods such as mass spectrometry, infrared spectroscopy, and radio emission. The composition of interstellar ices is compared with that of the cometary ices in Table 9.3. It is striking that cometary and interstellar ices have quite comparable relative molecular abundances. 

Munoz Caro, G. M., Meierhenrich, U. J., Schutte, W. A. et al. (2002). Amino acids from ultraviolet inadiation of interstellar ice analogues. Nature, 416, 403-6. 

Ambrogelly et al (2007) Natural expansion of the genetic code. Nature Chem Biol 3 29-35 Bernstein MP, Dworkin JP, Sanford SA et al (2002) Racemic amino acids from the ultraviolet photolysis of interstellar ice analogues. Nature 416 401 —403 Blackmond DG, Klussmann M (2007) Spoilt for choice assessing phase behaviour models for the evolution of homochirality. Chem Commun 3990-3996 Breslow R (1959) The mechanism of the formose reaction. Tetrahedron Lett 21 22-26 Breslow R et al (2010) Initiating prebiotic homochirality on Earth. Orig Life Evol Biosph 40 11-26... 

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