Amino interstellar medium

Figure 5.19 Formation of amino acids on ice surfaces irradiated in the laboratory (Nature Nature 416, 403-406 (28 March 2002) doi 10.1038/416403a-permission granted). Data were obtained from analysis of the room temperature residue of photoprocessed interstellar medium ice analogue taken after 6 M HCl hydrolysis and derivatization (ECEE derivatives, Varian-Chrompack Chirasil-L-Val capillary column 12 m x 0.25 mm inner diameter, layer thickness 0.12 pirn splitless injection, 1.5 ml min-1 constant flow of He carrier gas oven temperature programmed for 3 min at 70°C, 5°C min-1, and 17.5 min at 180°C detection of total ion current with GC-MSD system Agilent 6890/5973). The inset shows the determination of alanine enantiomers in the above sample (Chirasil-L-Val 25 m, single ion monitoring for Ala-ECEE base peak at 116 a.m.u.). DAP, diaminopentanoic acid DAH, diaminohexanoic acid a.m.u., atomic mass units.

Both pathways probably involved quite different conditions, the main difference being the absence of liquid water in the interstellar medium. Nevertheless the basic building blocks and chemical reactions should have been roughly similar, thus leading to important connections between these two routes. While a wide variety of amino acids are prebiotically relevant (as attested by either Urey-Miller experiments or meteorite analysis), we shall focus in this section on a-amino acids (as the most relevant to biochemistry) and closely related compounds. 

PAHs are believed to be a major class of carbon-bearing molecules in the interstellar medium 1138], They are found in carbonaceous chondrites tliat have fallen to Earth (see section 4.2.1) and in interplanetary dust particles [28]. Shock and Schulte [139] suggested that amino acids could be syntliesized by aqueous alteration of precursor PAHs in carbonaceous chondrites. We directed attention to shock reaction of PAHs [135,140,141], and conducted shock reactions using benzene, tire simplest aromatic hydrocarbon, as a starting material to simulate possible reactions occurring in interstellar space. Furtliermore, we examined the mechanism of shock reaction on the basis of quantum chemistry and discussed the implication for cosmocheniistiy. 

The present work is only the very first step towards a comprehensive and systematic understanding of the fundamental elementary processes involved in the chemistry of hydrocarbon-rich planetary atmospheres and interstellar medium. Our experiments explicitly identified synthetic routes to nitriles — the alleged precursor molecules to amino acids. The experimental data can be employed to set up a systematic database of reaction products and can predict the formation of hitherto unobserved gas phase molecules. The applications of the crossed beam method to astrochemical problems have just begun. Many interesting problems remain to be studied. In the coming century, laboratory experiments of the kind we have presented here combined with observations and planetary-space missions will undoubtedly unravel the complex chemical processes which extend from atoms and simple molecules to large molecules and aggregates. 

This debate is not without its practical consequences. If amino acids exist only on Earth, then life as we know it may also exist only on our planet. If, on the other hand, amino acids exist in the interstellar medium, then it is at least possible that Earth-like life may exist almost anywhere in the universe. 

Another approach to the study of the interstellar medium includes experiments carried out here on the Earth s surface. In such experiments, researchers attempt to simulate the conditions found in various regions of the ISM and to determine if chemical reactions hypothesized for those regions actually do occur. The study by scientists at SETI and NASA on amino acids, reported earlier in this chapter, is an example of such experiments. Researchers general approach in such experiments is to enclose certain fundamental substances (such as hydrogen, oxygen, and carbon monoxide) within a sealed container at the low temperatures and pressures common to the ISM and then expose those substances to the type of radiation that may be found in some particular region of the ISM, such as ultraviolet radiation or cosmic-ray-like radiation. The substances formed in such experiments can then be compared with those actually observed in the ISM. 

Some of these classes of compounds, such as amino acids and nucleobases are essential in terrestrial biochemistry. For example, amino acids are integral components of proteins and enzymes, and nucleobases are components of DNA and RNA. Others, such as carboxylic acids and sugar-related molecules 21), have been detected in the interstellar medium (22). 

The detection of diamino acids in the Murchison meteorite (41) has stirred up the speculation that these compounds are perhaps direct products of interstellar synthesis processes. In particular, laboratory experiments aimed at the ultraviolet (UV)-irradiation of interstellar ice analogs leads to the formation of refractory material which, after hydrolysis with 6 M HCl, has been shown to contain mono-amino acids as well as diamino acids (42). Although it is evident that some organic compounds that were formed in the interstellar medium, either in the gas phase or in interstellar grains, survived exposure to the interstellar... 

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