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I—The Chemistry of Carbon, 1 Biotic and Abiotic Carbon Compounds
Pages 9-20

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From page 9...
... They record events that occurred during the formation of the solar system and during the division of materials among planets. In that sense, variations in the chemical structures of carbon compounds are some of our ultimate historical documents.
From page 11...
... Chains can bend around to form rings, and rings can be fused to form sheets of atoms for which the bonding diagrams look like chicken wire. In such cases, the four bonds at each carbon atom are often arranged to provide double or even triple linkages (see the next section, "Aromaticity")
From page 12...
... Knowing the limits on conditions could provide clues that might reveal a biotic or an abiotic origin for compounds sampled. Aromaticity One of the most important characteristics of organic compounds is their ability to form molecules with a distinctive stability resulting from aromaticity, with the electrons of neighboring carbon atoms shared "in resonance." An aromatic compound is one in which at least two equivalent structures exist.
From page 13...
... The natural products (e.g., cholesterol) are biosynthesized from lipid precursors.7 Homologous aliphatic organic compounds and biomarkers can be distinguished by organic geochemists as being derived from abiotic or biotic sources.8 The homologous compounds with the biomarkers should be considered as the intermediary organics between the CH4 chemistry on planetary bodies and the formation of aromatics in the solar and interstellar medium as additional suitable tracers for evidence of life.
From page 14...
... of the nine different carbon skeletons possible for a molecule with seven carbon atoms and no double bonds or rings. The asterisks mark chiral centers (see Figure 1.3)
From page 15...
... The products are a molecule of water and an organic molecule in which the two carbon skeletons are connected by an ester linkage. Such reactions, in which two smaller molecules combine to form a larger product with the release of some small molecule, are termed "condensation reactions." Polymers exemplify both structural complexity (or, at least, great molecular size)
From page 16...
... The alcohol and carboxylic acid shown as reactants are examples of carbon skeletons similar to those introduced in Figures 1.4 and 1.5 bearing two of the functional groups shown in Figure 1.6. For each carbon skeleton, the addition of the functional group has created a new chiral center by introducing asymmetry at a formerly symmetrical carbon position.
From page 17...
... . The carbon skeletons of the amino acids used in proteins contain up to 10 carbon atoms and up to six double bonds or rings.
From page 18...
... Given the structures and relative abundances of organic compounds from samples of cosmochemical interest, it may be possible to work backward and to reconstruct the physical and chemical conditions prevailing at the times of their synthesis and throughout their subsequent history. With regard to the indicators that might differentiate between a biotic and an abiotic origin for particular organic compounds, the task group found that the most compelling indicators of an abiotic origin include the following: · The presence of a smooth distribution of organic compounds in a sample, e.g., a balance of even versus odd numbers of carbon atoms in alkanes; · The presence of all possible structures, patterns, isomers, and stereoisomers in a subset of compounds such as amino acids; · A balance of observed entantiomers; and · The lack of depletions or enrichments of certain isotopes with respect to the isotopic ratio normally expected.
From page 19...
... Fetzer, "High Molecular Weight Polycyclic Aromatic Hydrocarbons in Hydrothermal Petroleums from the Gulf of California and Northeast Pacific Ocean," Organic Geochemistry 24: 1065-1077, 1996.


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