CARBONACEOUS CHONDRITES: A RECORD OF THE ORGANIC CHEMICAL EVOLUTION OF THE EARLY SOLAR SYSTEM

Studies of carbonaceous chondrites have provided evidence that interstellar organic matter has survived processes associated with the formation of the solar system and has been incorporated into grains and larger objects. Within the asteroidal parent bodies, the presumably simpler interstellar organic compounds were transformed into more complex organic compounds by aqueous and thermal processing.3-6 A remarkably broad range of organic compounds has been identified in carbonaceous chondrites.7,8 This organic ensemble includes amino acids, purines, and pyrimidines. The presence of these particular compounds demonstrates that the solar system provided at least one environment in which recognizable biomolecules were synthesized abiotically. Furthermore, the presence of such compounds in meteorites indicates that impacts by meteorites, comets, and dust must have delivered potentially biologically useful organic compounds to early Earth and the other terrestrial planets. Carbonaceous chondrites thus currently provide a potentially rich and accessible source of information regarding the organic compounds naturally present during prebiotic evolution of the terrestrial planets as well as other solid bodies such as the moons of Jupiter and Saturn (e.g., Europa, Enceladus, or Titan).

Murchison Meteorite: A Timely Gift from the Solar System

In September 1969, just as many chemists were beginning the search for organic compounds in lunar samples, a new carbonaceous chondrite fell near Murchison, Victoria, Australia. Rich in volatiles, it contains more than 10 percent water and about 2.2 percent carbon by weight. Initial research was directed toward determining the constituents of the aqueous and solvent-soluble fractions of this meteorite. It was soon determined that Murchison contained a spectacularly complex suite of small molecules (Table 3.1). For example, to date, more than 70 different amino acids have been identified in this meteorite. The distribution of amino acids is similar to that produced in certain abiotic syntheses and, where mirror-image structures (i.e., stereoisomers) are anticipated, both chiral forms are found to be nearly equal in abundance. This balance is one signature of abiotic synthesis. Moreover, the amino acids extracted from the Murchison meteorite contain distinctly higher levels of 13C than do any terrestrial amino acids. Thus, even though many of the soluble compounds in Murchison are recognizable as biochemically significant, e.g., the amino acids, their stereochemical and isotopic characteristics clearly identify them as both extraterrestrial and nonbiological.9

The classes of soluble meteoritic organic compounds that have familiar biochemical counterparts include the amino acids, fatty acids, purines, pyrimidines, and sugars.10,11 Additional soluble constituents include alcohols, aldehydes, amides, amines, mono- and dicarboxylic acids, aliphatic and aromatic hydrocarbons, heterocyclic aromatics, hydroxy acids, ketones, phosphonic and sulfonic acids, sulfides and ethers.12-14 Concentrations of the major representatives of these classes vary widely from less than 10 parts per million (amines) to tens of parts per million (amino acids) to hundreds of parts per million (carboxylic acids).15

Cooper et al.16 identified a complex suite of sugars and sugar derivatives present in the soluble fraction of Murchison at concentrations comparable to that of the amino acids (~60 ppm) in the Murchison and Murray meteorites. Chromatographic analyses of virtually all classes of acyclic compounds reveal complex molecular assemblages containing homologous series of compounds up to C12 in some cases (carboxylic acids). The relative abundances of the heavy stable isotopes of carbon and hydrogen (13C and 2H) in these compounds differ significantly from those of terrestrial materials and provide decisive evidence that these materials are not terrestrial contaminants.

Distinctive patterns of structural variation are seen in the Murchison organics but they differ from those found in living systems.17 As a class, the amino acids exemplify this contrast. Specifically:

  • The abundances of amino acids decrease with increasing carbon number,18

  • The abundances of branched-chain isomers far exceed those of the straight-chain isomers, and

  • Structural diversity dominates at the lower carbon numbers (e.g., acyclic, monoamino acids).



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