TABLE 5.1 Carbon in the Murchison Meteorite

Total carbon

2.12%, 1.96%

Carbon as interstellar grains

 

Diamond

400 ppm

Silicon carbide

7 ppm

Graphite

<2 ppm

Carbonate minerals

2-10% of total carbon

Macromolecular carbon

70-80% of total carbon

SOURCE: Modified after J.R. Cronin, “Clues from the Origin of the Solar System: Meteorites,” pp. 119-146 in The Molecular Origins of Life: Assembling Pieces of the Puzzle, A. Brack A. (ed.), Cambridge University Press, Cambridge, U.K., 1998.

Similar experiments have generated nonbiological routes for the synthesis of other organic molecules, including some molecules that are used in our own biochemistry. For example, the Oró-Orgel synthesis exploits the reactivity of HCN to make adenine (C5H5N5), one of the five nucleobases used to store information in DNA and RNA. Analogous synthesis generates adenine from formamide.

The complexity of the products of adding energy to simple organic mixtures, including the complexity of tholins, has a disadvantage. The diversity of products is so great in such experiments in prebiotic chemistry that they do not greatly limit the inventory of organic species that might have been present on early Earth.

5.2
NATURAL AVAILABILITY OF BIOLOGICAL-LIKE MOLECULES

5.2.1
Biological-like Molecules from the Cosmos

There is little doubt that natural processes generate organic molecules analogous to those generated by the laboratory experiments described above. Amino acids are found in natural specimens, including meteorites, that are almost certainly not influenced by biological processes. They include many amino acids that are not part of the human-like standard collection of encoded amino acids.

Some chemical fragments of DNA and RNA can also be found in meteorites (Tables 5.1 and 5.2). For example, some meteorites have been reported to contain small amounts of adenine, one of the nucleobases found in RNA and DNA. The current view is that the Murchison meteorite contained adenine, guanine, their hydrolysis products hypoxanthine and xanthine, and uracil. The reported concentration of all those substances, however, is low, about 1.3 ppm. The Murchison and other meteorites may also contain ribitol and ribonic acid, the reduced and oxidized forms of ribose, respectively, but ribose itself has not been found.6

TABLE 5.2 Organic Compounds in the Murchison Meteorite

Amino acids

60 ppm

Purines and pyrimidines

1.3 ppm

Aliphatic hydrocarbons

>35 ppm

Basic N-heterocycles

7 ppm

Aromatic hydrocarbons

15-28 ppm

Amines

8 ppm

Carboxylic acids

>300 ppm

Amides

55-70 ppm

Dicarboxylic acids

>30 ppm

Alcohols

11 ppm

Hydroxycarboxylic acids

15 ppm

Aldehydes and ketones

27 ppm

SOURCE: Data from Cronin, J.R., and Pizzarello, S. 1986. Amino acids of the Murchison meteorite. III. Seven carbon acyclic primary alpha-amino alkanoic acids. Geochim. Cosmochim. Acta 50:2419-2427.



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