FIGURE 3.2.1 The ability of atoms in organic molecules to assume multiple configurations in three-dimensional space is demonstrated by these three forms of tartaric acid. Structures A and B and A and C are superimposable mirror images of each other and so are termed diastereomers. Structures B and C are non-superimposable mirror images of each other and are, thus, enantiomers (see Box 3.1). Courtesy of Roger E. Summons, Massachusetts Institute of Technology.

FIGURE 3.2.2 Structure of cholesterol with its eight asymmetric carbon atoms identified with their position number. Theoretically, this compound could exist in as many as 256 (28) possible stereoisomers, and yet biosynthesis produces only the one illustrated.

carbon, nitrogen, or sulfur isotopic data in Archean sediments, for example, subject to debate.24-27 Although not likely to yield unambiguous biosignatures in the near future, isotopic analyses of martian sediments and atmospheric gases will be important for discerning their evolution and for establishing comparative data, as they do on Earth. Identification of a suite of supporting isotopic data in a reaction pathway, and its environmental context, is the most effective approach to identifying an isotopic biosignature. Elucidation of the isotopic systematics of

BOX 3.3 Structural Isomers

The propensity of carbon compounds to exist with multiple ring systems and unsaturations means that the generic organic compound CpHqNrOsPtSu, can assume an enormous variety of possible structures, known as structural isomers.1 Despite the potential for variety, researchers observe that naturally synthesized biochemicals fall into patterns, and the number of known compounds is but a small subset of what is chemically feasible. Moreover, the biomolecule may be the thermodynamically least favored structure within a set of possible isomers if this aspect enhances its functional capacity.

Structural isomers are readily separated using chromatography. In many, but not all cases, their mass spectra are also distinctive. As with other forms of isomerism, combinatorial instruments such as gas chromatographs-mass spectrometers and liquid chromatographs-mass spectrometers provide the most sensitive and diagnostic tools for trace analysis.

1E.L. Eliel, S.H. Wilen, and L.N. Mander, Stereochemistry of Organic Compounds, Wiley, New York, 1994.

the C-cycle on Earth has been underway for more than 50 years, and much remains to be understood.2829 An added complication for studies of Mars is the unknown degree to which nonbiological atmospheric processes fractionate isotopes.

An example of an isotopic biomarker that might be used in the search for life on Mars is the 18O/16O ratio in phosphates.30 Phosphorus in the form of phosphates (PO43-) is utilized in genetic material and cell membranes, and as a cofactor and energy-transporting molecule in terran biology. On Earth, the ultimate source of PO43- is apatite that is dissolved, biologically processed, and redeposited as various sedimentary PO43- phases and as biogenic calcium phosphate deposits (phosphorites). Biologically processed PO43- on Earth has a strong biotic O-isotopic signature that is highly evolved from abiotic apatite baseline values. On Mars, evolution of the 18O/16O ratios in phosphates from this abiotic baseline could be used as a biomarker. Furthermore, the 18O/16O ratio of PO43- records temperature and high-temperature exchange reactions with water, also making PO43- a potential indicator of past hydrothermal activity on Mars.31

An additional example of an isotopic effect concerns the tendency in biological processes for large molecules to be synthesized by the repeated addition of subunits of two or five carbon atoms (see Box 3.4). The lipid building blocks acetate (C2) and isopentenyl pyrolphosphate (C5) are, for example, isotopically inhomogeneous. Acetate provides one of the best examples because it shows very significant differences in the 13C contents of its methyl and carboxyl carbons.32 The most overt consequences are isotopic ordering in fatty acids and a major isotopic difference between acetogenic and polyisoprenoid lipids. In a single organism, the isotopic differences between acetogenic and polyisoprenoid lipids depend on how many of the polyisoprenoid carbon atoms arise from acetate versus carbohydrate metabolism.33

Morphological Biosignatures

Morphological biosignatures represent the class of objects that can be interpreted as indicative of life based on their size, shape distribution, and provenance. Features of interest occur at both the macroscopic (e.g., stromatolites and microbially induced sedimentary structures) and the microscopic (e.g., microfossils) scale. If they were discovered on Mars, macroscale morphological features such as stromatolites, although being the subject of some contention as a definitive indicator of biogenicity,34 would prove to be highly desirable targets for further study and/or sample return.35-37

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