Cooh

HsGWj

nh2

HOCX?

L-alanine

D-alanine

FIGURE 3.1.1 The atoms in the a-amino acid alanine can assume two different configurations in three-dimensional space. The two forms, L-alanine and D-alanine, are called enantiomers because they are non-superimposable mirror images of each other. Abiotic processes produce equal mixtures of both L and D enantiomers, but terran life preferentially uses the L or D form. For example, most organisms on Earth make exclusive use of the L form of a-amino acids. Chemical bonds oriented out of and into the plane of the page are shown as solid or dashed wedges, respectively. Courtesy of Roger E. Summons, Massachusetts Institute of Technology.

FIGURE 3.1.1 The atoms in the a-amino acid alanine can assume two different configurations in three-dimensional space. The two forms, L-alanine and D-alanine, are called enantiomers because they are non-superimposable mirror images of each other. Abiotic processes produce equal mixtures of both L and D enantiomers, but terran life preferentially uses the L or D form. For example, most organisms on Earth make exclusive use of the L form of a-amino acids. Chemical bonds oriented out of and into the plane of the page are shown as solid or dashed wedges, respectively. Courtesy of Roger E. Summons, Massachusetts Institute of Technology.

BOX 3.2 Diastereomeric Preference

Diastereomeric preference is another manifestation of the ability of atoms in certain molecules to assume different orientations in space. If the two spatial arrangements of atoms are not mirror images of each other, then the different molecular forms are known as diastereomers or diastereoisomers (Figure 3.2.1). Unlike enantiomers, diastereoisomers have different physical and chemical properties and can be separated by chromatography or other processes that exploit subtle differences in polarity. Simple sugars are good examples of diastereoisomers and the more complex the molecule, the more possibilities there are to form diastereomers. Thus, for example, the steroid cholesterol (see Figure 3.2.2) can exist in 256 different structural configurations, but living systems make use of only one of them.1

1K.E. Peters, J.M. Moldowan, and C.C. Walters, The Biomarker Guide, Cambridge University Press, 2004.

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