Xwj

Isobutane

The different bonding relationships of butane and isobutane are distinguished in their structural formulas, and the two compounds have different chemical properties.

Optical isomers occur whenever a carbon atom has four different atoms or groups attached to it. This pattern allows two different ways of making the attachments, each the mirror image of the other (Figure 2.21). Such a carbon atom is an asymmetrical carbon, and the pair of compounds are optical isomers of each other. You can imagine your right and left hands as optical isomers. Just as a glove is specific for a particular hand, some biochemical molecules can interact with one optical isomer of a compound, but are unable to "fit" the other.

The a carbon in an amino acid is an asymmetrical carbon because it is bonded to four different functional groups. Therefore, amino acids exist in two isomeric forms, called D-amino acids and L-amino acids. d and l are abbreviations for the Latin terms for right (dextro) and left (levo), respectively. Only L-amino acids are commonly found in most organisms, and their presence is an important chemical "signature" for life.

Now that we have covered the major properties of all molecules, let's review them in preparation for the next chapter, which focuses on the major molecules of biological systems.

► Molecules vary in size. Some are small, such as H2 and CH4. Others are larger, such as a molecule of table sugar (sucrose, C12H22O11), which has 45 atoms. Still other molecules, especially proteins such as hemoglobin (the oxygen carrier in red blood cells), are gigantic, sometimes containing tens of thousands of atoms. The formation of large molecules from simpler ones in the environment was a key precursor to the emergence of life during the Archean.

► All molecules have a specific three-dimensional shape. For example, the orientation of the bonding orbitals around the carbon atom gives the methane molecule (CH4) the shape of a regular tetrahedron (see Figure 2.10c). In carbon dioxide (CO2), the three atoms are in line. Larger molecules have complex shapes that result from the numbers and kinds of atoms present and the ways in which they are linked together. Some large molecules, such as hemoglobin, have compact, ball-like shapes. Others, such as the protein called keratin that makes up your hair, are long, thin, ropelike structures. Their shapes relate to the roles these molecules play in living cells.

► Molecules are characterized by certain chemical properties that determine their biological roles. Chemists use the characteristics of composition, structure (three-dimensional shape), reactivity, and solubility to distinguish a pure sample of one molecule from a sample of a different molecule. The presence of functional groups can impart distinctive chemical properties to a molecules, as does the physical arrangement of atoms into isomers.

Molecule

Hand Mîctot image

Mirror image

Hand Mîctot image

Mirror image

2.21 Optical Isomers (a) Optical isomers are mirror images of each other. (b) Molecular optical isomers result when four different groups are attached to a single carbon atom. If a template is laid out to match the groups on one carbon atom, the groups on that molecule's mirror-image isomer cannot be rotated to fit the same template.

2.21 Optical Isomers (a) Optical isomers are mirror images of each other. (b) Molecular optical isomers result when four different groups are attached to a single carbon atom. If a template is laid out to match the groups on one carbon atom, the groups on that molecule's mirror-image isomer cannot be rotated to fit the same template.

Between the small molecules discussed in this chapter and the world of the living cell stand the macromolecules. These huge molecules—proteins, lipids, carbohydrates, and nucleic acids—are the subject of the next chapter.

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