AU-irares-Retmal filling models, the radius of each atom is proportional to its van der Waals radius, and the contours of the model define the space occupied by the molecule (the volume of space from which atoms of other molecules are excluded).

Configuration is conferred by the presence of either (1) double bonds, around which there is no freedom of rotation, or (2) chiral centers, around which substituent groups are arranged in a specific sequence. The identifying characteristic of configurational isomers is that they cannot be interconverted without temporarily breaking one or more covalent bonds. Figure 1-18a shows the configurations of maleic acid and its isomer, fumaric acid. These compounds are geometric, or cistrans, isomers; they differ in the arrangement of their substituent groups with respect to the nonrotating double bond (Latin cis, "on this side"—groups on the same side of the double bond; trans, "across"—groups on opposite sides). Maleic acid is the cis isomer and fumaric acid the trans isomer; each is a well-defined compound that can be separated from the other, and each has its own unique chemical properties. A binding site (on an enzyme, for example) that is complementary to one of these molecules would not be a suitable binding site for the other, which explains why the two compounds have distinct biological roles despite their similar chemistry.

In the second type of configurational isomer, four different substituents bonded to a tetrahedral carbon atom may be arranged two different ways in space—that is, have two configurations (Fig. 1-19)—yielding two stereoisomers with similar or identical chemical properties but differing in certain physical and biological properties. A carbon atom with four different substituents is said to be asymmetric, and asymmetric carbons are called chiral centers (Greek chiros, "hand"; some stereoisomers are related structurally as the right hand is to the left). A molecule with only one chiral carbon can have two stereoisomers; when two or more (n) chi-ral carbons are present, there can be 2n stereoisomers. Some stereoisomers are mirror images of each other; they are called enantiomers (Fig. 1-19). Pairs of stereoisomers that are not mirror images of each other are called diastereomers (Fig. 1-20).

As Louis Pasteur first observed (Box 1-2), enan-tiomers have nearly identical chemical properties but differ in a characteristic physical property, their interaction with plane-polarized light. In separate solutions, two enantiomers rotate the plane of plane-polarized light in opposite directions, but an equimolar solution of the two enantiomers (a racemic mixture) shows no optical rotation. Compounds without chiral centers do not rotate the plane of plane-polarized light.

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