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Stereochemistry

Stereochemistry


Stereochemistry is the study of molecules in three dimensions.

The major factor influencing whether or not inorganic chemical reactions will proceed is temperature. In biological reactions, the shape of the molecules is more important. Enzymes are able to make reactions kinetically favorable at room temperature. The deciding factor for whether a biological reaction will occur is the fit between the substrate(s) and the binding site(s) on the enzyme.

In this Hypertext, we will emphasize 3 main concepts of stereochemistry: symmetry, handedness and chirality.

Note: To really get a feel for molecules in 3 dimensions, you may want to use a chemistry modeling set to build yourself models of molecules as we go along.

Symmetry

You are probably already familiar with the concept of symmetry. An object is bilaterally symmetrical if it can be bisected with a line or a plane, and the two sides are identical, but mirror images of each other. If you could fold the object along the plane of symmetry, the two sides would overlap perfectly. In addition to bilateral symmetry, an object can have symmetry that is best seen when divided into 3 parts, or five parts, etc. Some examples of symmetrical objects: circles, butterflies, the letter E. A starfish is also symmetrical but has five-fold symmetry.

images of symmetrical objects

An object that is not symmetrical is called asymmetric. Examples of asymmetric objects are: spirals, the number 2, and most geopolitical boundaries (in this image, the state of Massachusetts).

images of asymmetrical objects

Handedness

Just as your left hand and your right hand are mirror images of each other, many chemical compounds can exist in two mirror image forms. For example, when the amino acid alanine is synthesized in the laboratory, a mixture of the two possible structures is formed. However, when alanine is produced in a living cell, only one of the two forms is seen.

The naturally occuring form of alanine is called L-alanine, and its mirror image is called D-alanine. Comparison of the 20 common amino acids will show that only the "L" form is used in protein synthesis. The enzymatic machinery used in protein synthesis has an asymmetric binding site the amino acids must fit into. Your right hand won't fit properly into a left handed glove, and an amino acid of the wrong shape won't fit into an enzyme. Of all the naturally occuring amino acids in proteins, only Glycine--NH2-CH2-COOH-- has a plane of symmetry (along its "spine").

Chirality

Chirality is a special case of asymmetry. A molecule is chiral if there is no internal plane of symmetry, and the molecule and its mirror image are not superimposable.

Even after rotating one of the molecules it remains different from its stereoisomer. They are not superimposable.

When studying carbohydrates, we often refer to a chiral carbon molecule. A carbon molecule is chiral if it has 4 different substituents, or R groups. To distinguish between the two possible arrangements of 4 substituents around a chiral carbon molecule, a system of nomenclature was developed.

Stereoisomers

A chiral molecule and its mirror image molecule are called stereoisomers or enantiomers. Pairs of stereoisomers are sometimes indistinguishable in chemical reactions, but can be distinguished by examining a physical property of the molecule. A pure solutions of a stereoisomer will rotate the plane of plane polarized light. The other enantiomer will rotate polarized light the same number of degrees, but in the opposite direction. For this reason, stereoisomers are also called optical isomers. A solution that contains a mixture of the two optical isomers will not change the plane of plane polarized light, because the effects of the two isomers cancel each other out.
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