Handedness and Optical Isomerism

What is not as well known is that a large number of the molecules in plants and animals are chiral, and usually only one form of the chiral molecule (left-handed or right-handed) is found in nature. For example, all but 1 of the 20 naturally occurring amino acids in our proteins are chiral, and they have the same handedness. Most of the natural sugars exhibit the opposite handedness when compared to amino acids. 

A chiral molecule and its non-super-imposable mirror image are called enantiomers. Enantiomers are two different molecules, just as your left hand and right hand are different. To have enantiomers a molecular structure must be asymmetric (without symmetry). The simplest case is a tetrahedral carbon atom bonded to four different atoms or groups of atoms. 

Such a carbon atom is asymmetric and is said to be chiral. A molecule that contains a chiral atom is likely to be a chiral molecule, although this is not always so. 

Some compounds are found in nature in both enantiomeric forms. For example, both forms of lactic acid are found in nature. During the contraction of muscles the body produces only one enantiomer of lactic acid the other enantiomer is produced when milk sours. Let s look at the structure of lactic acid to see why two different isomers or chiral molecules might be possible. The central carbon atom of lactic acid has four different groups bonded to it —CHj, —OH, —H, and —COOH. 

One must be able to specify the handedness of chiral centers in order to distinguish between them. Enantiomers are mirror-image molecules that have 

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