Chirality

Introduction

Only a few small molecules that participate in the chemical dance of life can be superimposed on their own mirror image. Every nucleotide and every amino acid except glycine exists as a stereospecific isomer.

Chirality arises when an atom (usually a carbon atom) has a different atom or group of atoms attached through each of four valence electrons. If two of the constituents switch positions, a different stereoisomer results.

Press the view button below to display two simple molecules. Try to rotate the two molecules so that they superimpose on each other perfectly. Give it a few attempts to convince yourself whether they are superimposable or not.

Are these two molecules the same stereoisomer?

Chiral Centers and Enantiomers

Now press the view button below. This will bring up another pair of molecules. Try to rotate the two molecules so that they superimpose on each other perfectly. Give it a few tries before you are convinced.

These two molecules are an example of enantiomers. Enantiomers are mirror images of each other.

Now you have seen what is meant by a chiral center. A chiral center is an atom around which one can form two different molecules by switching the positions of two substituents.

Alanine

L-alanine (also called (R)-alanine) is the physiologically active form of alanine. Its enantiomer, D-alanine (also called (S)-alanine) does not occur in nature. If you press the view button below, L-alanine can be seen on the top and D-alanine can be seen on the bottom. If you rotate the molecules around, you will see that they are different from each other in the same way as the second pair of molecules we explored earlier.

Conclusions

In this structural tour we have explored the concept of chirality. Chirality arises whenever different isomers of a molecule are not superimposable. Enantiomers are a special case of chirality: an enantiomeric pair are mirror images of each other.