There are two stereoisomers of alanine:
Both of these compounds are three-carbon carboxylic acids with an amino group attached to the α carbon.
However, they are not the same compound!
Notice that the α carbon is a .
What is the difference between your left hand and your right hand? They look similar, yet your hands are not identical -- they can not be superimposed on top of each other. They are non-superimposable (nonidentical) mirror images of each other. If you put your right hand in front of a mirror, it would look like a left hand in the mirror. Your right hand is a mirror image of your left hand but it does not fit into a left-handed glove. Chirality is the property of "handness." Objects that have left- and right-handed forms are called chiral from the Greek word for "hand."
What is it about about a molecule that makes it chiral? One feature (but not the only one) that lends chirality is a chiral carbon atoms, also called a chiral center. A chiral center is defined as an sp3 hybridized carbon that is bonded to four different groups. When you return to the tutorial, notice how the tetrahedral arrangement around a chiral carbon atom makes it a chiral object.
Examine the structures closely. Rotate each molecule so that the methyl group is pointed up along the Y-axis, with the amino group to the right. Notice where the hydrogen attached to the chiral center ends up.
In L-alanine, the hydrogen is in front of the chiral center, pointing directly at you.
In D-alanine, the hydrogen is in behind the chiral center, pointing away.
L-alanine and D-alanine are, in fact, mirror images. Rotate one of the structures 180° around the Y-axis in order to see this relationship clearly. These two compounds are called enantiomers, that is, they are nonsuperimposable mirror images.
The two molecules can not be superimposed on top of one another. No matter how you twist and turn the two structures, they can never look the same. They are stereoisomers. They have the same molecular formula, the same connectivity, but the arrangement of their atoms in three dimensions is different. Therefore, they are different molecules.
In 19 of the 20 amino acids used for the biosynthesis of proteins, the α-carbon atom is chiral. The exception is glycine, which has two hydrogen atoms attached to the central carbon atom. All 19 chiral amino acids are in the L configuration, where "L" signifies that the amino acid has the same configuration as L-glyceraldehyde.
Click here to learn how to recognize the L configuration.