As we discussed in class, the amino acid valine, like all other amino acids except glycine, is capable of existing in two stereoisomeric forms. The carbon bearing the NH₂ (amino) group is a stereogenic center, making the molecule chiral.

The line structure on the left provides no information about 3-D structure except what we can infer by recognizing the stereogenic center. The next structure uses the wedge/dash system to attempt a 3-D representation of one enantiomer; by switching the positions of two ligands on the stereogenic center we obtain the other enantiomer, shown at right.

Here are ball-and-stick models of the two stereoisomers:

The orientation of the two is not quite the same, but they are placed in approximately mirror image positions. For some people, the solidity of the bonds and atoms will create a more three-dimensional impression than the line structures.

If you are using a Chime-enabled browser, you will see below two Chime windows containing the enantiomers of valine. You can rotate, resize, and alter the type of model using your mouse. Drag with the left button to rotate, drag while holding down the Shift key to resize, and right-click to use the menu to change the model type.

For most people, the molecules instantly spring into three dimensions the moment they begin to move. Try to orient the molecules into exactly mirror image positions and see how they look both as ball-and-stick and space-filling models.

The structure on the left is the "naturally"-occurring stereoisomer. In the outmoded designation used by biochemists, this is the L- isomer. To a modern organic chemist, using the system approved by the International Union of Pure and Applied Chemistry (IUPAC), this is the S- stereoisomer.