Left and Right Handed Molecules

When four different atoms or molecules are attached to a carbon atom, there are two different configurations possible. These are referred to as (+) or (-) (depending on whether they rotate polarized light clockwise or counterclockwise), d or l (as in dextrorotary and levorotary), or as S or R (from the latin sinister or rectus).

The simplest example of an amino acid with handedness is alanine, the central carbon of which is bonded to 1) a hydrogen atom (H), 2) a methyl group (CH3 — represented by a solid line), 3) an amino group (NH2), and 4) a carboxyl group (COOH). The D-alanine and L-alanine are mirror images of each other.

Just as we cannot put a left-handed glove on our right hands, the D-form of alanine cannot be rotated and superimposed on the L-form. When the central carbon and any two arms are aligned identically, the other two arms are always pointing in opposite directions. Similarly, the surfaces (as defined by any three arms) are not superimposable. This means that handedness is as important as molecular structure in determining how a substrate or drug reacts with an enzyme or receptor.——SWF