Holland Clinic

The main reason for posting these chemical structures is to give
people some idea of what we mean when we say that certain
medications or drugs are "related". Related drugs share certain
chemical structures in common, usually these are the "business
ends" of the chemical or the part that is biologically active.
Therefore, a good organic chemist might not know the name of a
particular one of the medications shown above, but once she sees
the basic backbone structure, she can pretty well tell how the
drug will affect people. Notice how subtle differences can be: look
at the structures of pseudoephedrine at the top and ephedrine
below it. Every atom is in the same place except for the "OH"
group on the carbon attached to the ring. In the case of
pseudoephedrine, the "OH" group projects downwards (indicated by
the dashed lines expanding towards the "OH" group", while in the
case of ephedrine, the "OH" group extends upward out of the plane
of the drawing as indicated by the black triangle. Thus these two
very similar molecules are mirror images of each other like our right
and left hands are mirror images of each other. Thus
pseudophedrine and ephedrine are called "stereo-isomers" or
"enantiomers" and the carbon atom around which the mirror images
pivot is called the "chiral carbon". This is more than just sophistry
too.

Just like your right hand won't fit into a left handed glove, one
enantiomer won't fit into a receptor protein and the other will.
Many medications contain a 50-50 mix of enantiomers (50-50
because when the drugs are made in the lab, each molecule
randomly assembles into one or the other form). Furthermore,
often only one of the isomers has a beneficial effect while the
other may have no effect or even cause unpleasant side effects.
This is the case for an antidepressant medication called "Celexa"
wich is a 50-50 mix of two optical isomers of a chemical called
citaprolam shown below> One enantiomer eases depression while
the other causes side effects. For this reason, drug companies
have purified out the active enantiomer and it is called
"escitaprolam" or "s-citaprolam" or more commonly, Lexapro.