Ask A Scientist© Chemistry Archive

name        Mel R.
status      student
age         19

Question -  why are optical isomers important in organic chemistry?
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Mel,

As you already know, certain molecular structures can exist in mirror-image
pairs. The optical isomeric members in such a series may not all have
identical chemical properties. This is especially important regarding
bio-molecules. As you may know, living systems as we recognize them, are
almost exclusively built with the L-form of amino acids and the D-form of
sugars. The opposite isomers are not significantly useful in the metabolic
processes of life-forms with which science is familiar.

Regards,
ProfHoff 288
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Optical isomers have the property of "handed-ness" just like your hands and
gloves, or your feet, and shoes. These pairs of molecules are
non-superimposable mirror images of one another just like your hands,
gloves, feet, and shoes are. Just like you are able to tell very easily when
you try to put a left hand glove on your right hand, or a left shoe on your
right foot, so the stereochemistry of d- and l- handed molecules is
different. All biologically active molecules, hence all of life is "left
handed". Your nerve receptors are optically active, so they respond
differently to the different isomers, if they respond at all.

Some of organic chemistry is not sensitive to this property of molecules,
other fields of organic chemistry is controlled by this property of
molecules.

Louis Pasteur was the first scientist to recognize optical activity. He was
a consultant to a winery and was examining crystals of tartaric acid, a
chemical found in wines. He noticed that crystals of tartaric acid that he
examined under his microscope were composed of non-superimposible pairs. He
separated the two types with tweezers! And as the saying goes, "The rest is
history."

In the last decade catalysts have become commercially available that favors
the formation of one of the pairs over the other. The pharmaceutical
industry is actively conducting research on synthesizing pure optically
active molecules, because the biochemistry of the pairs can be very
different. You can see that at the very least you might be able to double
the activity of a drug if you could make the isomer that was doing the job.

You may have heard of the drug phthalidomide (spelling may be thalidomide??)
which was prescribed to pregnant women to control morning sickness. Very
unfortunately, it also resulted in babies with profound deformaties of the
arms and legs. Later research showed that one of the optically active pairs
controlled morning sickness, and the other-handed one of the pair caused the
birth defects.

Optically active compounds are also being used in conjunction with a medical
diagnostic technique called PET (positron emission tomography) to follow the
activity of optically active molecules in the body in "real time".

So you can see, optically active molecules are very important.

Vince Calder
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