Structure and Optical Isomerism
A very important feature of the structure of amino acids (and other kinds of compounds
as well, for that matter) is called optical isomerism. It applies to all
amino acids except glycine.
|Look at the number-two carbon atom. You should notice that in one
direction it is bonded to an amino group. In another direction, it is bonded to a
carboxylic group. It is also bonded to a hydrogen atom and an alkyl group or some other
kind of group. Except in the case of glycine where -R is a -H, that number two carbon atom
is bonded to four different groups. A carbon atom which is attached to four
different groups is called an asymmetric carbon atom or
sometimes a chiral carbon atom. The importance of this depends on some
structural properties that we will investigate in this section.
If you are in the lab you get a model kit and follow along with the diagrams shown
here. Get a carbon atom and attach to it four different groups. For convenience just use
different colored units, rather than actually building an amino group and a carboxylic
acid group and an isopropyl group or something like that. Then make the other models as
they are shown bleow. If you are not in the lab now, you should work with the models to do
this exercise when you are in the lab.
|Here is a model of a carbon atom with four different groups attached.
|Here is another model constructed to be the mirror image of the first
model. To do this, construct a model that would appear just as the first model that you
made would look like if you were looking at it in a mirror.
|Here you can see why these are called mirror images of one another.
|We can demonstrate that these two structures are not identical to one
another by trying to superimpose one structure on another and get all of the same colored
units to be in the identical places. You can see that is not possible.
|The two structures are different. They are isomers
of one another. It so happens that they are called optical isomers of one
another because they have optical properties that are different from one another. We will
discuss that particular property a little bit more when we discuss carbohydrates in a
When asymmetric carbon atoms are present in a molecular compound, there are two ways in
which the groups attached to that carbon can be arranged in the three dimensions, as we
have just shown with the two models above. It is generally true, if not universally true,
that only one of these optical isomers is biologically active. In other words, when these
compounds are made by a plant or animal, only one of the two forms is made. When it comes
time for these molecules to interact with an enzyme, only one of these molecules would
react. The other would not. Both shape and orientation in biological compounds are
Chemically, optical isomers behave the same. Biologically,
they do not. One will react properly, but the other will not. Optically,
there is also that difference which will be pointed out when we deal with carbohydrates in
a later lesson.
|We can use these models to illustrate why you need to have four different
groups bonded to the central atom. One group (the black group) has been removed from the
model on the left and replaced it with a duplicate of one of the other three groups (the
white group). We now have a model with the central atom bonded to four groups, but they
are not all different. The same has been done to the mirror image
(unfortunately, you cannot see that).
|By turning the second model in the right way you can see that it is
identical to the first one.
|Consequently, this central atom is not an asymmetric carbon atom, the
molecule is not an optically active molecule, and these are identicalcompounds and not
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©2001, 2003 Clackamas Community College, Hal Bender