Optical Isomerism
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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.

65str03.JPG (3090 bytes)

 

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.

Model of an asymmetric carbon atom. [65mod07.JPG]

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.

Models of mirror images of asymmetric carbon atoms. [65mod08.JPG]

Here you can see why these are called mirror images of one another.

Model of asymmetric carbon atom and its image in a mirror. [65mod09.JPG]

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.

Models of assymetric carbon atoms superimposed on one another. [65mod11.JPG]

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 later lesson.

Models of optical isomers of asymmetric carbon atoms. [65mod12.JPG]

 

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 extremely important.

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).

Models of mirror images of carbon atoms which are not asymmetric. [65mod13.JPG]

By turning the second model in the right way you can see that it is identical to the first one.

Superimposed models of mirror images of carbon atoms which are not asymmetric.  [65mod14.JPG]

Consequently, this central atom is not an asymmetric carbon atom, the molecule is not an optically active molecule, and these are identicalcompounds and not optical isomers.

Models of mirror images of carbon atoms which are not asymmetric, a different arrangement. [65mod15.JPG]

 

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E-mail instructor: Kerry Cotter

Clackamas Community College
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