Optical Activity

One of the physical properties of molecules that have asymmetric carbon atoms (like these four in the middle of glucose marked with *) is that the molecules will rotate the direction in which the light is vibrating as it passes through them. This property is called optical activity.

67037.jpg (3404 bytes)

Polarized Light

With white light you cannot tell this is happening. That is because the light is vibrating in all directions to begin with, so when it rotates it's still vibrating in all directions. Consequently, this property only becomes apparent when you work with polarized light, so let me take a moment to describe polarized light.

A polarizing filter such as this will allow light to pass through if the light is vibrating parallel to the alignment of the filter. It will block light that is vibrating perpendicular to the alignment of the filter. (A more accurate description is that the component of the vibrations that are aligned with the filter will pass through, and the component of the vibrations that are perpendicular to the filter alignment will not pass through.)

Light passing through single polarizing filter. [Mvc-011f.jpg]

A second filter with the same alignment still lets light pass through.

Light passing through two polarizing filters (aligned). [Mvc-016f.jpg]

When the alignment of the second filter is turned it blocks much of the light, making that portion of the image darker.

Light passing through two polarizing filters (at 45 degrees). [Mvc-014f.jpg]

When the second polarizing filter is rotated 90^o (degrees) to the first, the polarized light that came through the first filter is blocked out.

Light passing through two polarizing filters (at 90 degrees). [Mvc-015f.jpg]

Rotating Polarized Light

When a chemical containing asymmetric carbon atoms (such as glucose or fructose) is placed between the polarizing filters, it is possible to observe that light polarized by the first filter is rotated before passing through the second filter.

Here, corn syrup is placed in a beaker between two polarizing filters aligned with the same orientation.

Two aligned polarizing filters with cornsyrup between them. [Mvc-018f.jpg]

Notice that when the filters are aligned at 90^o (degrees) to each other, the light is blocked out except where it passes through the corn syrup. The filter would have to be turned a little bit past 90^o in order to block out the polarized light that has been rotated.

Two crossed polarizing filters with corn syrup between them. [Mvc-019f.jpg]

Something else that you might be able to pick up and might not is that there is a color change through there. The light coming through the glucose between the crossed filters has a blue color. This is because different colors of light are rotated different amounts. At 90^o there is a little bit of blue light getting through but the other colors are blocked out.

We have a demonstration of this ability to rotate polarized light set up in the lab for you to take a look at when you are in the lab. When you work with this demonstration, you will be able to see a variety of colors at various degrees of rotation of the top polarizing filter.

Optical Isomers

This optical property of compounds that have asymmetric carbon atoms is what has prompted us to call these compounds optically active. The mirror images of these compounds are referred to as optical isomers.

Structure of D-glucose and its mirror image. [67038.jpg]

The reason that this is so important, however, is not because these particular compounds can rotate light. Instead, it has to do with the way that biological reactions take place. Biological reactions generally are catalyzed by enzymes and the enzymes are set up to specifically handle a particular orientation of atoms within a molecule. An enzyme that would be able to take D-glucose, and hydrolyze it, or oxidize it, or do something else to it would not be able to carry out that same reaction on the mirror image of D-glucose. That is because it wouldn't fit properly up against the enzyme as is necessary to carry out the reaction. As a consequence, different arrangements of the atoms in molecules, even just one or two of the OH's switched in position in the molecules shown above will make a biological difference. Therefore, it's very important to be very familiar with the orientations of the functional groups within a particular molecule.

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E-mail instructor: Sue Eggling

Clackamas Community College
Š2001, 2003 Clackamas Community College, Hal Bender