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Components of Cells
The Macromolecules
Polarized Light

Polarized Light

Jean-Baptiste Biot had a taste for adventure. During the early part of the French revolution he took part in a riot directed against the Convention (the people in Paris trying to rule the country)and was taken prisoner by government forces. If a powerful friend had not rescued him, he might have died and we would now not know as much about the glucose molecule as we do.

Later in life, when he was not going up in the first hot-air balloon with the other famous scientist of his time, Gay Lussac (1804), he was experimenting with the way polarized light behaves as it passes through clear liquids and solutions.

Light coming from the sun, or from a light bulb, can be thought of as a wave-form that "wiggles" up and down. All the different waves coming from any given source, "wiggle" or oscillate, in all different directions. The result is a very chaotic mixture of light waves that vibrate in all directions at once. This is the natural light we use to see things, or see through things.

If this chaotic mixture of light waves is passed through a special filter, only those waves oscillating in one direction (such as "up" and "down") will be allowed to pass through, and those waves oscillating in all other directions will be blocked and not allowed through the filter. This is the principle behind some of the more expensive sunglasses on the market, which use polarizing filters to reduce the sun's glare.

[Note: technically a polarizing filter lens will only transmit a single component of a light beam in which both the electrical and magnetic field vectors oscillate in a single plane].

Rotation of Polarized Light

If a beam of polarized light is shone through vessel containing pure water, it comes out at the other side of the vessel still oscillating in the same direction it went in. However, if the beam of polarized light encounters dissolved molecules in the water, strange things begin to happen, and the light waves are rotated. When they leave, the vessel, therefore, they are no longer oscillating in the same direction as before.

This change in direction of the plane of oscillation of the electric and magnetic vectors of polarized light can be measured in a special instrument called a polarimeter, and the results can tell scientists a lot about the structural properties of the molecules which have caused the change.

When the molecules in the polarimeter are achiral (are symmetric), such as water molecules, nothing happens to the beam of polarized light. But if the molecules are chiral, such as glucose, then the beam of polarized light will be rotated in either a clockwise direction (termed "positive") or an anticlockwise direction (termed "negative")

The amount of the rotational change can be measured at the other end of the polarimeter by using a second filter. For full light intensity, this second filter will have to be rotated until it allows all the light to go through, and if the light has been bent by the sample, then the second filter will have to be turned by that amount or the light will be blocked.

The angle of this second filter corresponds to the amount of light rotation by the sample.


Enantiomers

Some molecules, such a glucose, come in different structural forms which are termed enantiomers. These versions of the molecule differ in the ways that atoms are bonded to one another, but are similar in all other ways, such as their physical and chemical properties. It is thus very difficult to distinguish between enantiomers of a molecule since they are almost identical.

Except when exposed to polarized light.

What Biot and others found was that right-handed and left-handed enantiomers of a molecule that had a chiral center (four different groups attached to one carbon atom, for example) bent polarized light in opposite directions. This kind of behavior was unique to these kinds of chiral molecules and is nowadays called optical activity.

The enantiomer that rotates polarized in a clockwise direction is termed dextrorotatory (usually written +), while the opposite enantiomer (the mirror image version) will rotate the light in the counterclockwise direction, termed levorotatory (usually written -). For ease of writing these terms are usually abbreviated to (d) - from the Latin dexter, meaning "right" and (l) - from the Latin laevus meaning "left".

In this way all kinds of chiral compounds can be examined. The results are usually expressed in the form of a number called the specific rotation, which is calculated this way:

[Note: the wavelength of light used for these measurements is 589 nm (from a sodium lamp); the observed rotation is the angle measured in the polarimeter; the concentration is in grams/milliliter; and the length of the vessel is in decimeters].

Interestingly, if a molecule with a chrial center (such as a carbon atom) is made synthetically, in a test tube, a mixture of enantiomers is obtained in which both versions are equally represented. But chrial compounds are also found extensively in living organisms (like the organic carbohydrates) and when taken out of the living cell, they are found to be optically active, with one enantiomer either the sole form or the predominant form. This is the result of catalytic activity and the fact that active sites of enzymes have to be formed very specifically and cannot accommodate to opposite forms of the same molecule.

Life, it appears, is basically chiral!

Jean Baptiste Biot was made a chevalier of the Legion of Honor in 1814 and a commander of the order in 1849. He was also given the prestigious "Rumford medal" by the Royal Society in 1840. He died in Paris in 1862.


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© 2004, Professor John Blamire