Chirality+and+Polarization+-+Ryan+Seery

= = Ryan Seery toc

= History = Jean-Baptiste Biot was the first scientist to discover the effects of chiral molecules on polarized light in 1815. He performed experiments using white light, which consists of waves vibrating on many different planes. When it was directed through a filter (see Figure 1), components of the light that were polarized parallel to the filter passed. This polarized light was then passed through a chamber filled with a sample solution of molecules to be analyzed. Finally, the light went through another filter, but this one was adjustable. Biot rotated the filter's plane until he found the strongest beam of light coming through, and measured the degree of rotation. He discovered that certain solutions were capable of rotating light. The dissolved substances were referred to as "optically active". He also noted that the degree of rotation was directly proportional to the concentration of the solution.

In 1848, Louis Pasteur became the first scientist to separate an optically inactive substance into two optically active parts, using tartaric acid. Each component still retained properties identical to the original substance, including density, freezing/boiling point, and solubility. However, they had opposite effects on polarized light. One component rotated the light clockwise (+), while the other rotated the plane counterclockwise (-) by the same degree. From these results, he postulated that the components of tartaric acid were mirror images of each other. This remains the base principle of stereochemistry today, and these two components have to be known as enantiomers. Further research by Pasteur showed that microorganisms could process one of the enantiomers, but not the other. He then concluded that the biological properties of chemical compounds are reliant on both the molecular formula as well as their arrangement.

=** Terms **=
 * __Isomers__ - molecules that have the same molecular formula, but different spatial arrangements.
 * __Chirality__ - the property exhibited by some molecules describing the ability (or lack thereof) to superpose the molecule's mirror image on itself.
 * __Stereoisomers__ - molecules that have their atoms bonded together in the same order, but still retain isomerism due to their chiral components.
 * __Enantiomers__ - two molecule which are non-superposable mirror images of each other.
 * __Racemic mixture/Racemate__ - a solution of optical isomers containing an equal amount of both enantiomers.
 * __Optical isomers__ - chiral molecules that rotate a beam of polarized light in opposite directions.
 * __Superposition__ - the placement of one object on top or above another.

= The Basics = Chirality is the property of two mirror-image objects that cannot be superposed on one another. A perfect example of this is the human hand. They are alike in the respect that they both have 4 fingers, a thumb, a back, and a palm. However, no matter how you rotate or flip your left hand, you cannot make it congruent to your right hand. If your fingers line up, your hands will be facing opposite ways. If you put one hand directly on top of the other, your fingers will not line up. This same concept shows up in molecular structure, especially in organic compounds composed of a carbon atom surrounded by four atoms of different species. (Figure 2) An object that cannot be superposed on its mirror image is called // chiral, // whereas an // achiral // object can be. A rule of thumb to tell if a molecule is chiral or not is this: If the molecule has no plane of symmetry (rotational symmetry does not apply), then the molecule is chiral; otherwise it is achiral.



= Organic Chiral Molecules = Molecules that are used organically in your body, such as DNA, proteins, amino acids, and sugars are all chiral molecules. Isomeric amino acids are classified as either L- or D-amino acids. Human proteins are made exclusively from L-amino acids. The explanation of why it is that only L-amino acids are used in our bodies is still a mystery. D-amino acids can pass through your body with little or no effect, or cause certain illnesses and disorders, depending on the kind of molecule. For instance, Thalidomide was advertised as a sedative drug for pregnant women during the 1960s. However, it had drastic side effects. It worked well as a sedative, but also caused horrific birth defects in infants worldwide. This was because the solution advertised was a 50/50, or racemic, mixture of the two mirror images of the molecule. One version of the chiral molecule was a sedative, while the other side created birth defects. When molecules interact with one another in your body, they recognize handedness, meaning that if an L-amino acid were to interact readily with another L-amino acid, then it probably would not have much effect on a D-amino acid. This phenomenon is analogous to trying to shake hands with somebody using opposite hands.

= Polarization = Light is a form of electromagnetic radiation, which means that its waves consist of an electric and magnetic component. These two components vibrate on different planes. As light travels toward you, it vibrates in all directions. For our purposes, we will average out all the planes of vibration, resulting in half of them being vertical and half horizontal. However, it is still propagating, or traveling, in one direction. This light is said to be unpolarized. When it comes in contact with a polarization filter, we can see that it only lets past the light that is vibrating parallel to the orientation of the filter. This results in a beam of light with only one plane of polarization. Now we will place another filter after the first in the path of the light. If it is the same orientation as the first (or the exact opposite), then the light will pass through unchanged. But, if we rotate it, we get a gradual decrease in the intensity of the light that passes through. This trend will continue until the second orientation is perpendicular to the first, at which point no light will pass through the second filter.

This whole concept of polarization is observed in various molecular structures that serve as the polarization filters. Crystals also exhibit this phenomenon. Chiral molecules, however, do something a little bit different. Instead of filtering the polarization planes, they // rotate // them. Thus, if you were to take the example above and put a solution of chiral molecules in between the perpendicularly oriented filters, the light // would // pass through the second filter (see Figure 1).

Polarization is perhaps best explained by visual demonstration. Here is a link to an applet describing how polarization works. Notice that some light is still able to get through when the filters are not exactly lined up. The video below shows this concept in action: media type="youtube" key="QgA6L2n476Y" height="307" width="384" align="left"

=** Importance **= The handedness of molecules plays an important role in our biological interactions with organic compounds. For instance, all life on Earth utilizes left-handed, or (-) amino acids. If a protein were to be assembled with even one right-handed amino acid, the entire function of the protein would be negated. Likewise, the human body can metabolize (+) glucose, but does not interact with (-) glucose. This poses an interesting philosophical question: why is it that all life on the planet is made of homochiral molecules? Why did nature choose (+) over (-), or vice versa, as the case may be? Scientists still do not have an answer.

As for polarization, this concept can be utilized in many different fields, from astronomy to skin cancer research and treatment. In astronomy, polarization has been used to reveal how the light was created, the distance of the source, and interesting symmetrical effects produced by supernovae (left). Polarized light scanning allows scientists and doctors to see growths underneath the skin without probing (right).