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Chiral enantiomers are mirror images of each other that can rotate polarized light in different directions. Optical activity is measured using a polarimeter and is important in identifying the dominant chiral form of molecules, such as in pharmaceuticals. Optical activity occurs in both organic and inorganic molecules.
The molecules of a single substance often have the same properties, but may be oriented differently as if they were right-handed or left-handed. Light, polarized in a single direction, can pass through a structure such as a crystal and be rotated in one direction or another. If they tend to scatter light in this way, have the same characteristics, and appear to be mirror images of each other, these molecules are often called chiral enantiomers. Optical activity refers to how a chiral substance can rotate the polarization of light and make it dimmer. A polarimeter is an instrument that is often used to measure this activity, using a sample of the material to test its optical activity firsthand.
The molecular structure of a substance typically determines the relationship between chirality and optical activity. Depending on the material, the plane of polarized light can be oriented clockwise or counterclockwise. Two identical molecules that affect light in opposite directions are called enantiomers. An equal amount of each usually nullifies the effect. If one chiral substance dominates the other, however, its properties will rotate light waves in the respective direction.
A polarimeter is often used to measure optical activity. The front filter polarizes the incoming light passing through a tube filled with the sample to be measured. There is usually an analyzer at the end of the tube and beyond, that’s where the human eye can see the effect. If plane polarized light is rotated then it is generally weaker, while the effect can be counteracted by rotating the analysis lens. The level of optical activity can be determined using a formula the number of degrees of rotation of this lens.
Optical activity is often measured in organic chemistry, typically in carbon atoms. It can also be seen in optically active compounds such as sugar and even glycine, an amino acid. There are two nearly identical forms of a molecule in each of these substances. Knowing what that is requires experimentation, but is often important with pharmaceuticals. Some depression and arthritis drugs have had adverse health effects in their chiral forms, so one form of the molecule must be filtered out.
Discovered in the mid-1800s, optical activity was first used to identify tartaric acid crystals in French wine. It is studied in modern times regarding quantum mechanics as well as electric and magnetic fields. Optical activity occurs in both organic and inorganic molecules.
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