Optical Activity
Optical Activity
The simplest method to distinguish between enantiomers in a laboratory is the use of polarimetry. The two enantiomers rotate the plane polarized light in opposite directions. For example, the (S) enantiomer of thyroxine (T4) (thyroid hormone) rotates the plane polarized light to the left whereas, the (R) enantiomer rotates plane polarized light to the right.
A beam of light oscillates in a limitless number of planes at right angle to the course of light travel. When this light is passed through polarizer, it only allows light with single plane to pass. This light with single plane is called as plane polarized light. When plane polarized light is passed through a solution containing organic compound with chiral centers it is rotated through an angle α. Such organic compounds are termed as optically active compounds. The instrument used to measure the angle of rotation is called polarimeter. Following figure shows the schematic illustration of polarimeter.
The working of polarimeter is quite simple. The light source is passed through polarizer. Polarizer only allows light in single plane. The polarized light is then passed through optically active compound solution. The optically active solution rotates the plane polarized light either to the right or to the left. The extent of rotation is examined by looking at the analyzer.
Those compounds which rotate the plane polarized light to the right (clockwise direction) are called dextrorotatory and are assigned (+) sign by convention. Whereas optically active compounds involved in rotating the plane polarized light to the left (counterclockwise) are called levorotatory and are assigned (-) sign. The extent of plane polarized light rotation depends upon the following factors.
- 1) Number of optically active molecules:
The greater the number of optically active molecules encountered by plane polarized light the greater is the extent of rotation. The number of active compounds, in turn, is directly proportional to sample concentration and pathlength of the sample. The greater the concentration or sample pathlength the greater will be the extent of rotation and vice versa.
- 2) Wavelength of the light:
The extent of rotation also depends upon the wavelength of the light source used. Every 0.03 nm change in wavelength of light shifts the value of rotation for 0.01%.
The use of polarimeter in a meaningful way so that comparisons can be made between optically active compounds, a specific rotation [α]D is used. The parameters used in measuring specific rotation are.
Wavelength of Light = 589.6 nm (sodium D line)
Sample pathlength = l = 10 cm = 1 dm
Sample concentration = c = 1 g/cm3
The specific rotation measured at standard conditions is a physical constant for an optically active compound. Following table shows specific rotation of different optically active organic compounds.
Polarimetry has many applications and widely used in pharmaceutical industry, chemical industry, food and beverages industry, and cosmetics and fragrance industry. In pharmaceutical Industry polarimetry is widely used for the determination of purity, concentration, and identification of different biologically active enantiomers. In chemical industry this method is used to monitor the production of optically active compounds. In beverages, food, fruit, and dairy industry the concentration of optically active sugars is monitored via polarimetry. In cosmetic industry, the polarimetry is mainly employed to control purity and identify optically active essences and essential oils.
