OPTICAL ACTIVITY
Some compounds recognized as optical isomers show optical activity by interacting with plane-polarized light. Such optical isomers are also called enantiomers. Enantiomers are non-superimposible mirror images of one another. They are called Chiral or the compound with one asymmetric carbon atom having four different substituent.
- A simple example of non superimposible mirror image is our hands. Our left hand is a mirror image of our right, yet there is no way our left thumb can be over our tight thumb it our palms are facing the same way and placed over one another.
- The number of optical isomers is 2n, where ‘n’ is the number of asymmetric centers.
- Optical isomers also have a unique property that they do not have axis of symmetry, which means that there is no line that bisects the compound such that the left half is a mirror image of the right half. Substances that rotate the plane of polarized light to clock-wise direction as view towards the light source are dextrorotatory (d) or (+) optical isomers. Those that rotate light in the opposite or anti-clock-wise direction are levo rotator (1) or (-) optical isomers. Substances which do not rotate the plane polarized light are the racemic mixture (mixture of equal quantity of d and 1 optical isomers) or called raesemate.
Optical isomers have same properties like melting points, boiling points, solubility, etc but there is profound difference in biological activity. There are drugs, called enantiopure drugs that have different effects based on whether the drug is a raecemic mixture or purely one enantiomer. For example, D-ethambutol treats tuberculosis, while L-ethambutol causes blindness. Physical properties of raecemate are different from the enantiomers.
MEASUREMENT OF OPTICAL ROTATION BY POLARIMETRY
The rotation of polarized light by optically active substances as a measure of their concentration in a solution is the function of polarimetry. The instrument used is called a polarimeter. It is the rotation of polarized light by a substance in a solution that is measured.
First unpolarized light from the light source is polarized. This polarized light passes through a cell containing sample solution. if the sample is optically active, the plane of the polarized light waves is rotated. The rotation is analyzed and reported as observed rotation. The schematic diagram of polarimeter showing the conversion of unpolarized light to polarized light and its flow through sample and the rotation of plane.
- There are many factors affecting the angle of rotation like; type of sample, concentration of optically active compounds, length of the sample tube, wavelength of light source, and temperature of sample.
Where, la is the specific rotation at wavelength 1, ‘I’ is the temperature and ‘a’ is the observed rotation in (),’1 is the path length in decimeters and ‘c’ is the concentration of the analyte in g per 100 ml. Thus, [a] is 100 times the measured value in (°), for a solution containing 1 g per 100 ml in a cell having path length of 1.0 decimeter under defined conditions of incident wavelength of light and temperature.
A graph of 100a on y-axis versus cl on the x-axis will give a slope of la, which is equal to the specific rotation of the sample. It is defined as the observed angle of optical rotation a when plane-polarized light is passed through a sample with a path length of 1 decimeter and a sample concentration of 1 g per ml. It is the main property used to quantify the chirality of a molecular species or mineral. The specific rotation of a pure material is an intrinsic property of that material at given wavelength and temperature. Values of specific rotation should be accompanied with temperature at which measured and the solvent in which the material dissolved. If the temperature is not specified; in these case it is assumed to be room temperature. The unit of specific rotation is deg dm-1 cm3 g-1 but usually only degree is used.
Specific rotations of some of the common sugars are as follows showing whether the sugars are dextrorotatory (+) or levo-rotatory (-).