Question

Write‌ ‌a‌ ‌short‌ ‌note‌ ‌on‌ ‌the‌ ‌optical‌ ‌activity‌ ‌of‌ ‌amino‌ ‌acids.‌

Answer 1

Hint: Optical activity of a substance is its ability to rotate the plane of polarization of a beam of light that is passed through it. The compounds that have this ability are called optically active compounds. Optical activity is measured using a polarized source and polarimeter.

Complete answer:
Optical activity occurs only in chiral materials. Chiral materials are those compounds where there is no microscopic mirror symmetry. We can also say that the materials which are non-superimposable on their mirror images are said to be chiral and this property is called chirality.
In case of the amino acids, except glycine, all of them have their \[\alpha - \] carbon bonded to different functional groups like carboxyl, amino, \[R - \] and hydrogen atom. Hence, we can say that the \[\alpha - \] carbon is a chiral carbon and therefore the molecule is optically active. Because of the tetrahedral arrangement of the bonding orbitals around the \[\alpha - \] carbon atom, the four different functional groups can occupy two unique spatial arrangements.
We know that the molecules which have a unique arrangement in space are called stereoisomers. Hence, this new arrangement of amino acids will be the stereoisomer of normally arranged amino acids. This stereoisomer will also be optically active and these two optically active compounds will be non – super imposable mirror images of each other. And hence will be called an enantiomer. Both of these will rotate the plane-polarized light in opposite directions, that is, in right or left and are accordingly affixed as \[D( + )\] or \[L( - )\].
The amino acid residues in protein molecules are exclusively \[L\] stereoisomers. While \[D - \] Amino acid residues have been found only in a few small peptides, including some peptides of bacterial cell walls and certain peptide antibiotics. The absolute configurations of simple sugars and amino acids are specified by the \[D,L\] system related to the configuration of \[D\] and \[L\] glyceraldehyde.

Note:
One should note that the rotation of the plane of polarization of light may also occur due to Faraday effect which involves a static magnetic field. However, this is not classified under optical activity and is a distinct phenomenon.
Also remember that optical activity is a reciprocal phenomenon, i.e., it is the same for opposite directions of wave propagation through an optically active medium.