JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B. Pharmacy III MEDICINAL CHEMISTRY-I Previous Question Paper (August/September-2021 - R17)

Chelation is a process in which a polydentate ligand (chelating agent) forms two or more coordinate bonds with a single central metal ion, resulting in the formation of a stable, ring-like heterocyclic structure known as a chelate. This process significantly increases the stability of the metal complex compared to complexes formed with monodentate ligands due to the chelate effect.

A classic example is the formation of a complex between EDTA (Ethylenediaminetetraacetic acid) and a calcium ion (Ca²⁺). EDTA acts as a hexadentate ligand, wrapping around the metal ion to form multiple five-membered rings. This effectively sequesters the metal ion, preventing it from participating in normal chemical reactions, which is a principle utilized in treating heavy metal poisoning.

Optical isomerism is a form of stereoisomerism where molecules have the same molecular formula and connectivity but differ in their spatial arrangement, resulting in the ability to rotate the plane of plane-polarized light. This phenomenon occurs in molecules that possess a chiral center, typically a carbon atom bonded to four different groups, rendering the molecule non-superimposable on its mirror image.

Key aspects include:

• Enantiomers: Pairs of optical isomers that are non-superimposable mirror images of each other. They possess identical physical properties except for the direction in which they rotate plane-polarized light.
• Dextrorotatory (+): Isomers that rotate plane-polarized light to the right (clockwise).
• Levorotatory (-): Isomers that rotate plane-polarized light to the left (counter-clockwise).
• Racemic Mixture: An equimolar mixture of two enantiomers, which is optically inactive because the rotations cancel each other out.

In medicinal chemistry, optical isomerism is critical as enantiomers often exhibit different pharmacological activities, potencies, or toxicological profiles due to the stereospecific nature of drug-receptor interactions.