side-chain theory

The "side-chain theory" was a largely outdated concept in chemistry that attempted to explain immunological phenomena. It proposed that living organisms are complex molecular aggregations. These molecules were thought to react with each other through their "side chains," which had specific structural correspondences. This interaction was exemplified by cell receptors that could bind to foreign molecules, stimulating the cell to produce more receptors, some of which would function as antibodies.1

In the context of protein structure, side chains (also known as R groups) of amino acids play a crucial role in determining the protein's tertiary structure. These side chains can be charged, polar, or nonpolar, and their interactions—such as salt bridges, hydrogen bonds, and hydrophobic interactions—dictate how the protein folds into its three-dimensional shape.2