A side chain is a group attached to a main molecular framework. In a protein, the amino-acid side chain is the variable group attached to the backbone alpha carbon; in a polymer, it is a substituent branching from the backbone.
The choice of backbone is part of the description. Amino-acid side chains influence charge, hydrophobicity, and molecular recognition, while polymer side chains can alter flexibility and solubility without changing the main-chain connectivity.
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