Which part of a standard amino acid most directly distinguishes one amino acid from another?

The **R group**, or side chain, varies among amino acids in structure, size, and charge. The common amino acids share the α-amino acid framework, while their side chains give them distinct identities and influence solubility. **Key takeaway:** Side-chain variation differentiates amino acids. **Glossary:** α-carbon: central carbon bonded to amino and carboxyl groups.