In proteins, what structural feature primarily makes one amino acid different from another?

• A. The identity of the backbone atoms (N, $C$${\alpha }_{}$, and C) which vary between amino acids
• B. The side chain (R group) attached to the $C$${\alpha }_{}$-carbon
• C. The presence of an amino group and a carboxyl group on every amino acid
• D. The peptide bond that links amino acids together

Answer: (B)