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)

Verified step by step guidance

1. Recall the general structure of an amino acid, which includes a central alpha carbon (C\_\(\alpha\)) bonded to four groups: an amino group (NH\_2), a carboxyl group (COOH), a hydrogen atom, and a variable side chain (R group).
2. Understand that the backbone atoms (N, C\_\(\alpha\), and C) are consistent across all amino acids, forming the common framework of proteins.
3. Recognize that the amino group and carboxyl group are present in every amino acid and are responsible for forming peptide bonds, but they do not differentiate one amino acid from another.
4. Focus on the side chain (R group) attached to the alpha carbon, which varies in size, shape, charge, and chemical properties among different amino acids.
5. Conclude that the unique identity of each amino acid is primarily determined by its side chain (R group), making it the key structural feature that distinguishes one amino acid from another.