Which of the following best describes the side chains (R groups) among amino acids?

• A. They are always nonpolar.
• B. They are identical in all amino acids.
• C. They are always charged at physiological pH.
• D. They are variable and determine the unique properties of each amino acid.

Answer: (D)

Verified step by step guidance

1. Understand the structure of amino acids: Each amino acid consists of a central carbon atom (α-carbon) bonded to four groups: an amino group (-NH₂), a carboxyl group (-COOH), a hydrogen atom, and a side chain (R group). The R group is what differentiates one amino acid from another.
2. Recognize the variability of R groups: The side chains (R groups) can vary widely in their chemical properties. They can be nonpolar, polar, charged (positively or negatively), or aromatic, depending on the specific amino acid.
3. Analyze the incorrect options: The statement 'They are always nonpolar' is incorrect because some R groups are polar or charged. The statement 'They are identical in all amino acids' is incorrect because the R group is what makes each amino acid unique. The statement 'They are always charged at physiological pH' is incorrect because not all R groups are charged at physiological pH; some are neutral.
4. Understand the correct answer: The correct statement is 'They are variable and determine the unique properties of each amino acid.' This variability in R groups is responsible for the diverse chemical and functional properties of amino acids in proteins.
5. Relate this concept to protein function: The unique properties of amino acids, determined by their R groups, influence protein folding, structure, and function. For example, hydrophobic R groups tend to cluster inside the protein structure, while hydrophilic R groups are often exposed to the aqueous environment.