Which of the following statements about amino acid side chains is correct?

• A. Basic amino acids have uncharged side chains at physiological pH.
• B. All nonpolar amino acids contain sulfur in their side chains.
• C. Aromatic amino acids are always hydrophilic.
• D. Acidic amino acids have side chains that are negatively charged at physiological pH.

Answer: (D)

Verified step by step guidance

1. Step 1: Understand the context of the question. The problem is asking about the properties of amino acid side chains, specifically at physiological pH (approximately 7.4). Amino acids are classified based on the chemical nature of their side chains, such as acidic, basic, nonpolar, and aromatic.
2. Step 2: Analyze the first statement: 'Basic amino acids have uncharged side chains at physiological pH.' Basic amino acids, such as lysine, arginine, and histidine, typically have positively charged side chains at physiological pH due to their ability to accept protons. This statement is incorrect.
3. Step 3: Analyze the second statement: 'All nonpolar amino acids contain sulfur in their side chains.' Nonpolar amino acids include glycine, alanine, valine, leucine, isoleucine, phenylalanine, and methionine. While methionine contains sulfur, not all nonpolar amino acids do. This statement is incorrect.
4. Step 4: Analyze the third statement: 'Aromatic amino acids are always hydrophilic.' Aromatic amino acids include phenylalanine, tyrosine, and tryptophan. While tyrosine and tryptophan have hydrophilic properties due to hydroxyl and nitrogen groups, phenylalanine is hydrophobic. This statement is incorrect.
5. Step 5: Analyze the fourth statement: 'Acidic amino acids have side chains that are negatively charged at physiological pH.' Acidic amino acids, such as aspartic acid and glutamic acid, have carboxyl groups in their side chains that lose protons at physiological pH, resulting in a negative charge. This statement is correct.