Which of the following is a basic amino acid at physiological pH (7.4)?
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Correct answer: Lysine (positively charged side chain)
Lysine, along with arginine and histidine, is classified as a basic amino acid because its side chain carries a positive charge at physiological pH (7.4). The epsilon-amino group of lysine (pKa ~10.5) is fully protonated at pH 7.4. These basic amino acids frequently interact with negatively charged molecules such as DNA and phospholipids.
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