Answer: The difference in classification between lysine and tryptophan occurs because of the difference in the basicity of the additional N atom in their R groups. The additional N atom in lysine’s R group is a primary amine, which is more basic than the indole ring in tryptophan’s R group.
Explanation: Amino acids are classified as acidic, basic, or neutral based on the chemical properties of their side chains. Basic amino acids have side chains that are positively charged at physiological pH (around 7.4), while acidic amino acids have side chains that are negatively charged at physiological pH. Neutral amino acids have side chains that are neither positively nor negatively charged at physiological pH.
Lysine is classified as a basic amino acid because its side chain contains an additional basic N atom that can accept a proton (H+) to form a positively charged ion. This additional N atom is a primary amine (-NH2), which has two hydrogen atoms attached to the nitrogen atom.
Tryptophan is classified as a neutral amino acid because its side chain contains an indole ring that is neither positively nor negatively charged at physiological pH. The indole ring contains two nitrogen atoms, but neither of them has a lone pair of electrons available for protonation.
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