Amino acids are organic compounds that combine to form proteins and are classified based on the nature of their side chains. They can be categorized as acidic, basic, or neutral. Basic amino acids have side chains that are positively charged at physiological pH (around 7.4).
Let’s analyze each amino acid option:
- Lysine: A basic amino acid with a side chain that has an extra amino group, making it positively charged at physiological pH.
- Arginine: Another basic amino acid characterized by its guanidino group, which can accept additional protons, thus carrying a positive charge at physiological pH.
- Histidine: A basic amino acid that contains an imidazole group in its side chain, which can acquire a positive charge by accepting a proton.
- Leucine: A branched-chain amino acid with a neutral, hydrophobic side chain. It does not have a side chain that is ionizable under physiological conditions, making it non-basic.
Among the given options, Leucine does not qualify as a basic amino acid because it lacks any group in its side chain capable of carrying a positive charge. Instead, Leucine is categorized as a neutral and hydrophobic amino acid.
Thus, the correct answer is:
Leucine