Free amino acids dissolved in cytoplasm at pH 7 usually carry a full negative charge at one end of the molecule. Which functional group produces that charge?

Core Concept

The carboxyl group is a weak acid: around neutral pH it has largely given up its hydrogen ion, leaving electrons behind on oxygen as a negatively charged carboxylate. That stable negative charge attracts water and lets the group participate in peptide bond formation and ionic interactions. The amino group behaves oppositely, accepting protons and turning positive, while hydroxyl and sulfhydryl groups stay essentially uncharged at pH 7.