Which amino acid has a side chain that can form an ionic bond at physiological pH?

Amino acids can be categorized based on the properties of their side chains (R-groups). These properties include whether they are nonpolar, polar uncharged, polar positively charged (basic), or polar negatively charged (acidic) at physiological pH (~7.4). Leucine is a nonpolar, hydrophobic amino acid. Its side chain does not ionize and cannot form ionic bonds at physiological pH. Aspartate has a side chain that contains a carboxyl group (-COOH). At physiological pH, this group loses a hydrogen ion (H⁺), becoming negatively charged (-COO⁻). This negative charge allows aspartate to form ionic bonds with positively charged entities, including the side chains of basic (positively charged) amino acids like lysine, arginine, and histidine. Proline has a unique cyclic structure that locks its nitrogen in a ring, making it rigid. It is nonpolar and does not form ionic bonds at physiological pH. Glycine, the simplest amino acid, has a hydrogen atom as its side chain. It is nonpolar and does not have a chargeable side group capable of forming ionic bonds at physiological pH. Thus, aspartate, with its ability to carry a negative charge at physiological pH due to its carboxylate side chain, is capable of forming ionic bonds, making it the correct choice among the options given. Problem: Which amino acid has a side chain that can form an ionic bond at physiological pH? A) Leucine B) Aspartate C) Proline D) Glycine