Which amino acid is neutral at physiological pH?

The correct choice is histidine, which has a side chain that can act as either a proton donor or acceptor, depending on the pH of the environment. At physiological pH (around 7.4), histidine is approximately 50% protonated and 50% unprotonated. This unique property allows histidine to exist in a neutral state, making it particularly important in enzymatic reactions and as a buffer in biological systems.

Arginine and lysine are both positively charged (basic) at physiological pH due to their side chains containing amino groups that are protonated, while tyrosine, although it has a polar side chain and can participate in hydrogen bonding, is still considered neutral overall but can carry a slight negative charge under specific conditions. Therefore, histidine is the only amino acid in this list that is neutral at physiological pH, allowing it to play a versatile role in various biochemical processes.