Which of the following amino acids contains nitrogen in its side chain?

Histidine is the correct answer because it contains an imidazole side chain that includes nitrogen atoms. In biological systems, this nitrogen can participate in acid-base reactions, making histidine particularly important in enzyme active sites and protein stability.

Asparagine and glutamine also contain nitrogen in their side chains; however, histidine is notable for its unique ability to participate in biochemical reactions due to its pKa being close to physiological pH, allowing it to exist in both protonated and deprotonated forms. Serine does not contain nitrogen in its side chain, having only a hydroxyl group, which distinguishes it from the other amino acids listed.