Which of the following amino acids is negatively charged?

Glutamic acid is considered a negatively charged amino acid due to its side chain containing a carboxylic acid functional group (-COOH). At physiological pH, which is around 7.4, this carboxylic acid group loses a proton (H+), resulting in a carboxylate anion (-COO−). This makes glutamic acid negatively charged at physiological conditions.

In contrast, alanine, serine, and leucine possess non-polar or neutral polar side chains without carboxylic acid groups that would lead to a negative charge under similar conditions. Specifically, alanine has a simple aliphatic side chain, serine contains a hydroxymethyl group, and leucine has a branched aliphatic side chain. None of these side chains contribute to a negative charge, leading to their classification as neutral amino acids. Thus, among the options provided, glutamic acid is the only amino acid that exhibits a negative charge, particularly under physiological conditions.