Afunctional group on an amino acid that is polar and can become negatively charged:

Carboxylic acid.

Explanation:

Amino acids are made up of the amine group, carboxyl group, hydrogen and R hydrophobic chain. Amino acids are the monomers of the protein and they are coded by the particular codons.

Amino acids contains both the amino group and the carboxyl group. The carboxylic group is a polar group and becomes negative charged when the pH of the solution is greater than the pKa value of the amino acids.

Thus, the answer is carboxylic acid.

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