D-aspartic acid is not used for protein synthesis but is incorporated into some peptides and plays a role as a neurotransmitter/neuromodulator.

Like all other amino acids, aspartic acid contains an amino group and a carboxylic acid. Its ?-amino group is in the protonated -NH form under physiological conditions, while its ?-carboxylic acid group is deprotonated -COO^- under physiological conditions.