These are models of aspartic acid in its fully protonated form. Aspartic acid has a second carboxyl group on its side chain which can also lose its proton. Aspartic acid is an acidic amino acid which exists with a negative charge at neutral pH. Proteins that bind metal ions often have acidic amino acids in their binding sites.
Aspartic acid can also act as a nucleophile in some enzyme reactions.
Aspartic acid is a strong beta sheet breaker and is indifferent to alpha helix.