amphoteric

Of a chemical compound, being able to behave either as an acid or as a base depending on their environment. For example, the metals aluminium and zinc, and the oxides and hydroxides, act as bases in acidic solutions and as acids in alkaline solutions.

Amino acids and proteins are also amphoteric, as they contain both a basic (amino, NH2) and an acidic (carboxyl, COOH) group. In proteins, these groups are neutralized by the formation of the peptide bond between the amino acids (except for the ends of the chain). However, most proteins are still amphoteric, as they contain amino acid residues with basic and others with acidic side chains.