Aminopolycarboxylic acid
file:Metal-EDTA.svg|thumb|left|120px|a metal complex with the EDTA anion
file:Asparaginsäure - Aspartic acid.svg|thumb|120px|Aspartic acid is an aminodicarboxylic acid and precursor to other ligands.
An aminopolycarboxylic acid is a chemical compound containing one or more nitrogen atoms connected through carbon atoms to two or more carboxyl groups. Upon deprotonation of their carboxylic acids, aminopolycarboxylates form strong complexes with metal ions. This property makes aminopolycarboxylic acids useful complexone in a wide variety of chemical, medical, and environmental applications. Some such ligands produced on a large commercial scale include EDTA and NTA.
Structure
The parent of this family of ligands is the amino acid glycine, H2NCH2COOH, in which the amino group, NH2, is separated from the carboxyl group, COOH, by a single methylene group, CH2. When the carboxyl group is deprotonated, the glycinate ion can function as a bidentate ligand, binding the metal centre through the nitrogen and one of two carboxylate oxygen atoms, to form chelate complexes of metal ions.Replacement of a hydrogen atom on the nitrogen of glycine by another acetate residue, −CH2COOH, gives iminodiacetic acid, IDA, which is a tridentate ligand. Further substitution gives nitrilotriacetic acid, NTA, which is a tetradentate ligand. These compounds can be described as aminopolycarboxylates. Related ligands can be derived from other amino acids other than glycine, notably aspartic acid.
[file:mida.svg|thumb|Binding of a metal complex by the iminodiacetate anion]
Higher density is achieved by linking two or more glycinate or IDA units together. EDTA contains two IDA units with the nitrogen atoms linked by two methylene groups and is hexadentate. DTPA has two CH2CH2 bridges linking three nitrogen atoms and is octadentate. TTHA has ten potential donor atoms.