Aminopolycarboxylic acid

An aminopolycarboxylic acid (sometimes abbreviated APCA) is a chemical compound containing one or more nitrogen atoms connected through carbon atoms to two or more carboxyl groups. Aminopolycarboxylates that have lost acidic protons form strong complexes with metal ions. This property makes aminopolycarboxylic acids useful complexone in a wide variety of chemical, medical, and environmental applications.

The parent of this family of ligands is the amino acid glycine, H₂NCH₂CO₂H, in which the amino group, NH₂,is separated from the carboxyl group, CO₂H 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, –CH₂CO₂H 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.

Higher denticity 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.

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