diphosphates

Diphosphates are compounds containing two phosphate units. The central inorganic representative is the pyrophosphate ion, P2O7^4−, in which two phosphate groups are linked by a high-energy P–O–P anhydride bond. The term also applies to organic diphosphate esters in which two phosphate groups are bound to a molecule, such as adenosine diphosphate (ADP). In many contexts, “diphosphate” is used interchangeably with “pyrophosphate.”

Inorganic pyrophosphate forms by condensation of two phosphate units with loss of a water molecule. In aqueous solution, it exists in multiple protonation states and, upon hydrolysis, yields two inorganic phosphate ions (Pi). This hydrolysis is highly exergonic and is catalyzed by pyrophosphatases in biological systems. The reversible formation and hydrolysis of the diphosphate bond play a key role in driving various biosynthetic polymerizations, including nucleotide and nucleic acid assembly, by providing a favorable drive to the forward reaction when PPi is removed as Pi.

Biological occurrence: Pyrophosphate arises as a byproduct of reactions that build nucleotides, nucleic acids, and certain other macromolecules. The rapid enzymatic hydrolysis of PPi helps prevent reversal of these biosynthetic steps and can help regulate cellular energy flow.

Industrial and commercial use: Diphosphates and their salts, such as sodium or disodium pyrophosphate, are used as food additives (leavening agents, emulsifiers, buffering/sequestering agents), in detergents, and in water treatment. The term appears in chemical nomenclature and in discussions of energy-carrying phosphate groups in biology.