Polyphosphate

A polyphosphate is a salt or ester of polymeric oxyanions formed from tetrahedral PO4 (phosphate) structural units linked together by sharing oxygen atoms. Polyphosphates can adopt linear or a cyclic ring structures. In biology, the polyphosphate esters ADP and ATP are involved in energy storage. A variety of polyphosphates find application in mineral sequestration in municipal waters, generally being present at 1 to 5 ppm. GTP, CTP, and UTP are also nucleotides important in the protein synthesis, lipid synthesis, and carbohydrate metabolism, respectively. Polyphosphates are also used as food additives, marked E452. Image:Triphosphorsäure.svg|Structure of [[triphosphoric acid]] Image:Polyphosphoric acid.svg|[[Polyphosphoric acid]] Image:Trimetaphosphat.svg|Cyclic tri[[metaphosphate]] Image:Adenosindiphosphat protoniert.svg|[[Adenosine diphosphate]] (ADP) The structure of tripolyphosphoric acid illustrates the principles which define the structures of polyphosphates. It consists of three tetrahedral PO4 units linked together by sharing oxygen centres. For the linear chains, the end phosphorus groups share one oxide and the others phosphorus centres share two oxide centres. The corresponding phosphates are related to the acids by loss of the acidic protons. In the case of the cyclic trimer each tetrahedron shares two vertices with adjacent tetrahedra. Sharing of three corners is possible. This motif represents crosslinking of the linear polymer. Crosslinked polyphosphates adopt the sheet-structure Phyllosilicates, but such structures occur only under extreme conditions. Polyphosphates arise by polymerization of phosphoric acid derivatives. The process begins with two phosphate units coming together in a condensation reaction. 2 H(PO4)2− (P2O7)4− + H2O The condensation is shown as an equilibrium because the reverse reaction, hydrolysis, is also possible. The process may continue in steps; at each step another (PO3)− unit is added to the chain, as indicated by the part in brackets in the illustration of polyphosphoric acid.