Pyrophosphate

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"Diphosphate" redirects here. It can also refer to any salt containing two phosphate groups.
Pyrophosphate
Pyrophosphate anion
  Phosphorus, P
  Oxygen, O
Names
IUPAC name
Diphosphate
Systematic IUPAC name
Dipolyphosphate
Other names
Pyrophosphate
Phosphonatophosphate
Identifiers
3D model (
JSmol
)
ChEBI
ChemSpider
DrugBank
E number E450 (thickeners, ...)
26938
UNII
  • InChI=1S/H4O7P2/c1-8(2,3)7-9(4,5)6/h(H2,1,2,3)(H2,4,5,6)/p-4 checkY
    Key: XPPKVPWEQAFLFU-UHFFFAOYSA-J checkY
  • InChI=1/H4O7P2/c1-8(2,3)7-9(4,5)6/h(H2,1,2,3)(H2,4,5,6)/p-4
    Key: XPPKVPWEQAFLFU-XBHQNQODAI
  • [O-]P(=O)([O-])OP(=O)([O-])[O-]
Properties
P2O4−7
Molar mass 173.941 g·mol−1
Conjugate acid
 Pyrophosphoric acid
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
checkY verify (what is checkY☒N ?)

In

inorganic phosphate, as for dimethylallyl pyrophosphate. This bond is also referred to as a high-energy phosphate
bond.

Acidity

Pyrophosphoric acid is a tetraprotic acid, with four distinct

pKa's:[1]

H4P2O7 ⇌ [H3P2O7]− + H+, pKa1 = 0.85
[H3P2O7]− ⇌ [H2P2O7]2− + H+, pKa2 = 1.96
[H2P2O7]2− ⇌ [HP2O7]3− + H+, pKa3 = 6.60
[HP2O7]3− ⇌ [P2O7]4− + H+, pKa4 = 9.41

The pKa's occur in two distinct ranges because deprotonations occur on separate phosphate groups. For comparison with the pKa's for phosphoric acid are 2.14, 7.20, and 12.37.

At physiological pH's, pyrophosphate exists as a mixture of doubly and singly protonated forms.

Preparation

Disodium pyrophosphate is prepared by thermal condensation of

sodium dihydrogen phosphate or by partial deprotonation of pyrophosphoric acid.[2]

Pyrophosphates are generally white or colorless. The alkali metal salts are water-soluble.[3] They are good complexing agents for metal ions (such as calcium and many transition metals) and have many uses in industrial chemistry. Pyrophosphate is the first member of an entire series of polyphosphates.[4]

In biochemistry

The anion P2O4−7 is abbreviated PPi, standing for inorganic pyrophosphate. It is formed by the hydrolysis of ATP into AMP in cells.

ATP → AMP + PPi

For example, when a nucleotide is incorporated into a growing DNA or RNA strand by a polymerase, pyrophosphate (PPi) is released. Pyrophosphorolysis is the reverse of the polymerization reaction in which pyrophosphate reacts with the 3′-nucleosidemonophosphate (NMP or dNMP), which is removed from the oligonucleotide to release the corresponding triphosphate (dNTP from DNA, or NTP from RNA).

The pyrophosphate anion has the structure P2O4−7, and is an acid anhydride of phosphate. It is unstable in aqueous solution and hydrolyzes into inorganic phosphate:

P2O4−7 + H2O → 2 HPO2−4

or in biologists' shorthand notation:

PPi + H2O → 2 Pi + 2 H+

In the absence of enzymic catalysis, hydrolysis reactions of simple polyphosphates such as pyrophosphate, linear triphosphate, ADP, and ATP normally proceed extremely slowly in all but highly acidic media.[5]

(The reverse of this reaction is a method of preparing pyrophosphates by heating phosphates.)

This hydrolysis to inorganic phosphate effectively renders the cleavage of ATP to AMP and PPi irreversible, and biochemical reactions coupled to this hydrolysis are irreversible as well.

PPi occurs in synovial fluid, blood plasma, and urine at levels sufficient to block calcification and may be a natural inhibitor of hydroxyapatite formation in extracellular fluid (ECF).[6] Cells may channel intracellular PPi into ECF.[7] ANK is a nonenzymatic plasma-membrane PPi channel that supports extracellular PPi levels.[7] Defective function of the membrane PPi channel ANK is associated with low extracellular PPi and elevated intracellular PPi.[6] Ectonucleotide pyrophosphatase/phosphodiesterase (ENPP) may function to raise extracellular PPi.[7]

From the standpoint of

high energy phosphate accounting, the hydrolysis of ATP to AMP and PPi requires two high-energy phosphates, as to reconstitute AMP into ATP requires two phosphorylation
reactions.

AMP + ATP → 2 ADP
2 ADP + 2 Pi → 2 ATP

The plasma concentration of inorganic pyrophosphate has a reference range of 0.58–3.78 

µM (95% prediction interval).[8]

Terpenes

Isopentenyl pyrophosphate converts to geranyl pyrophosphate, the precursor to tens of thousands of terpeness and terpenoids.[9]

Isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP) condense to produce geranyl pyrophosphate, precursor to all terpenes and terpenoids.

As a food additive

Various diphosphates are used as

raising agents, sequestrants, and water retention agents in food processing.[10] They are classified in the E number scheme under E450:[11]

In particular, various formulations of diphosphates are used to stabilize whipped cream.[12]

See also

References

  1. PMID 34603734
    .
  2. ISBN 9780470132340
    .
  3. ^ C.Michael Hogan. 2011. Phosphate. Encyclopedia of Earth. Topic ed. Andy Jorgensen. Ed.-in-Chief C.J.Cleveland. National Council for Science and the Environment. Washington DC
  4. ISBN 978-0-08-037941-8
    .
  5. doi:10.1021/ja01607a011
    .
  6. ^
    PMID 10894769
    .
  7. ^
    PMID 11159191
    .
  8. PMID 223577
    .
  9. ISBN 9783527609949
    .
  10. ^ Codex Alimentarius 1A, 2nd ed, 1995, pp. 71, 82, 91
  11. ^ D. J. Jukes, Food Legislation of the UK: A Concise Guide, Elsevier, 2013, p. 60–61
  12. ^ Ricardo A. Molins, Phosphates in Food, p. 115

Further reading

External links

Look up diphosphate or pyrophosphate in Wiktionary, the free dictionary.
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