Acid-base and Complexation Properties
Polyphosphates are weak bases. A lone pair of electrons on an oxygen atom can be donated to a hydrogen ion (proton) or a metal ion in a typical Lewis acid-Lewis base interaction. This has profound significance in biology. For instance, adenosine triphosphate is about 25% protonated in aqueous solution at pH 7.
- ATP4- + H+ ATPH3-, pKa 6.6
Further protonation occurs at lower pH values.
ATP forms chelate complexes with metal ions. The stability constant for the equilibrium
- ATP4- + Mg2+ MgATP2-, log β 4
is particularly large. The formation of the magnesium complex is a critical element in the process of ATP hydrolysis, as it weakens the link between the terminal phosphate group and the rest of the molecule.
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