 Phosphate monoester

A phosphate monoester is a phosphate ester with one organic group joined to phosphorus through an oxygen atom. ^[ref] A neutral-acid representation is $\mathrm{RO}-\mathrm{P}(=\mathrm{O})(\mathrm{OH}{)}_2$. Ionization changes the O–H groups and charge without changing the count of ester linkages. “Mono” counts the one esterified group at that phosphate unit, not the number of carbon atoms in $R$.

For example, an ethyl group in $\mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2\mathrm{O}-\mathrm{P}(=\mathrm{O})(\mathrm{OH}{)}_2$ contains two carbons, but the compound is still a phosphate monoester. A phosphate diester instead has two organic groups linked through oxygen to the same phosphorus, represented as $\mathrm{RO}-\mathrm{P}(=\mathrm{O})(\mathrm{OH})-\mathrm{O}{\mathrm{R}}^{\prime }$. This structural counting distinguishes the classes without requiring a particular pH.

Hydrolysis of a phosphate monoester releases the corresponding hydroxyl-containing compound and inorganic phosphate. Alkaline phosphatase is an enzyme activity classified by that reaction, with broad substrate specificity.^[ref]

An experimentally useful example is 4-Nitrophenyl phosphate. Its hydrolysis releases 4-Nitrophenol, whose colored anionic form permits an optical assay. That readout follows cleavage of the ester linkage; it does not count every possible phosphorus-containing compound in the sample.^[ref]