Phosphoric ester

A phosphoric ester is an ester of phosphoric acid: an organic group is joined through oxygen to phosphorus. Replacing one, two or three of the acid's O–H hydrogens by organic groups gives a phosphate monoester, phosphate diester or phosphate triester. A phosphoric anhydride instead contains an acid-derived linkage, such as P–O–P, rather than an additional P–O–C ester linkage.^[ref]

For a structural counting example, compare

C H_{3} O - P (= O) (OH)_{2} and (C H_{3} O)_{2} P (= O) (OH) .

(1)

The first has one organic O–P attachment; the second has two. Removing an O–H proton changes the charge but leaves that attachment count unchanged. Conversely, adding a second separate monoester group elsewhere on a large molecule does not turn the first phosphorus center into a diester.

This distinction matters in biochemical measurements. 4-Nitrophenyl phosphate is a phosphate monoester whose cleavage can be followed optically; the released inorganic phosphate is a different chemical species. An alkaline phosphatase assay detects a specified conversion under specified conditions, rather than counting all phosphorus-containing material indiscriminately.^[ref]

Table of contents
- Phosphate triester Phosphoric ester
- Phosphate diester Phosphoric ester

Phosphate triester

 0  0

A phosphate triester is a phosphoric ester in which three organic groups are attached through oxygen to one phosphorus center. A general connectivity formula is

O = P (OR) (O R^{'}) (O R^{''}) .

(1)

Unlike a neutral-acid phosphate monoester or phosphate diester, this representation contains no remaining P–OH group. The organic groups may nevertheless contain their own ionizable sites; “triester” does not guarantee that every molecule in the class is electrically neutral.

Trimethyl phosphate supplies a concrete example:

O = P (OC H_{3})_{3}

has formula

C_{3} H_{9} O_{4} P

. Its three methyl groups each contribute one carbon and three hydrogens, while the central unit and linking oxygens contribute one phosphorus and four oxygens.^[ref]

An original connectivity comparison replaces one

- OC H_{3}

group by

- OH

. The resulting fragment,

O = P (OC H_{3})_{2} (OH)

, is a diester because only two organic groups remain attached through oxygen. Merely removing a proton from that O–H group changes charge without restoring a third ester linkage.

The suffix counts esterification at one phosphorus center. Three separate phosphate monoesters elsewhere on a large molecule do not constitute one phosphate triester. This local counting rule helps distinguish a change in connectivity from a change in pH, protonation or total phosphorus content.

Phosphate diester

 0  0

A phosphate diester is a phosphoric ester with two organic groups linked through oxygen to the same phosphorus. Its neutral-acid representation is

RO - P (= O) (OH) - O R^{'}

; deprotonating the remaining O–H group gives

RO - P (= O) (O^{-}) - O R^{'}

without breaking either ester linkage.^[ref]

For an original connectivity example, let the two organic groups be methyl groups. The fragment

C H_{3} O - P (= O) (OH) - OC H_{3}

contains one phosphorus center but two organic O–P connections. Two separate molecules of

C H_{3} O - P (= O) (OH)_{2}

instead contain two phosphorus centers, each monoesterified. Counting phosphorus atoms alone therefore cannot determine the ester class.

A diester can bridge two different organic units or join two positions within one unit. In a cyclic nucleotide, the two attachments close a ring; they need not belong to two separate molecules.^[ref] In DNA and RNA, successive sugar units are joined through phosphodiester bonds. Cleaving one ester linkage changes chain connectivity, which is why the same local chemistry can govern the integrity of a much longer nucleic acid.

The word “diester” specifies connectivity at the phosphoric group. It does not mean a P–O–P chain of two phosphate units, and it does not by itself specify a reaction rate or the conditions required for hydrolysis.