Dictionary of Chemistry [6th Ed.]

ionized to give the ion H 2 PO 2 –. The
acid and its salts are readily oxidized
to the orthophosphate and conse-
quently are good reducing agents.

phosphite See phosphonic acid.

phosphodiester bond The cova-
lent bond that links a phosphate
group and a sugar group, by means
of an oxygen bridge, in the sugar–
phosphate backbone of a nucleic acid
molecule. See dna.

phosphoglyceric acid (PGA;
3-phosphoglycerate)See glycerate
3-phosphate.

phospholipids(phosphatides)A
group of lipids having both a phos-
phate group and one or more fatty
acids. Glycerophospholipids (or phos-
phoglycerides) are based on *glyc-
erol; the three hydroxyl groups are
esteriÜed with two fatty acids and a
phosphate group, which may itself
be bound to one of a variety of sim-
ple organic groups (see illustration).
Sphingolipidsare based on the alco-
hol sphingosine and contain only one
fatty acid linked to an amino group.
With their hydrophilic polar phos-
phate groups and long hydrophobic
hydrocarbon ‘tails’, phospholipids
readily form membrane-like struc-
tures in water. They are a major com-
ponent of cell membranes.

phosphonateSee phosphonic

acid.

phosphonic acid (phosphorous

acid; orthophosphorous acid) A

colourless to pale-yellow deliques-

cent crystalline solid, H 3 PO 3 ; r.d.

1.65; m.p. 73.6°C; decomposes at

200 °C; very soluble in water and sol-

uble in alcohol. Phosphonic acid may

be crystallized from the solution ob-

tained by adding ice-cold water to

phosphorus(III) oxide or phosphorus

trichloride. The structure of this ma-

terial is unusual in that it contains

one direct P–H bond and is more cor-

rectly written (HO) 2 HPO. The acid is

dibasic, giving rise to the ions H 2 PO 3 –

and HPO 3 2–(phosphonates; formerly

phosphites), and has moderate reduc-

ing properties. On heating it gives

phosphine and phosphoric(V) acid.

phosphonium ionThe ion PH 4 +,

or the corresponding organic deriva-

tives of the type R 3 PH+, RPH 3 +. The

phosphonium ion PH 4 +is formally

analogous to the ammonium ion

NH 4 +but PH 3 has a much lower pro-

ton afÜnity than NH 3 and reaction of

PH 3 with acids is necessary for the

production of phosphonium salts.

phosphonium saltsSee phospho-

nium ion.

phosphorA substance that is ca-

411 phosphor

p

O

H 3 C

H 3 CC

H

C

H

O

fatty acid

fatty acid

glycerol phosphate group choline

H 2 C

O

O– CH 3

CH 3

CH 3

(CH(CH(CH 222 ))) 141414 CCC OOO CHCHCH 222

(CH(CH(CH 222 ))) 777 (CH(CH(CH 222 ))) 777 CCC OOO CCC HHH

OOO PPP OOO CHCHCH 222 CHCHCH 222 NNN

Structure of phosphatidylcholine (lecithin), a glycerophospholipid