PLASTOQUINONES
A substituted quinone was
isolated in a lipid extract of lucerne by Kofler in 1946. The discovery of ubiquinone and
of its role in mitochondrial electron transport stimulated studies of related substances.
Crane FL (1959) suggested that this quinone was called plastoquinone becauce it appeared
concentrated in the chloroplasts of higher plants. Rapidly, it appeared that plastoquinone
plays a role analogous to that of ubiquinone in mitochondria but associated with the
transformation of light into chemical energy. They are also present in algae and
cyanobacteria.
The elucidation of the structure of plastoquinone was that of a
2,3-dimethyl-1,4-benzoquinone with a C45 side chain related to the isoprenoid alcohol, solanesol.
There
are several plastoquinones with side chains of different length in position 5. They are
designated as plastoquinone-n where n is the number of carbon atoms in the side chain or
better as PQ-n where n indicates the number of isoprenoid units. n varies from 6 to 9.
Plastoquinone is a yellow cristalline solid (PQ-9 has a MW of 748) soluble in most organic
solvents. It has a characteristic UV spectrum with a maximum at 255 nm (molar abs. coeff.:
15700). On reduction (with sodium borohydride) a single maximum of lower intensity at 290
nm (molar abs. coeff.: 3440) is observed.
Various heterocyclic quinones have been described in
sulphur-oxidizing archaebacteria, the benzothiophenoquinones.
Several species of these quinones
are present with menaquinones in the same bacteria. R1 may be -CH3 or -SCH3, and R2 is an isoprenoid
chain , saturated or monounsaturated, with 3 to 6 isoprenoid units.
