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 because 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 crystalline 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.
Several plastoquinone and plastohydroquinone
analogues have been described in brown algae (Reddy
P et al., Phytochemistry 2009, 70,
250). They differ primarily in the linear
terpene chain moiety which has hydroxy and carbonyl groups in various positions.
As an example, we give below the structures of the simplest (sargaquinone) and
one of the most complex (fallaquinone) described in Sargassum fallax.
Sargaquinone
Fallaquinone
Many of these molecules displayed significant antioxidant, potent antiviral or moderate antitumour activities.
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 mono6unsaturated, with 3 to 6 isoprenoid units.
Several alkylated hydroquinones have been described
in vegetals.
As an example, we give below the common structure of cytotoxic compounds
discovered in Tapirira guianensis, a tall tree from Mexico to Brazil,
used as folk medicine (m = 8 and n = 5) (David JM et al., J Nat Prod 1998,
61, 287) and in Lannea welwitschii (lanneaquinol
with m = 6 and n = 7) (Groweiss A et al., J Nat prod 1997, 60, 116).
These two products displayed cytotoxic activity against
human prostate cancer.
