648
R. H. THOMSON
restored by addition of vitamin K 1? the light petroleum extract does not
contain vitamin K, but rather an appreciable amount of plastoquinone.
[Other investigators (56) confirm that vitamin K is absent from chloroplasts. Dam's original work was evidently done on somewhat crudely
separated fractions.] It is generally accepted that a quinone participates
in photosynthetic electron transport (most probably in photosynthetic
phosphorylation), but the precise sequence of events is still obscure.
Various schemes have been proposed (see Vol. V of this treatise and
ref. 57) which should now be modified by substituting plastoquinone
for vitamin K. [The function of vitamin K is discussed further on p.
696.]
(4). Benzoquinones. All aromatic compounds of the anthracene
series found in plants are quinones, and most of the naphthalene compounds which have been isolated are also quinones. In contrast, few
benzoquinones have been discovered although benzenoid compounds,
including phenols, are common plant products. Of the eight quinones
shown on pp. 650-651, perezone (LXI), found in roots of two Compositae (58), is a sesquiterpene, and the pedicinins (LXVI) are two
of a group of closely related polyhydroxychalcones and flavanones
present as a yellow dust on the leaves of Didymocarpus
pedicellata
R.Br. (59). A major component of this mixture is a chalcone-quinol,
pedicin, from which the pedicinins are derived, but the pigments as a
group are probably best regarded as chalcones, two of which happen
also to be quinonoid.* Thymoquinone has been isolated, along with
thymoquinol (LVIII) from the essential oil of Monarda fistulosa Linn.
(Labiatae) (60) but is probably an artifact. The three orange pigments
(LXIII), (LXIV), and (LXV), occur chiefly in the berries of mysinaceous shrubs, but embelin is present also in the root of Embelia barbey ana Mez. (61) and rapanone has been found both in the bark of
Rapanea maximowiczii Koidz (Myrsinaceae) (62) and in the bulbs of
Oxalis purpurata var. jacquinii (Sonder) (Geraniaceae) (63). Structurally related phenols with long alkyl side chains also occur in higher
plants, not in the Myrsinaceae but in several genera of the Anacardiaceae. In particular, the oily exudate from Campnosperma auriculata
Hook.f. contains a mixture of alkylated quinols of which nonadecylquinol (LXII) has been identified (64). (Maesaquinone has also a C 19
side chain.) Mixtures of catechols with a Ci 5 side chain at position 3
occur in the oil exuded by Rhus toxicodendron Linn. (65), Semecarpus
anacardium Linn. (66), and other species, whereas those obtained inter
* There is a similar group of coloring matters in the flowers of Carthamus
tinctorius Linn. (Compositae) which includes the chalcone-quinone carthamone
(59a).
R. H. THOMSON
restored by addition of vitamin K 1? the light petroleum extract does not
contain vitamin K, but rather an appreciable amount of plastoquinone.
[Other investigators (56) confirm that vitamin K is absent from chloroplasts. Dam's original work was evidently done on somewhat crudely
separated fractions.] It is generally accepted that a quinone participates
in photosynthetic electron transport (most probably in photosynthetic
phosphorylation), but the precise sequence of events is still obscure.
Various schemes have been proposed (see Vol. V of this treatise and
ref. 57) which should now be modified by substituting plastoquinone
for vitamin K. [The function of vitamin K is discussed further on p.
696.]
(4). Benzoquinones. All aromatic compounds of the anthracene
series found in plants are quinones, and most of the naphthalene compounds which have been isolated are also quinones. In contrast, few
benzoquinones have been discovered although benzenoid compounds,
including phenols, are common plant products. Of the eight quinones
shown on pp. 650-651, perezone (LXI), found in roots of two Compositae (58), is a sesquiterpene, and the pedicinins (LXVI) are two
of a group of closely related polyhydroxychalcones and flavanones
present as a yellow dust on the leaves of Didymocarpus
pedicellata
R.Br. (59). A major component of this mixture is a chalcone-quinol,
pedicin, from which the pedicinins are derived, but the pigments as a
group are probably best regarded as chalcones, two of which happen
also to be quinonoid.* Thymoquinone has been isolated, along with
thymoquinol (LVIII) from the essential oil of Monarda fistulosa Linn.
(Labiatae) (60) but is probably an artifact. The three orange pigments
(LXIII), (LXIV), and (LXV), occur chiefly in the berries of mysinaceous shrubs, but embelin is present also in the root of Embelia barbey ana Mez. (61) and rapanone has been found both in the bark of
Rapanea maximowiczii Koidz (Myrsinaceae) (62) and in the bulbs of
Oxalis purpurata var. jacquinii (Sonder) (Geraniaceae) (63). Structurally related phenols with long alkyl side chains also occur in higher
plants, not in the Myrsinaceae but in several genera of the Anacardiaceae. In particular, the oily exudate from Campnosperma auriculata
Hook.f. contains a mixture of alkylated quinols of which nonadecylquinol (LXII) has been identified (64). (Maesaquinone has also a C 19
side chain.) Mixtures of catechols with a Ci 5 side chain at position 3
occur in the oil exuded by Rhus toxicodendron Linn. (65), Semecarpus
anacardium Linn. (66), and other species, whereas those obtained inter
* There is a similar group of coloring matters in the flowers of Carthamus
tinctorius Linn. (Compositae) which includes the chalcone-quinone carthamone
(59a).