colour, smell and taste – the beautiful world of plants and chemistry alison foster, university of...
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Colour, smell and taste – the beautiful world of plants and
chemistry
Alison Foster, University of Oxford Botanic Garden
Dubrovnik, Croatia, 2012
Colour
• Main pigment types
– Porphyrins (nitrogen containing)
– Flavonoids (incl. anthocyanins)
– Carotenoids (terpenes)
– Betalains (nitrogen containing)
Colour
O+HO
OH
OH
OH
O Glucose
β-carotene
cyanidin-3-glucoside
N+
HO
HO
H
COO-
NH
CO2HHO2CH
betanidin
ColourO+HO
OH
OH
OH
O Glucose
Anthocyanins:
• Found in many plant families – except those in Caryophyllales
• Found in all plant tissues• 109 tonnes produced per year in
nature• 2% of all carbon fixed via
photosynthesis is converted to anthocyanins
Ferns (and Fern allies)
Gymnosperms
Angiosperms
ColourO+HO
OH
OH
OH
O Glucose
Functions of anthocyanins:
• Pollinator attractant
• Sunscreen – succulents and autumn leaf colouration
• Anti-feedants – bitter tasting
Colour
Anthocyanins as pollinator attractant?
Bromeliaceae
Colour
Anthocyanins as sunscreen or antifeedant or both?
Colour
Anthocyanins as antifeedant ?
Colour
Metasequoia glyptostroboides
dawn redwood
Rhodoxanthin:
Colour
Daucus carrota
carrot
beta-carotene
Colour
Solanum lycopersicon
tomato
lycopene:
Colour
Betalains:
• Amino-acid derived• Nitrogen containing – metabolically more expensive
• Yellow to red pigments (betaxanthins and betacyanins)
• Only present in plants in order Caryophyllales
N+
HO
HO
H
COO-
NH
CO2HHO2C
H
Caryophyllales
Colour
Betalains:
• Caryophyllales:• Beetroot (Amaranthaceae)
• Bougainvillea (Nyctaginaceae)
• Pokeweed (Phytolaccaceae)
• Carnations (Caryophyllaceae)
N+
HO
HO
H
COO-
NH
CO2HHO2C
H
Smell
• Nicotiana attenuata from the SW United States– Releases benzyl acetone
when flower opens at night– Attracts hawkmoth pollinators
• Pollinators are also herbivores (larval stage)
• Upon herbivore attack, flowers no longer produce benzyl acetone nor open at night
• Day opening flowers are then hummingbird pollinated
O
Smell• Lavandula
angustifolia
camphor cineole
linalyl acetate
linalool
Smell• Amorphophallus titanum• Spadix emits smell of
rotting flesh• Pollinators are attracted
– Carrion-eating beetles– Flesh flies
• Smell made up of amines and disulfides
H2NNH2
H2N NH2
putrescine
cadaverine
Smell
• Cercidiphyllum japonicum– Senescing and abscising leaves– Green leaves contain maltol glucosides– Yellow brown/fallen leaves contain maltol
O
O
OH
Taste• Chilli peppers
(Capsicum species) contain capsaicin
• “Hotness” measured on the Scoville scale
• Test method devised in 1912 by Wilbur Scoville
• Now measured by HPLC
HO
OMe
NH
O
Taste
0
• Jalapeño pepper
• Scotch Bonnet
5,000
100,000
• Naga jolokia 1,040,000
• Sweet pepper
Chilli pepper Scoville Heat Units
Taste
• Euphorbia resiniferatoxin
•Resiniferatoxin
O
O
O
OH
OMe
OO
O
H
HO
16 Billion SHU
Taste
• Ginger – Zingiber officinalis, Zingiberaceae
• Active constituent of fresh ginger is gingerol
• Cooking ginger transforms it into zingerone
HO
OMe
O OH
HO
OMe
O
Taste• Ginger – Zingiber
officinalis, Zingiberaceae
• Dried ginger contains more of another molecule, shogaol
• The Japanese name for ginger is shōga
HO
OMe
O OH
Taste• Ginger – Zingiber
officinalis, Zingiberaceae
• Dried ginger contains more of another molecule, shogaol
• The Japanese name for ginger is shōga
HO
OMe
O OH
HO
OMe
O
Taste• Vanilla– Vanilla
planifolia, Orchidaceae
Taste
HO
OMe
NH
O
HO
OMe
O
HO
OMe
O OH
O
O
O
OH
OMe
OO
O
H
HO
Taste
caffeine
theophylline theobromine
Taste
theobromine caffeinetheophylline
Conclusions