Anthocyanins

Anthocyanins are red to blue flavonoid pigments used throughout the plant kingdom and by some bacteria. They are truly remarkable, versatile compounds. Here's a brief list of their uses and other interesting properties.

It is interesting to note that the other two classes of pigments the orange carotenes and yellow xanthophylls are actually present throughout the year; they become visible only when the dominant green chlorophyll finally breaks down in the fall. Thus are the spectacular golden aspen and cottonwood revealed.

They are perhaps most noticeable in bright red autumn foliage where, together with other common orange and yellow pigments such as carotenes and xanthophylls, they can turn entire mountainsides a patchwork of flaming colors. As the leaves break down their chlorophyll (in order to reclaim as much nitrogen as possible before the leaves fall [need ref]), they begin to produce anthocyanins. [Why?]

Interestingly, the absorption spectrum of anthocyanins is nearly perfect. DNA is most susceptible to damage due to UV in the 240-310nm range. The ozone layer in the upper atmosphere filters out nearly all UV below 290nm, leaving a narrow slice from around 290nm (depending on how big the ozone hole is) to 310nm. Anthocyanins filter out about 95-99% of this in the epidermis before it ever reaches the sensitive chloroplasts.

Interestingly, the absorption spectrum of anthocyanins is nearly perfect. [Can't find a plot anywhere, but supposedly it absorbs UV-B in particular.] DNA is most susceptible to damage due to UV in the 240-310nm range. The ozone layer in the upper atmosphere filters out nearly all UV below 290nm, leaving a narrow slice from around 290nm (depending on how big the ozone hole is) to 310nm. Anthocyanins filter out about 95-99% of this in the epidermis before it ever reaches the sensitive chloroplasts.

Not only do anthocyanins reflect red and blue light, they also absorb dangerous UV-B, making them an excellent natural sunscreen. In particular they are used by young leaves for this purpose before they are capable of producing more permanent (and costly) protective waxes on their cortex. In fact, as a testament to the uncanny efficiency of nature, UV-A (and sometimes blue light) actually induces the production of anthocyanins in most plants. That is, plants produce it when and to the extent that it is needed the more UV radiation strikes a plant, the more sunscreen it puts on. [1][2][3]

Sometimes leaves will produce anthocyanins in great quantity even while chlorophyll is present. This is somewhat unusual, as red and blue wavelengths contribute the majority of photosynthetically active radiation (PAR) that is, red and blue light are what photosynthesis requires to do its thing. Throwing up a red/blue filter over your chloroplasts makes no sense unless you're expecting a whole lot of direct sunlight. Whatever evolution (or sadistic horticulturists) had in mind, this effect is responsible for the reddish leaves of many ornamental trees and herbs, such as purple-leaf European beech (Fagus sylvatica var. atropurpurea), "Bloodgood" Japanese maple (Acer palmatum f. atropurpureum), and red-leaf cabbage (Brassica oleracea var. capitata f. rubra).

Another interesting example is the Confederate rose (Hibiscus mutabilis), which changes the color of its flower from white when they first open, to pink as they age, finally to bluish-pink when they die. Often all three color-phases may be visible at the same time on a given plant.

Another place anthocyanins are manifest is flowers, such as to name only a few snapdragons (Antirrhinum majus), larkspurs (Delphinium), violets (Viola), and morning glories (Ipomoea purpurea). But perhaps among the most interesting are hydrangeas (Hydrangea macrophylla). They change the color of their petals (actually sepals, but that's a topic for another day) from blue to pink depending on the soil pH: the higher the pH, the less soluble the aluminum in the soil is, and thus the less aluminum is taken up by the roots and transported to the flowers. Any aluminum in the flowers will bind with the pigment (delphinidin 3-monoglucoside) and change the color from pink to blue. (Got all that?) Add lime to increase pH and get blue flowers; add zinc-coated nails to lower the pH and restore your pink flowers; carefully balance the two and confuse the heck out of your poor plant! [4]

Yet another place we've all seen anthocyanins are in the vivid colors of countless supermarket fruits. Common examples include apples, eggplant, red grapes and currants, even blood oranges. They presumably serve the function of advertising to potential consumers when the fruit's precious cargo of seeds are ripe and ready for dispersal (in nature, at least). In fact, they are often present in quite large quantities. For example cherries and blueberries contain up to half a percent by weight!

However herbivores might do well to interpret the bright color as a warning, as the synthesis of anthocyanins is apparently often accompanied by less-palatable phenolic compounds, such as capsaicin and tannins. [Why??] While these compounds might be formidable deterrents for snails or insects, fortunately for us humans, they are merely unpleasant. Capsaicin, for example, is the agent in chillies that burns your mouth. Tannins are responsible for the astringent flavor of unaged red wine. [NOTE]

Anthocyanins themselves are powerful antioxidants. Among other things, this helps protect plants from radicals formed by UV radiation. And since they are not broken down during digestion, they are a good source of dietary antioxidants as well. This is one of the reasons colorful fruit are good for you. In fact, if you're really serious about it, apparently red and black beans top the list, containing up to two percent by weight fully 2000mg per standard 100g serving!

Apparently, there is some evidence that these antioxidants can be used to treat a number of health problems as well, including boosting insulin production, and fighting leukemia and lymphoma. [6] Reportedly, the concentrated black raspberry extracts used in one study gave 200 times the anti-inflammatory benefits of aspirin, but without the side effects. [7] However, other research has shown that the health benefits from consuming flavonoids of any sort (including presumably anthocyanins) is due to increased uric acid levels that result from the body trying to flush them out of the system as fast as possible! [8] As in all matters in life, you must ultimately decide for yourself.

Chemistry Experiment

An easy and popular introductory chemistry demonstration involves extracting anthocyanins from red cabbage leaves to form a crude acid-base indicator: boil the cabbage for a few minutes until the water turns dark purple, remove the cabbage, then add a few drops to white vinegar (acid) or baking soda dissolved in hot water (basic). Acids turn a nice hot pink, bases blue-green to yellow. Red cabbage contains both anthocyanins and flavonols dozens, actually, mostly cyanidin-glucosides. While anthocyanins, as noted, turn red in acids and blue in bases, flavonols (at least the ones present in red cabbage) are colorless in acids and yellow in strong bases (pH above 9 or 10). The combination makes our indicator turn anywhere between yellow and greenish blue, depending on the pH and relative abundances of the two pigments. Let your base evaporate in the sun to make it really strong it will turn nearly pure yellow. Try drying it on coffee filters to create "pH paper".

Chemistry Experiment

Left cup holds raw juice; middle holds several drops in white vinegar; right holds several drops in baking soda/water mixture.

From left to right: strong vinegar, weak vinegar, raw juice, weak lye, highly concentrated baking soda.

Chemically, anthocyanins are an extremely diverse group, with as many as 550 different compounds being reported as of early 2006. The chemical structure changes in response to variation in pH: changing from red in acids to blue in bases, much like hydrangeas. [9] Synthesis of anthocyanins is a complicated process requiring at least five separate specialized enzymes. On average apparently fully 2% of all hydrocarbons produced by photosynthesis are converted to flavonoids and derivatives such as anthocyanins. [10]

Related pigments: carotenoids, xanthocyanins, betacyanins, and other flavonoids such as quercetin.

Citations:
1. Bruns et al., 1986
2. Stapleton, 1992
3. www.plantcell.org
4. www.science.edu.sg
5. wikipedia: Tannin
6. wikipedia: Anthocyanin
7. www.columbusdispatch.com
8. wikipedia: Flavonoid
9. wikipedia: Glucosides of anthocyanidins
10. wikipedia: Anthocyanin
11. McDougall, Fyffe, Dobsona & Stewart, 2007
12. www.chemistry.org
13. www.haverford.edu
14. www.madsci.org

On the other hand, tannins are also responsible for preventing oxidation during aging. In the process, the tannins polymerize and precipitate out as sediment, removing the astringent flavor. This is why red wines are better aged. [5]

Red cabbage contains both anthocyanins and flavonols dozens, actually, mostly cyanidin-glucosides[11]. While anthocyanins, as noted, turn red in acids and blue in bases, flavonols (at least the ones present in red cabbage) are colorless in acids and yellow in strong bases (pH above 9 or 10). The combination makes our indicator turn anywhere between yellow and greenish blue, depending on the pH and relative abundances of the two pigments. Let your base evaporate in the sun to make it really strong it will turn nearly pure yellow. [12][13]