Instructor Side
JCE Classroom Activities: #2
Anthocyanins: A Colorful Class of Compounds by the Journal’s Editorial Staff R
R OH
Background HO
+ O
OH add OH
R' The red colors of leaves in the fall, and the lose OH colors of radish skins, beets, and some cabbages OH result from pigments known as anthocyanins. OH This activity makes use of the fact that these substances are also acid-base indicators (1). The earliest definition of acids was given by Robert Boyle, who stated that acids turned plant juices red. In his book Experiments upon Colours, published in 1663, Boyle described a means of making indicator paper from the juice of violets, cornflowers, roses, snowdrops, brazilwood, primroses, cochineal, and litmus. Use of such natural acid-base indicators predates the use of synthetic indicators such as phenolphthalein by over 200 years. More details about anthocyanins are provided in reference 2. The structures of anthocyanins in acidic and basic solutions are shown in the figure.
O
HO
OH R' OH
OH
Aglycone cyanidin
R
R'
OH
H H
peonidin
OCH3
delphinidin
OH
OH
petunidin
OCH3
OH
malvidin
OCH3
OCH3
pelargonidin
H
H
About the Activity Tree leaves contain three principal classes of pigments (3): chlorophyll (green), carotenoids (yellow and orange), and flavonoids (red, yellow, blue, orange, ivory). Chlorophyll and carotenoids are contained within plastids in the leaf cells. The flavonoids include anthocyanins and are distributed uniformly throughout the cells. They are soluble in water and alcohols and can be extracted by chopping and macerating the plant material, soaking it for a few minutes in hot water or rubbing alcohol, and straining out the remaining plant fiber. Similar extractions can be performed on flower petals and vegetable tissues to obtain indicators. This activity has been written as an investigation that could be done at home by an individual student. It can be adapted for classroom use by having each student (or group of students) prepare extract from a different kind of plant. Extracts can be refrigerated overnight if necessary. Dropper bottles can be prepared ahead of time containing vinegar, ammonia, and solutions of other colorless household items. (We suggest soda pop, antacids, laundry detergents, toilet-bowl cleaners, dishwasher detergents, soaps, shampoos, salt, sugar. Students should be warned not to mix these products with each other and especially not to mix bleach with other products.) Solids should first be dissolved in water. A microscale spot plate or small plastic ice-cube tray can be used for the tests, reducing the volumes of solutions needed and allowing dropwise measurement. If you have appropriate ventilation and are willing for your students to work with flammable solvents, this activity can be extended to include chromatographic separation of pigments (4).
Integrating the Activities into Your Curriculum This activity can be used to introduce the idea of separation of natural materials into their component substances, to introduce the idea that specific substances are responsible for colors of objects, as an introduction to the study of plant pigments, as an introduction to acid-base indicators, as part of a unit on the pH scale, as part of a unit on titration, or as a problem-solving collaborative activity in which students work out their own method of using plant pigment indicators to determine relative acidity or basicity of household products.
More Information 1. Forster, M. J. Chem. Educ. 1978, 55, 107–108; Mebane, R. C.; Rybolt, T. R. J. Chem. Educ. 1985, 62, 285; Szabadváry, F., translated by Oesper, R. E. J. Chem. Educ. 1964, 41, 285–287. 2. Curtright, R.; Rynearson, J. A.; Markwell, J. J. Chem. Educ. 1994, 71, 682–684; Curtright, R.; Rynearson, J. A.; Markwell, J. J. Chem. Educ. 1996, 71, 306–309; Alkema, J.; Seager, S. L. J. Chem. Educ. 1982, 59, 183. 3. Barry, D. M. J. Chem. Educ. 1997, 74, 1175–1177. 4. Kimbrough, D. R. J. Chem. Educ. 1992, 69, 987–988; Mewaldt, W.; Rodolph, D.; Sady, M. J. Chem. Educ. 1985, 62, 530–531. 5. A more advanced experiment on extracting plant pigments is available at http://mercury.aichem.arizona.edu/chem244/chem244a/l3a.html. 6. A discussion of fall colors appeared in Lear, B. ChemMatters October 1986, page 7.
This Activity Sheet may be reproduced for use in the subscriber’s classroom. Vol. 74 No. 10 October 1997 • Journal of Chemical Education
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JCE Classroom Activities: #2
Student Side
Anthocyanins: A Colorful Class of Compounds by the Journal’s Editorial Staff The reds and purples that make beautiful fall colors in the leaves of trees are produced by compounds called anthocyanins. Flowers have their distinctive colors because of their unique combinations of anthocyanins and another class of compounds, carotenoids. The colors of fruits and vegetables such as apples, plums, raspberries, beets, radishes, and red cabbages also depend on anthocyanins. Did you know that the colors of many of these items depend on how acidic they are? Here’s how to find out.
Try This 1. Make a liquid extract of anthocyanins from dark red, purple, or blue flower petals. Chop the petals as finely as possible and crush them using the back of a spoon or a mortar and pestle. When you have a volume of about 15 mL (1 tablespoon), transfer the chopped and crushed petals to a beaker or small glass container and add about 15 mL of isopropyl (rubbing) alcohol. (Read the cautions on the label and follow them when using the alcohol.) When the liquid is deeply colored, use cheesecloth or a strainer to remove the remaining solid material. 2. Use a medicine dropper to place about 10 drops of your liquid extract into each of three test tubes or other small glass containers. To the first container add 10 drops of vinegar; to the second container add 10 drops of household ammonia. (Read and follow precautions on the bottles of vinegar and household ammonia before using them. Wear goggles. Wash vinegar or ammonia off your skin immediately with water. Do this step in a wellventilated room or outdoors.) Compare the colors of the vinegar and ammonia solutions with the color of the original extract. Record your observations in a notebook. Vinegar is an acid and ammonia is a base. Based on your observations, explain why anthocyanins are called indicators. 3. To the sample with vinegar added, add ammonia a drop at a time. After each addition stir or mix the solution, observe it, and record your observation. Keep adding ammonia until you observe no further change. 4. Repeat steps 1, 2, and 3 above with other red, purple, or blue plant matter. Some things to try are apple skin, beets, radish skin, rhubarb skin, turnip skin, red cabbage, blueberries, grape juice, cherries, red onions, yellow onions, plum skin, tomato, other flower petals, green leaves from trees or shrubs, and colored fall foliage from trees or shrubs. Which plant extract provides the greatest range of colors between vinegar and ammonia solutions? 5. Based on your observations in the previous steps, devise a procedure by which you could evaluate how acidic or basic various household products are. (Include all precautions mentioned in step 2 above and follow all precautions on the product labels—some will require rubber gloves. Do not mix the household chemicals with each other.) Check your procedure with a teacher or parent. Once it is OK, apply it to as many household products as you can. 6. Make a list of such products, starting with the most acidic and going to the most basic. The ones that are strongly acidic and strongly basic are corrosive and are likely to be harmful to skin and especially eyes. Compare your findings with the precautions listed on the product labels. Do you think the labels are adequate? Why or why not?
More Information, Print and WWW Lear, Brad, ChemMatters, October 1986, page 7. http://a1.com/daylily/anthocya.html http://photoscience.la.asu.edu/photosyn/education/colorchange.html http://www2.njgarden.com/njgarden/foliage1196.html http://www.ag.uiuc.edu/~robsond/solutions/horticulture/docs/fallcolr.html http://www.esf.edu/pubprog/brochure/leaves/leaves.htm http://www.iumsc.indiana.edu/educational/chloroph/chloroph.htm http://www.iumsc.indiana.edu/educational/carot/carotene.htm
cyanidin-3-glucoside This Activity Sheet may be reproduced for use in the subscriber’s classroom. 1176B
Journal of Chemical Education • Vol. 74 No. 10 October 1997
