e d i t e d by: MICHAELR. SLABAUGH Weber State College Ogden. Utah 8440s
The Chemical Pigments of Plants Joy Alkema and Spencer L. Seagerl Box 2503, Weber State C o l l e g e , O g d e n , UT 84408
Each spring the predominantly drab colors of wintering plants give way to a sudden display of bright hues. The prevalent color of spring is of course green in a variety of shades. But, in addition, splashes of yellow, red, pink, and magenta are seen in the foliage of some plants. As time passes, nature's chromatic display continues. Flowers bloom and exhibit practically every color and shade that can he imagined. Fruits and berries rinen. . . and in so doing-thev.undereo dramatic color changes from the green characteristic of their unripe condition. During the autumn. much of the ereen foliaee changes color to become some shade of yellow, orange, red, scarlet, magenta, or purple, which finally is replaced by brown as winter approaches. Interestinelv. most of these colors and color chances result from the production, interaction, and breakdown of three classes of substances: porphyrins, carotenoids, and flavonoids. In Table 1 the types of compoundsincluded in each of these three classes are aiven toaether with the colors they produce in plants. The coyors depend in some instances up& factors such as pH or upon chemical interactions.
u PORPH IN
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Porphyrins Porphyrins are derivatives of porphin, which consists of four nitrogen-containing heterocyclic rings linked by four methene bridges in an alternating double-bond ring system. Many porphyrins have intense colors; the one of interest in this discussion is the dark green plant pigment chlorophyll. Plants contain both chlorophyll a and chlorophyll b, which have slightly different structures, but chlorophyll a predominates. o o r- ~ h i nand chloro~hvlla are In Fieure 1 the structures of . shown. It is apparent that chlorophyll a has the characteristic worohin structure but with a few differences. A maanesium A
cH&$
H
CHFCH CH3 CHLOROPHYLL a Figure 1. Molecular Structures of Porphin and Chlorophyll a.
Table 1. Classification of Color Producing Compounds. Class
Compound Type
Colors Produced
Porphyrin Carotenotd
Chlorophyll Carotene (a. y and lycopene) Xanthophyll Flavone Flavonol Anthocyanin
green yellow, orange. red yellow yellow yellow red, blue, purple, magenta
"
Flavonoid
groups are attachedto the fundamental porphk ring. Chlorophyll plays a vital role in the photosynthetic process hy which plants convert water, carbon dioxide, and sunlight into carbohydrates. Chlorophyll is produced by plants in response to availahle sunlight. Processes within the plant also break the chlorophyll down to simpler compounds. In order for photosynthesis to go on, chlorophyll is produced a t a rate equal to or greater than the rate of breakdown. However, as the amount (intensity and hours of daylight) of sunlight decreases, the rate of chlorophyll production also decreases. Carotenoids Com~oundsclassified as carotenoids are colored ternenes or derivatives of terpenes that occur in most plants. The terpene classification means that the carotenoids are made up
' Author to whom inquiries should be directed.
COOH
0.
of combinations of isoprene units. The structure of an isoprene unit is shown in Figure 2. The carotenoids we will consider to contribute significantly to plant colors include three forms of carotene (or, P, and y), lycopene, and the uxygen-containing xanthophylls. The structures of these compounds are given in Figure 3.
This feature presents relevant applications 61 chemistry to everyday life. The information presented might be used directly in class, posted on bulletin boards, or otherwise used to stimulate student invnlvement in activities related to chemistry. Contributions should be sent to the feature editor.
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March 1982
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CH2
FLAVONOID STRUCTURAL U N l l
COMPLETE FORMULA
SKELETON FORMULA Figure 2. The Isoprene Structure. The skeleton formula is oflen used.
FLAVONE STRUCTURE
LUTEOLIN
FLAVONOL STRUCTURE
MYRICETIN OH
BCAROTENE
X-CAROTENE ANTHOCYANIN STRUCTURE
CYANIDIN-3-GLUCOSIDE
Figure 4. Flavonoid Structures with Specific Examples
LYCOPENE
A XANTHOPHYLL
Figure 3. Structural Formulas of Carotenaids. The long connecting chain is represented by a dotied line in all but the a carotene formula.
The yellow-colored &carotene and xanthophylls are the predominant carotenoids in leaves and other photosynthesizing tissue. Lycopene, a red pigment, is found in peppers, tomatoes, watermelons, and certain other fruits. Smaller amounts of the other carotenoids are also present in some colored fruits and vegetables. Carrots, for example, contain small amounts of a-carotene in addition to the very abundant &carotene. In general, plants do not need light to produce carotenoids. Unlike chlorophyll, carotenoids, once produced, do not break down rapidly in a plant. Thus, carotenoids are present in leaves during their entire lifetime.
The yellow-colored flavones and flavonols occur in the leaves of some plants. However, most of their contributions to plant color appear in the petals of flowers. Anthocyanins are responsible for most red, blue, and purple colors in higher plants. Since they contain sugars, i t is not surprising that anthocyanin production in plants is related to the production and availability of carbohydrates. In some instances the amount of light available influences anthocyanin formation. For example, the side of a strawberry facing the sun gets red quicker than the side away from the sun. On the other hand, light seems to be of little importance in other plants such as radishes, which hecame uniformly colored even though buried. Cool weather also seems to enhance anthocyanin formation in some plants, but this might he a result of sugar availability as we will discuss later. The color of anthocyanins is significantly influenced by the pH of the medium in which they are dissolved. The following equations illustrate this behavior.
Flavonoids All flavonoids contain the structural unit shown in Figure 4. In the three types of flavonoid compounds we will discuss, the three-carbon chain connecting the rings is cyclized with oxygen to form a six-membered heterocvclic rine. In flavones OH pH
addition to the ~ a r b & ~ l b x y ~ eInn anthocyanins . ho carbony1 oxygen is found on the heterocyclic ring, and the position occupied by OH in flavonols is occupied by a glycosidic linkage formed by a reaction between the OH group and a sugar such as glucose. In addition, the anthocyanins occur in plants as salts (indicated by the positive charge on the heterocyclic ring). Specific compounds of flavones, flavonols, and anthocyanins also have OH groups located at various other locations on the rings of the molecule. These structural characteristics are shown in Figure 4. 184
Journal of Chemical Education
< 3. Red
PH 7-8, Violet
OH pH
> 11, Blue
Leaf Colors
The colors shown by leaves are produced by various combinations of the three classes of colored com~oundswe have described. In most plants chlorophyll is prdduced in ahundance durine the sun-filled erowine season. Its ereen color masks the &low carotenoi& and flavonoids, a i d the redto-hlue colors of anthocyanins. However, leaf colors other than green are observed in a t least three different instances: (I) Some new leaves of young or growing plants are edged or, in some cases, nearly completely colored with a red tint. (2) In a few types of plants the leaves display colors other than green throughout the growing season. (3) In the fall the greenleaves of many plant varieties change color before dropping. The flash of color in young leaves is the result of anthocyanin formation. It has been suggested ( 1 )that anthocyanins form during the early stage of leaf development because more sugars are formed by photosynthesis than are needed by the plants for growth. The high sugar concentration leads to the production of anthocyanins in amounts sufficient to overcome the maskine effect of the ereen chlorowhvll. . The transient color of theanthocyanins Zsappears as the plant grows, hut it is not known whether the iuvenile ~ s destroved . i e m e n t are by enzymes or simply "diluted" out as the plant cells di&e and multiply in number. Plants that display permanent leaf colors other than green during the growing season have considerable ornamentation value. Most often their colors are various shades of yellow, red, or magenta. The compounds responsible are mainly carotenoids and anthocyanins. All of these plants contain sufficient chlorophyll to carry out photosynthesis. But they also form high enough levels of carotenoids (and occasionally yellow flavonoids) oranthocyanins to allow their characteristic colors to be seen in the presence of chlorophyll. Anthocyanins also mask yellow colors quite well, so plants showing yellow coloration generally contain low levels of anthocyanins. In some plants such as the varigated coleus or varigated euonymus, such yellow colorations are not uniformly distrihuted. Anthocyanins are the source of the color seen in plants with red or purple foliage. These plants are frequently used to contrast with green-leaved varieties. Examples include certain maples (crimson king and Schwedlers), some varieties of Eurooean beech, and some Chinese and Jananese flowerine cherry, plum, and crabapple. All of these &nts have the ge: netic capacity to produce anthocyanins in quantities sufficient to mask or significantly modify the bright green of chlorophyll. Color variations can be seen between wlants of the same va-
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pH diffkrences in plant fluids. As we saw before, anthocyanin color is influenced by pH. The color changes in leaves that we enjoy in the fall result from changes in the relative amounts of chlorophyll, carotenoids, and anthocyanins in the leaves. We will outline in a general way how these changes occur. Creighton (2) gives more details of the nrocesses involved. ~hlorophyliformation slows down as the amount and intensity of sunlight decrease in the fall. However, the rate of chlorophyll decomposition does not decrease. The net effect is a eradual decrease in leaf chloro~hvlllevels as fall wrogress&. Yelloy carotenoids formed d i r k g the growing season do not decompose at this time. If the carotenoid color is not ample of ;piapt thLcIemonstrates thii behavior. Anthocyanin production ilicreases in leaves as the concentration of sugars increases. Several explanations have been offered to account for an increase in leaf sugar levels, and hence anthocyanin formatiori, in the fall. According to Creighton (21, warm sunny days lead to high photosynthetic activity and the formation of large amounts of sugars. When
such days are followed by very cool fall nights, the fluid movement out of the leaves slows down (remember, most processes slow with a drop in temperature). Since sugars are transferred out of the leaves by fluid movement, leaf sugar levels tend to increase. Creighton (2) and Winkelman (3) both point out that the development of the abscission layer between branches and leaf stems also slows fluid movement out of leaves. This spongy layer grows in response to decreasing temperatures and length of day. The layer is nature's way of preventing a plant from losing moisture through the leaves during the wmter. Eventually, the abscission layer completely seals off the leaves from the plant. When this happens, the leaves die and fall. But before that happens, the decreased fluid movement from the leaves arain contributes to increased sugar levels and anthocyanin formation. It has also been suggested ( 1 )that the starch breakdowm in dying leaves yields sugars which stimulate anthocyanin production. I t seems reasonable to assume that all three processes make a contribution at some time during the fall.Increased anthocyanin levels in leaves with depleted chlorophyll concentrations lead to most of the fall leaf colors. If carotenoids are also present in significant amounts, orange leaves result. In the absence of significant auantities of- ca. rotenoids, only the anthocyanins are seen. At times pH influences the resulting" color. Mawles have acidic s a n and the anthocyanin-rich leaves are red. Ashes, on the other hand, have more alkaline sap and display purple leaves. Purple colors are also thought to result when red anthocyanins interact with brown-colored pigments called tannins. We have not discussed the anthocyanins characteristic of specific plants. However, one that is very often found in autumn leaves is cyanidin-3-glucoside, with the structure given earlier in Figure 4.
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Flower Colors
Even the most casual observer must be impressed with the diversity of color displayed by flowers. A large part of this diversity results from the presence and interaction of carotenoids and flavonoids in the petals. Most yellow blossoms result from carotenoids (especially xanthophylls), while anthocyanins are responsible for most reds, blues, and purples. The anthocyanin colors can be modified by a number of factors including: (1) the presence of small quantities of other pigments, (2) copigmentation with other flavonoids (7,8,9), (3) metal chelation, and (4) the pH of cell fluid (8,9). When small amounts of other pigments are present with anthocyanins, their colors mix or blend. When copigmentation takes place, a chemical interaction occurs between the anthocyanin and the other flavonoid involved. This usually causes the anthocyanin color to become more blue. Metal chelation also involves a chemical reaction, hut it is between the anthocyaninsand metal atoms or ions. Table 2 (adapted from reference 7, page 251) illustrates the colors that result from various combinations and interactions of carotenoids and flavonoids.
Table 2. Examples of Carotenoid and Flavonoid Contributions to Flower Color Color Yellow Orange Scarlet
Magenta or Crimson Violet Blue
,
Pigment
Example
Carotenoid Flavonol Carotenoid Anthocyanin and Flavone Anthocyanin Anthocyanin and Carotenoid Anthocyanin Anthocyanin Anthocyanin and copigment Anthocyanin-metal complex
Rose Primmse Lily Snapdragon Geranium Tulip Camellia, Begonia Verbena Poppy, Leadwort Cornflower
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Number 3
March 1982
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Fruit and Vegetable Colors
The attractiveness of fruits and vegetahles depends to a large extent on their colors. In many cases these colors are the result of anthocyanin pigmentation. Carotenoids are not involved very often as the major pigment, hut they are important in several well-known examples such as lycopene in tomatoes, and 0 carotene in carrots and sweet potatoes. Chlorophyll, of course, is the pigment in green vegetables. The color-producing pigments are concentrated in the skin or rind of a number of fruits and vegetables, including the anthocyanins of apples, plums, and pears, and the carotenoids of oranges and bananas. In other examples, the pigments are distributed throughout the fruit or vegetable (e.g., anthocyanins in raspberries and carotenoids in carrots). The ripening of most fruits and some vegetahles is characterized bv-a raoid . decrease in chlorouhvll , levels c o d e d with a rapid increase in pigment levels. It is generally assumed that the same t ". w e s of anthocvanin reactions resuonsihle for flower colors are also important in some fruit and vegetable colors (7, page 274).
.
Summary
As you might suspect, there is a great deal more known about the compounds responsible for natural colors than we have included. We have said little about specific anthocyanins and other flavonoids. We have also limited our discussion of
186
Journal of Chemical Education
carotenoids to three or four specific examples, when in fact many are known to exist. However, some ideas and patterns have been revealed that lead to some useful generalizations: 1) Most natural colorants belong to one of three chemical classes-porphyrins, carotenoids, or flavonoids. 2) Yellow colors are often caused by carotenoids or nonanthocyanin flavonoids, either alone or in combinations. 3) Darker colors are very often caused by anthocyanins. The deeper and bluer colors frequently result from anthocyanins interacting with flavonoid eopigments, or metals.
We hope that the information we have included will enhance your chemical awareness as you observe the colors that abound in nature. Literature Cited (1) Hss8.H. B., Chrmtfch,525 (Sept. 1977). ( 2 ) Creizhton. H.B.. Horiirulturr. 62.10 119741 (3) Winkelman, C.,H u r l i ~ u l i v i e46.39 , (1968). (4) Asen, S.,Stewart,R. N.,andNorrir,K. H.,Phyloch~rn,LI, I139 (19721. (51 Asen,S..Norris,K.H.,Stewart,R. N.,and Serneniuk.P.,J Amrr Soc. H o r l Sci.98, 1"" ",",. (6) Asen. S.. Stewart. R. N.,and N0rrir.K. H.,Phhyturhem..14,9677 (19751. (71 Goodwin, T.W,"Chemiatryand Biochemistry of Plant Pigments,"U.S. ed., Academic Press, New York, 1965. (81 Hulme, A. C.,"The Biochemistry of F ~ u i t sand Their Products." U S . ed., Academic Presa, New York, 1970. (9) Harliurne, J. B., "Comparative Biochemistry of the Flrvonaids," U S . ed., Academic Presl, New York, 1967.
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