of the
NEW ENGLAND ASSOCIATION of
ClIEMlSTRY TEACHERS
Leaf Pigments' SAMUEL E. KAMERLING Bowdoin College, Brunswick, Maine
T
HERE are two major types of leaf pigments: acteristic feature is the strong reddish fluorescence of the so-called chromoplastid pigments and the cell- the molecule, a property which is responsible for the sap pigments. The chromoplastid group includes the moonlit appearance of a landscape photograph taken chlorophylls, the carotenes, and the xanthophylls; with a deep red filter. The chlorophylls are magnethe cel-sap pigments include the anthocyans, the antho- sium-containing molecules of a heterocyclic structure cyanidins, and the flavones. I t is the purpose of this somewhat similar to the iron-containing blood pigment paper to indicate the major features of the chemistry in that both chlorophyll and heme (of hemoglobin) are and physiological functions cf these groups of com- derivatives of porphin. Formula I gives Fischer's pounds. The chrcmoplastid pigments are water-insoluble substances found as granules in the thin layer of material known as cytoplasm located just within the cell walls of the leaf tissue. The bodies which contain the pigments may be green (the chloroplasts) or orange-red, red, or brownish red (the true chromoplasts). The chloroplasts contain the chlorophylls; since these are usually far more numerous than the chromoplasts which contain the "yellow" pigments, the leaves of healthy growing plants are usually green in color. It is the disappearance of the chloroplasts as the leaf ages or dies which is responsible for much of the characteristic yellowing or browning of leaves in the autumn. The green pigments, the chlorophylls, make up about five per cent of the chloroplasts, the balance being proI. Chlorophyll a tein and lipoid material. I t is the chloroplast portion of the leaf which is responsible for the assimilation of carbon dioxide and the photosynthetic conversion of latest structure for chlorophyll a; chlorophyll b diiers from a only in that ring I1 has the grouping -CHO carbon dioxide to carbohydrate material. The phytyl group of chlorophyll is The importance of chlorophyll has attracted the in place of -CHs. attention of investigators from the time of Berzelius worthy of special note; it is from phytol, an unsaturated to the present. Willstatter, Stoll, Hans Fischer, and alcohol of the ally1 alcohol type, CnoHs-,OH. The Conant have made the major contributions to our phytyl radical also occurs in vitamin K,. Chlorophyll knowledge of its chemistry and the reader is referred is very sensitive to acids, losing its magnesium and to the excellent review articles cited below for details. forming brownish products; the strong fluorescence is In brief, chlorophyll, which occurs to the extent of 3.2 also lost in this process. Alcoholic extracts of leaves, after destroying the per cent by weight of fresh leaves (8.2 per cent, dry chlorophyll with concentrated HC1 and removing the basis), is a mixture of two substances: chlorophyll a, CrsHnOsNaMg, and chlorophyll b, Ca5HmOeN4Mg;decomposition products, yield ether or benzene soluthe average ratio of a to b in plants is about three to tions which are yellow. The yellow pigments, the one. Chlorophyll a is bluish green; b is yellowish carotenes and the xanthophylls, are less abundant than green when dissolved in organic solvents. A char- the chlorophylls in the ratio of about one to six. Chemically, the carotenes are highly unsaturated hydrocarBased on an address delivered at the 232nd Meeting of the bons of the polyene type. There are three isomeric N.E.A.C.T., St. Paul's School, Concord. N. H., October 27, carotenes, known as alpha, beta, and gamma carotenes; 1945.
I / \ ~-CH=CH-LCH-CH=CH-L~H-CH=CH-CH=~-C CH%
H,C/
\
I
1I
H2C
C-CHs
CHI-C
\c/
I
II
CH2
\c/
Hz
H2 11. Beta-carotene
their molecular formula is CmHsa. The beta form is the most abundant; i t is the material which gives the carrot its color. In pure form beta-carotene is dark violet; its solution in organic solvents yields the familiar yellow-orange color. The structural formula for beta-carotene is given in Formula 11; i t will be noted that i t is a conjugated hydrocarbon with two unsaturated alicyclic rings. The structure of alphacarotene is similar to that of the beta form save for the location of the double bond in one of the alicyclic rings. Beta-carotene is of major importance because of its relation to vitamin A (Formula III), of which i t is the
when the cells are destroyed by boiling. Chemical investigations, notably by Willstatter and Robinson, have revealed the properties and structure of many of these substances. They behave as indicators in their response to acidity, being usually red-purple in acid and blue-green in alkali; many leaves and petals go through a striking color change on this account when exposed to ammonia vapor. They are phenolic and glycosidic derivatives of the parent heterocyclic structure, 2-phenylbenzopyrylium chloride (Formula V).
C1
/"\ I c-cH=cH-c=cH-cH=cH-LH-cHzoH I II
HIC
V. 2-Phenylbenzopyrylium chloride
H=b-CHs
For instance, the maple family yields a compound, C21HZ0011C1, known as asterin chloride whose chemical name is 3-8-glucosidylcyanidin chloride; Formula VI
H*
111. Vitamin A
precursor. The chemistry of the carotenoid pigments, of which there are very many occurring in animals and plants in addition to these leaf pigments, has been thoroughly investigated in the past 20 years; notable among the investigators have been Kuhn, Karrer, Standinger, Winterstein, and Zechmeister. The xanthophyll content of leaves is usually about twice the carotene content. Structurally the xanthophylls are oxygenated carotenoids containing one or more hydroxyl groups per molecule. Numerous xanthophylls have been obtained from various sources (egg yolk is a source of xanthophyll). Formula IV describes lutein, a xanthophyll related to beta-carotene; the molecular formula is C40HL~02. The functions of the carotenes and xanthophylls are not fully known but it is believed that they may be concerned with the breathing or assimilation processes of the plant; apparently vigor of growth and of the reproductive process in plants parallels carotene content. The cell-sap pigments are also of several types. Perhaps the most familiar is the anthocyan, or anthocyanin, type found in many leaves, petals, and fruits. For instance, red cabbage and beets release highly colored, water-soluble material of the anthocyan type CH,
HIC
Cl
H(t/
\/-+caH'105
indicates that this substance is phenolic in character with a glucose residue. The sugar residue in anthocyanins is not necessarily glucose. Such compounds are hydrolyzable by boiling alkali to the sugar free pigments; these are called anthocyanidins. The authocyanidins when fused with alkali yield polyhydric benzenes and hydroxybenzoic acids; e. g., asterin chloride yields by such treatment phloroglucinol and 3,4dihydroxybenzoic acid. The anthocyanins are believed to be respiratory pigments with some role in cellular oxidations and reductions. Since flowers on high altitude plants are brighter in color than those on the same plants a t low altitudes, it is thought that the anthocyanins may also serve to screen harmful radiation, and, by absorbing heat radiation, raise leaf temperatures.
CHJ
CHs
1
I1
H$ C-CHS HC \C/ Hs
OH \OH
0 H VI. A~terinchloride
I C-CH=CH-bCH-CH=CH-C=CH-CH=CH-CH-C-CH=CH-CH== I
0' r" /ON
CHs L-cH=cH
H'Sd" -c
CH.
CHx-J
IV.
Lutein
\C/ Hz
J
