Plant compounds that are perceived by humans to have color are generally referred to as 'pigments'. Their varied structures and colors have long fascinated chemists and biologists, who have examined their chemical and physical properties, their mode of synthesis, and their physiological and ecological roles. Plant pigments also have a long history of use by humans. The major classes of plant pigments, with the exception of the chlorophylls, are reviewed here. Anthocyanins, a class of flavonoids derived ultimately from phenylalanine, are water-soluble, synthesized in the cytosol, and localized in vacuoles. They provide a wide range of colors ranging from orange/red to violet/blue. In addition to various modifications to their structures, their specific color also depends on co-pigments, metal ions and pH. They are widely distributed in the plant kingdom. The lipid-soluble, yellow-to-red carotenoids, a subclass of terpenoids, are also distributed ubiquitously in plants. They are synthesized in chloroplasts and are essential to the integrity of the photosynthetic apparatus. Betalains, also conferring yellow-to-red colors, are nitrogen-containing water-soluble compounds derived from tyrosine that are found only in a limited number of plant lineages. In contrast to anthocyanins and carotenoids, the biosynthetic pathway of betalains is only partially understood. All three classes of pigments act as visible signals to attract insects, birds and animals for pollination and seed dispersal. They also protect plants from damage caused by UV and visible light.

https://onlinelibrary.wiley.com/doi/epdf/10.1111/j.1365-313X.2008.03447.x

Biosynthesis of plant pigments: anthocyanins, betalains and carotenoids

13C NMR spectra of pentacyclic triterpenoids--a compilation and some salient features

Three major groups of pigments, the betalains, the carotenoids, and the anthocyanins, are responsible for the attractive natural display of flower colors. Because of the broad distribution of anthocyanins (synthesized as part of the flavonoid pathway) among the flowering plants, their biosynthesis and regulation are best understood. However, over the past few years, significant progress has been made in understanding the synthesis and participation of carotenoids (derived from isoprenoids) and betalains (derived from tyrosine) in flower pigmentation. These three families of pigments play important ecological functions, for example in the attraction of pollinating animals. Anthocyanins in particular have also been the target of numerous biotechnological efforts with the objective of creating new, or altering the properties of existing, coloring compounds. The focus of this review is to examine the biosynthesis, regulation, and contribution to flower coloration of these three groups of pigments.

The genetics and biochemistry of floral pigments

Pigments are present in all living matter and provide attractive colors and play basic roles in the development of organisms. Human beings, like most animals, come in contact with their surroundings through color, and things can or cannot be acceptable based on their color characteristics. This review presents the basic information about pigments focusing attention on the natural ones; it emphasizes the principal plant pigments: carotenoids, anthocyanins, and betalains. Special considerations are given to their salient characteristics; to their biosynthesis, taking into account the biochemical and molecular biology information generated in their elucidation; and to the processing and stability properties of these compounds as food colorants.

Natural Pigments: Carotenoids, Anthocyanins, and Betalains — Characteristics, Biosynthesis, Processing, and Stability

Anthocyanins and betalains play important roles both in plant physiology, visual attraction for pollinators and seed dispersers, but also in food mainly defining its aesthetic value. Since anthocyanin and betalain structures allow to predict only part of their appearance, additional chemical and anatomical functions are required to modulate the appearance of plants and coloured food. Physiological effects that the same pigments exert in plants are supposedly similar to those which they show in humans following ingestion of coloured food. Therefore, anthocyanins and betalains both in fresh and also processed fruit and vegetables serve two functions: They improve the overall appearance, but also contribute to consumers' health and well-being.

Functional properties of anthocyanins and betalains in plants, food, and in human nutrition

An enzyme is extracted from the red peel of Amanita muscaria which cleaves the C(2)–C(3) and the C(4)–C(5) bond of the aromatic ring of L-dopa (1) to form a mixture of 4,5-secodopa (= salt of 6-amino-2-hydroxy-4-(2′-oxoethylidene)hept-2-enedioic acid; 2) and 2,3-secodopa ( = salt of 7-amino-5-formyl-2-hydroxyocta-2,4-dienedioic acid; 3), two hitherto hypothetical biosynthetic intermediates (see Scheme). Though isolation of these products has not been possible, structural evidence is inferred from reaction products, kinetics, and spectroscopical characteristics in comparison with known compounds. Secodopas 2 and 3 are characterized in dilute solution by HPLC and UV/VIS spectroscopy (anions; λmax 424 and 414 nm, resp., ϵ420 = 25500; on acidification, shift to 380 and 372 nm, resp.). They cyclize without enzyme catalysis, optimally at pH 4.5–5; 3 produces muscaflavin (5) and 2 betalamic acid (4). The products arc identified by direct comparison with authentic samples in HPLC, by 1H-NMR of 5, and by condensation of 4 with L-proline to form the well known betalain indicaxanthin (7). The enzymatic conversion of L-dopa (1) via2 to betalamic acid (4; (S)) and its condensation with L-proline leads to pure natural indicaxanthin (7; (2S,115)); correspondingly, the enzymatic conversion of D-dopa to (R)-betalamic acid and its condensation with L-proline produces isoindicaxanthin ((2S,11R)) which is unknown in nature. Particularly relevant is the fact that the same enzyme cleaves pyrocatechol to produce a solution of the enolate form of the known 2-hydroxy-6-oxohexa-2,4-dienoate (secopyrocatechol; 9; see Fig. 5). Dissociation constants of the corresponding enolic functions in the cleavage products are determined by spectrometric titration and compared to those of known systems.

2,3- and 4,5-Secodopa, the Biosynthetic Intermediates Generated from L-Dopa by an Enzyme System Extracted from the Fly Agaric, Amanita muscaria L., and Their Spontaneous Conversion to Muscaflavin and Betalamic Acid, Respectively, and Betalains

The synthesis of the methylester of cyclodopa (5.6-dihydroxy-indoline-2-carboxylic acid) is achieved by oxidative conversion of dopa methyl ester to dopachrome methyl ester and its subsequent reduction at pH 8, using respectively potassium hexacyanoferrate(III) and sodium dithionite. The efficiency of the oxidation step was found to be dependent on the rather low concentration of the reactants and the very short reaction time involved. After its production cyclodopa methyl ester was stabilized by immediate protonation followed by acetylation leading in a high yield to O,O,N-triacetyl cyclodopa methyl ester. By partial hydrolysis this derivative gives an O,N-diacetyl cyclodopa methyl ester and further N-acetyl cyclodopa methyl ester. Cyclodopa methyl ester and cyclodopa are obtained by prolonged anaerobic hydrolysis. Micro-separation procedures, nmr-, uv-spectra and chiral-optical properties of cyclodopa and its derivatives are reported and discussed. It is shown by nmr evidence that under acidic conditions the aromatic proton on C(7) of cyclodopa slowly exchanges with deuterium. When an alcoholic solution of cyclodopa methyl ester and semicarbazide is allowed to oxidize in the air, the semicarbazone of dopachrome methyl ester forms.

Die Synthese von Cyclodopa (Leukodopachrom)

Cyclodopa glucoside ( = (2S)-5-(β-D-glucopyranosyloxy)-6-hydroxyindoline-2-carboxylicacid; 1a) is prepared from betainin (2a) via a base exchange reaction. Its purification and physical properties are described. The compound is N-formylated in dilute HCOOH solution. Cyclodopa glucoside is contained in considerable amount in redbeet juice varying from 0.07 to 1.9 mmol/kg or from 12 to 46% relative to teh content of betanin. Teh occurrence of free cyclodopa glucoside supports its role as intermediate in betanin biosynthesis.

Cyclodopa glucoside ( = (2S)-5-(β-D-glocopyranosyloxy)-6-hydroxyindoline-2-carboxylic acid) and its occurrence in red beet (beta vulgaris var. rubra L.)

The structures and stereochemistries of two novel skeletons euferol [19(10→9)abeo-8α,9β,10α-tirucall-5-en-3β-ol] and melliferol [19(10→9)abeo-5α,8α,9β-tirucall-1 (10)-en-3β-ol], found in Euphorbia mellifera, have been established from spectral data and single crystal X-ray analyses.

Hugo Wyler

Papers

2,3- and 4,5-Secodopa, the Biosynthetic Intermediates Generated from L-Dopa by an Enzyme System Extracted from the Fly Agaric, Amanita muscaria L., and Their Spontaneous Conversion to Muscaflavin and Betalamic Acid, Respectively, and Betalains

Die Synthese von Cyclodopa (Leukodopachrom)

Cyclodopa glucoside ( = (2S)-5-(β-D-glocopyranosyloxy)-6-hydroxyindoline-2-carboxylic acid) and its occurrence in red beet (beta vulgaris var. rubra L.)

Euferol and melliferol: two novel triterpenoids from Euphorbia mellifera

Pelargonidin 3-malonylsophoroside from the red Iceland poppy, Papaver nudicaule