Angew. Chem. Int. Ed. Engl. 30: 654-672 (1991)
The Chemistry of Rose Pigments
Conrad Hans Eugster* and Edith Märki-Fischer

[*] Prof. Dr. C. H. Eugster, Dipl.-Chem. E. Märki-Fischer Organisch-chemisches Institut der Universität Winterthurestrasse 190. CH-8057 Zurich (Switzerland)

In this article we present a survey of the pigments found in the flowers and fruits of old and modern varieties of' roses. The yellow colors are produced by carotenoids, the reds by anthocyanins, and the modern oranges by a mixture of the two. The great structural diversity of the carotenoids contrasts with a surprisingly small number of anthocyanins. For the carotenoids found in roses, a clear correspondence exists between the structure and the breeding partners used the old yellow roses, which arose from crosses with Chinese varieties, mainly contain carotenoids from early stages in the biosynthesis, while in the modern yellow roses, which are descended from Central Asian foetida types, hydroxylations, epoxidations, and epoxide transformations readily occur. A recently elucidated carotenoid degradation sequence follows the scheme C40 → C13 + C27 → C13 + C14. The C13 compounds are odoriferous substances that contribute to the scent of roses. In the physiological pH region, copigmentation with flavonol glycosides is crucial for stabilization of the anthocyanin chromophores. Many roses, including the "apothecary's rose", which was once used medicinally, contain large amounts of strongly astringent ellagitannins, monosaccharide esters of gallic acid.

1. Introduction

The word "rose" is laden with meaning and evokes different images for everyone. These range from the fragrance of roses to the fine-sounding name of rosebushes in our own or a neighbor's garden, or in rosariums throughout the world they include cut roses as gifts or for decorating the table on festive occasions, but also the symbolism of roses.[1] their role in history. architecture, heraldry, painting, and literature, especially valuable, old illuminated works.[2] Possibly they also encompass rose societies and their journals, as well as the breeding. propagation, trade, and uses of roses in horticulture.[3]

All garden roses are descended from wild roses (botanical roses consist of species and their natural hybrids). Selection, mutation, and hybridization have gradually led to a large number of garden roses (varieties are genetically identical plants which are propagated vegetatively). In Jaeger's Rosenlexikon,[4] some 15000 varieties are described, while Modern Roses[5] includes some 14000 old and new registered varieties. Specialists estimate the total number of garden rose varieties bred to date at 50 000.[3b] Even though many varieties, especially the more modern ones, disappear again after a few decades, the rose is still the only garden plant where the results of so many hybridizations have been retained. The development of some varieties reaches back into the late Middle Ages.[7] This unique situation has never yet been used for a comparative scientific study.

Garden roses and wild roses possess three structurally distinct groups of pigments: chlorophylls, flavonoids (including anthocyanins), and carotenoids. Yellow colors were unknown in old European garden roses; the flowers of the classical provins, alba, damask, cabbage, and moss roses were always white, pink, or red, in all their variations. The advent of yellow in garden roses, from about 1820 on, was greeted with great enthusiasm, as can be seen in old books on roses. Today, we find the illustrations and descriptions those books correspondingly exaggerated. The development of yellow colors up to the present day will be explored in detail in Section 7.7.

2. Carotenoids in Roses

2.1. General Considerations

The assumption that the yellow pigmentation al rose petals is due to flavonol glycosides has persisted up to the present day.[8] It is only since 1963 that qualitative tests (solubility in ether, blue staining with SbCl3, etc.) have led to the proposal that carotenoids are present.[9] With one exception,[9b] identification was never attempted. In fact, all rose flowers, including yellow ones, contain quite a lot flavonoids. However, only when they are present in concentrations, together with carotenoids, do they contribute to visible absorption, giving rise to a bronze color.[10] This has been proven experimentally for 'William Allen Richardson' (Ducher, 1878) and 'Whisky Mac' (Tantau, l967),[11, 12] whose flowers are brownish yellow.

Carotenoids are widely distributed in the anthers of roses. Their function is still a mystery. Moreover, some carotenoids in the anthers have not yet been found in the petals (See Section 2.2).

Our studies on about 40 different yellow or yellowish rose varieties and species have led to the identification of about 75 carotenoids[13] Fig. 1). The individual structures will be described in more detail in the following sections.

2.2 Analysis and Identification

In 1977, our first analysis of the carotenoids from the yellow rose R. foetida, using classical adsorption chromatography, led to the identification of seven carotenoids.[13a] Seven years later, a further study of material from the same plant, using more refined techniques, allowed us to identify nearly 40 components.[13d] This was primarily due to the introduction of new HPLC methods, which, together with high-field NMR and other spectroscopic techniques permit us to analyze even very minor components. Comparison with standard compounds, which we have synthesized parallel to the isolation studies, was also helpful. It quickly became obvious that, in submilligram amounts, the isolates always contained contaminants that made the interpretation of the spectra very difficult. The most urgent question concerned the steric configurations of the 5,6- and 5,8-epoxides, large amounts of which are present in the more modern rose varieties.[14] The commonly used constitutional terms "aurochrome", "luteoxanthin", and "auroxanthin" concealed numerous stereoisomers which had not yet been identified. Thus, "auroxanthin" contains three pairs of constitutionally identical chiral centers. An all-E configuration would lead to a total of twenty possible diastereoisomers (sixteen enantiomeric pairs and four meso forms). If there is just a single, noncentral Z double bond, the numbers of isomers rises to thirty-two enantiomeric pairs! Even if biogenetic arguments are used to justify 3S,5S,3'S,5'S chirality, numerous isomers remain to be identified. Noteworthy in this connection is the surprising presence of 3'-epilutein (15) in the anthers of R. gallica officinalis and other roses.[131] If the usual precautions are taken (material as fresh as possible, degassed solvents, avoidance of direct light, rapid work, inert atmosphere (whenever possible), then the isolation of carotenoids from flowers presents no great difficulties. The lability of the 5,6-epoxides towards acids must be noted. We always add sufficient quantities of finely powdered CaCO3 to the material to be extracted. Before chromatography, phytosterols are precipitated from acetone solution by cooling. Preliminary purification of the saponified carotenoids is usually carried out on silica (Merck, 40-63m) using hexane ether or hexane acetone mixtures containing 0.05% Et(iPr)2N (beware of possible acetone condensation with aldehydes!). For small amounts we also use silica plates (e.g.. Merck 60, F254, 0.25 or 0.5 mm). When major components are present, these must be removed by crystallization (check by HPLC!).

The subsequent HPLC separations were performed as follows: hydrocarbons on Spherisorb ODS-5m with acetonitrile THF mixtures or on Sphensorh NH2-5m with hexane Et(iPr)2N; fractions with monohydroxy, dihydroxy, hydroxyepoxy, neoxanthin, and latoxanthin polarity were each purified separately on Spherisorb S-5 CN with hexane containing 0.1 % Et(iPr)2N and a gradient of various dichloromethane methanol mixtures.[13, 15]

For conclusive identification of rare carotenoids or those only present in trace amounts it is necessary to record UV/VIS, NMR, mass, and CD spectra and to carry out cochromatography with authentic samples. A library of spectra is extremely useful here; compilations of UV /VIS,[16] NMR,[17] CD,[18] and mass spectra[17a, 19] are available.

3. White Roses

The petals of white roses reflect a large proportion of the incident light. They are therefore more or less free of compounds that absorb light of wavelength 400-700nm. Nonetheless, the petals are not transparent. This is due to the many air-filled spaces between the cells, the so-called intracellular spaces. When the air is displaced, for example, by dipping a petal into acetone, the luster und reflection are rapidly diminished. On the other hand, the petals contain significant amounts of UV-absorbing substances, above all large amounts of flavonol glycosides. Up till now, these have only been studied in a very few varieties; thus, 'Niphetos' (Bougère, 1843) contains a great deal of quercetin, some isoquercetin, rutin, spireoside, and other, unknown flavonoids (see Section 8.5.2).

Fig. 2. Left: 'Merveille de Lyon' (1:5.5 scale; bush rose, Pernet Père, 1882), in which pigment synthesis is practically fully blocked. Middle: 'Schneeschirm' (1:5; small shrub, Tantau, 1946). Right: 'Nevada' (shrub, Pedro Dot, 1927). Temperature-dependent, partial pigment synthesis occurs in both Schneeschirm' and 'Nevada'.

More is known about the carotenoids[12] (Table 1). All the white roses we have studied contain significant amounts of carotenoids: examples include R. pimpinellifolia, 'Niphestos', 'Mme Plantier' (Plantier, 1835), 'Alberic Barbier' (Barbier, 1900), 'Yvonne Barbier' (Turbat, 1910), 'Nevada' (Dot, 1927). 'Virgo' (Mallerin, 1947). 'Iceberg' = 'Schneewittchen' (Kordes, 1958), although colorless hydrocarbons predominate. The few colored carotenoids also found presumably come from the base of the petals. Even epoxides are present. The apocarotenols, which comprise primarily esterified rosafluene (40), are remarkable.[13b] Rosafluene, like phytofluene (2), has a fluorescence emission maximum in the blue-green (λex= 359, λem≈ 525 nm). If the apocarotenols are present in sufficient amounts and their fluorescence is not quenched, these compounds act as natural brighteners. Rosafluene is a catabolite of any carotenoid from carotene onwards (see Section 7.6). For the determination of the degree of whiteness in roses, see Ref.[13i]. Beautiful examples of white roses are shown in Figure 2.

4. Green Roses

Roses with green petals are very rare. The best-known example is R. chinensis viridiflora (Fig. 3), which is said to have arisen by mutation in South Carolina in 1833.[20] Its pale green, often lightly russet flowers doubtless contain chlorophylls, but no detailed studies have been performed.

Various modern, white roses occasionally reveal a definite green tinge, such as 'Green Ice' (Moore, 1971). 'Greensleeves' (Harkness, 1980), and 'White Success' (Jelly, 1955). These, too, presumably contain chlorophylls.

Fig. 3. Rosa chinensis viridiflora. Green Ice Greensleeves

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