According to molecular and morphological data, the tribe Cimicifugeae Torrey & Gray belongs to the family Ranunculaceae Juss. and includes four recognized genera and ~49 species:
Actaea L. (32 species),
Anemonopsis Siebold et Zucc. (one species),
Beesia Balf. f. et W. W. Sm. (two species), and
Eranthis Salisb. (14 species)
[1][2][3]. Most of these species occur mainly in the northern hemisphere and are perennial herbs
[4]. The taxonomic position of the genera
Eranthis and
Beesia has been a matter of systematic uncertainty within the tribal rank in the Ranunculaceae family. According to morphological information,
Beesia has been assigned to three different tribes (Helleboreae DC., Actaeeae Spach, and Trollieae Schröd.) by intuitive taxonomic techniques but has seldom been included in cladistic analyses
[5][6][7]. By contrast, the
Eranthis genus has consistently been assigned to the Helleboreae tribe or as the only genus to the tribe Eranthideae T. Duncan & Keener in morphological classifications but always has been a sister taxon to plants of the Actaeeae tribe in cladistic analyses
[8]. The genus
Eranthis consists of 8–14 species growing in southern Europe and temperate Asia
[9][10][11]. Traditionally, the genus has been subdivided into two sections:
Eranthis sect.
Eranthis and
E. sect. Shibateranthis (Nakai) Tamura
[12]. The type section
Eranthis is characterized by plants with tubers, yellow sepals, and emarginate or slightly bilobate upper petal margins without pseudonectaries (
Figure 1)
[6][11]. The section
Eranthis in Europe includes
E. hyemalis (L.) Salisb. and
E. bulgarica (Stef.) Stef., whereas in Southwest and West Asia, it includes
E. cilicica Schott et Kotschy,
E. kurdica Rukšāns,
E. longistipitata Regel, and
E. iranica Rukšāns et Zetterl.
[13][14][15][16]. The section Shibateranthis has long-lived tubers, white sepals, and bilobate or forked petal margins with pseudonectaries (
Figure 1)
[6][17]. Representatives of this section have a natural geographic range in temperate North and East Asia (
E. albiflora Franch.,
E. byunsanensis B.Y. Sun,
E. lobulata W.T.Wang,
E. pinnatifida Maxim.,
E. pungdoensis B.U. Oh,
E. sibirica DC.,
E. stellata Maxim., and
E. tanhoensis Erst)
[10][11].
2. Phytocomponents Identified in Eranthis Plants and Their Chemotaxonomic Significance
2.1. Chromones
Since the 1960s, from some Eranthis species, a series of substances has been isolated that represents an important class of oxygen-containing heterocyclic compounds that are derivatives of benzo-γ-pyrone: chromones. Their isolation has been performed by various chromatographic methods, and the structures of individual compounds have been investigated by 1-dimensional (H-NMR) and 2-dimensional nuclear magnetic resonance (C-NMR) spectroscopy. In structure, chromones are similar to flavonoids and coumarins but are substantially less common in the wild. Chromones can give rise to hydroxy- and methoxy-derivatives and can attach a sugar moiety, whereas after condensation with benzene, pyran, or furan rings, they can be transformed into a variety of benzo-, pyrano-, or furochromones, respectively. Compounds from the class “simple furochromones and chromones” are most often found in Eranthis species; chromones have been detected in underground parts, whereas furochromones have been found in underground and aboveground parts (Table 1).
Table 1. Chemical constituents of the genus Eranthis (all classes of metabolites identified to date: vertical subdivisions in the table).
Structurally, the chromones found in
Eranthis species can be categorized into several classes (
Figure 2). Structures of compounds
1–
6 are similar in the carbon backbone—containing an oxepin ring—but differ from one another in substituents at positions C-2 and C-9. These substituents can be methyl and hydroxymethyl groups as well as mono- and diglycosides. These compounds have been registered in the underground parts of
E. cilicica Schott & Kotschy and
E. hyemalis (L.) Salisb.
[21][22][23]. The first publications on the isolation of chromones of this subclass date back to the end of the 1970s, when 5-hydroxy-9-hydroxymethyl-2-methyl-8,11-dihydro-4H-pyrano[2,3-g][1]benzoxepin-4-one [named eranthin (
2)] and its β-D-glucoside (
3) were isolated
[22].
Figure 2. Structures of furochromones and chromones from Eranthis species.
Structures similar to this type (
7 and
8), in contrast to the above compounds, have a displaced double bond [from C-9(10) to C-10(11)] on the oxepin ring; in addition, at position C-9, there is both an oxymethylene group bearing various substituents and a hydroxyl group. These compounds have so far been found only in the underground part of
E. cilicica [21].
1H and
13C nuclear magnetic resonance data have allowed us to identify the structural features of compounds
9–
14 and to determine that their oxepin ring is open; a 4′-hydroxy-3′-methylbut-2′-enyl group has been found at position C-8. Compounds
11 and
12 additionally contain a D-glucose residue at the C-7 position, whereas compounds
13 and
14 contain it at the C-4′ position. These chromones have been detected in the underground parts of
E. cilicica and
E. hyemalis [21][22]. All of the above chromones are specific to the genus
Eranthis.
2.2. Furochromones
These compounds of
Eranthis species are formed by the condensation of a simple chromone with a furan ring at positions C-6 and C-7 and, in contrast to the aforementioned chromones, are relatively common in the plant kingdom. For instance, the first representative of this subclass of compounds called khellin (
16) has long been used in folk medicine to relieve ureteral pain during colic. For the first time, khellin was found in a seed extract of
Ammi visnaga (L.) Lam. and was isolated as far back as the end of the 19th century
[36]. Currently, khellin’s ability to act directly on smooth-muscle fibers is widely used in clinical practice
[37]. Khellin, aside from species of the genus
Ammi, has been found in other representatives of the family Apiaceae Lindl., for example, in
Dioscorea L. sp. and
Pimpinella L. sp.
[38][39][40], and among
Eranthis species, in
E. hyemalis and
E. longistipitata Regel
[24][25]. The diversity of the structures in the furochromone subclass, which includes khellin, is mostly determined by the presence of substituents at the C-4, C-7, and C-9 positions. At the C-4 position, methoxy or hydroxyl groups can serve as a substituent; at position C-7, methoxy groups and glucose; and at position C-9, a methoxy group, or—as in
15,
17,
19, and
21—the substituent may be absent. Khellol (
17) represents an aglycone of khellol glucoside (
18), in which the sugar moiety is attached at position C-7. In the genus
Eranthis, most research on furochromones of this subclass has been conducted on samples of the aerial parts (leaves, stems, and flowers) of
E. pinnatifida Maxim.,
E. hyemalis, and
E. longistipitata [24][25][26][27], and only compound
21 has been detected in an underground part (tubers) of
E. cilicica [21].
Recently, new compounds not previously found in
Eranthis species were discovered in samples of
E. longistipitata from Central Asia (Kyrgyzstan): methoxsalen (
22), 5-
O-methylvisammioside (
26), and visamminol-3′-
O-glucoside (
27)
[25]. Methoxsalen (
22) is often seen in the plant extracts of such families as Apiaceae, Rutaceae Juss., Fabaceae Lindl., and Brassicaceae Burnett
[41][42], whereas the last two compounds of this subclass (
26 and
27) have been registered only in extracts from an underground part of
Saposhnikovia divaricata (Turcz.) Shischk. (Apiaceae)
[43][44][45].
Because
Eranthis species synthesize chromones during normal physiological processes, preliminary conclusions have been made that the genus
Eranthis is closest to the genera
Cimicifuga and
Actaea (in whose extracts, chromones have also been found), and not
Helleborus L. (for example), whose species do not synthesize chromones but are distinguished by the accumulation of cardenolides and bufadienolides
[23][46].
2.3. Triterpene Saponins
A phytochemical analysis of a methanol extract from tubers of
E. cilicica has revealed two new bisdesmosidic triterpenes, named eranthisaponins A (
28) and B (
29)
[28]. The new saponins are based on the structural backbone of hederagenin, which is a triterpenoid first isolated from seeds and leaves of
Hedera helix L.
[47]. A distinctive feature of eranthisaponin A (
28) is a branched tetraglycoside attached at the C-3 position of the aglycone, whereas a feature of eranthisaponin B (
29) is a linear hexaglycoside attached at the C-28 position of the aglycone. Such sugar forms in triterpene saponins have not been described previously. In addition, as one of the substituents, eranthisaponin A (
28) contains D-allopyranose: a monosaccharide that is extremely rare in plant saponins
[28].
Furthermore, in Eranthis plants, a number of known triterpene saponins (30–35) have been discovered that are (just as eranthisaponins B and A) based on the backbone of hederagenin with substituents at positions C-3 and C-28; extremely rarely (only in 31), the substituent (a hydroxyl group) is located at the C-23 position. Other substituents include di- and triglycosides composed of glucose, arabinose, and rhamnose residues.
The research continued by K. Watanabe with coauthors
[29] has allowed to subsequently isolate a new oleanane glycoside (
34) from the tubers of
E. cilicica. Another oleanane glycoside (
35) had been discovered earlier in the underground part of
Anemone coronaria L. (Ranunculaceae)
[48]. These substances (
34 and
35) contain triglycosides only at position C-3 of the carbohydrate part of the molecule. All of the above triterpene saponins belong to the oleanan type.
In the same study
[29], when fractionating a methanol extract from the tubers of
E. cilicica, Watanabe et al. isolated several cycloartane-type compounds (
36–
48). There were 13 such triterpene saponins, all of which had not been characterized before. Compounds
37 and
45 are aglycones of
36 and
44, respectively. The new compounds can be categorized into two very similar subclasses:
36–
43 and
44–48. In terms of their structure, rings A–D are similar, and differences lie in rings E and F (
Figure 3).
Figure 3. Structures of triterpene saponins of Eranthis species.
The compounds of both subclasses differ among themselves in the presence of various sugar moieties at the C-3 position of the aglycone as well as in the presence of a hydroxy, oxo, or methyl group at position C-28.
2.4. Alkaloids
In the tubers and aerial parts of
E. hyemalis, trace amounts of an alkaloid called corytuberine (
49) have been found
[30].
2.5. Coumarins
In ongoing studies on
E. longistipitata, coumarins have been discovered in aqueous-ethanol extracts from the leaves of this species: this class of compounds was registered in the genus
Eranthis for the first time
[25] but is widespread in the family Ranunculaceae
[49]. 5,7-Dihydroxy-4-methylcoumarin (
50), scoparone (
51), and fraxetin (
52) are affiliated with the subclass “simple coumarins”, which are based on a coumarin molecule with substituents in the form of methyl, hydroxy, and methoxy groups. Luvangetin (
53) can be assigned to linear pyranocoumarins, in which—aside from various substituents—a pyran ring is present in the backbone.
2.6. Flavonoids
In contrast to the sets of chromones and triterpene saponins, the set of flavonoids in
Eranthis species mainly contains known substances. For instance, a study on the aqueous-ethanol extracts from the leaves of four
Eranthis species has led to the identification of several flavonoids: quercetin (
54) (
E. longistipitata,
E. stellata Maxim., and
E. tanhoensis), kaempferol (
62) (
E. longistipitata,
E. stellata, and
E. tanhoensis), vitexin (
66) (
E. sibirica DC.), and orientin (
67) (
E. sibirica and
E. stellata)
[31]. In addition, in
E. longistipitata, from the class of flavonols, researchers have identified isoquercitrin (
55), hyperoside (
56), reynoutrin (
57), quercetin-3-sambubioside (
59), peltatoside (
60), rutin (
61), juglalin (
63), and trifolin (
64); from flavanones, aromadendrin (
65) and 6-methoxytaxifolin (
71); from C-glycoside flavones, there is carlinoside (
68); from the class of flavans, investigators have identified cianidanol (
69) and auriculoside (
70); and from chalcones, aspalathin (
72), phloridzin (
73), and phloretin (
74)
[32]. Flavonoids in the leaves of
E. hyemalis are represented by the glycosides of quercetin and kaempferol, the detailed structures of which have not been determined
[27]. The heterogeneity of the qualitative and quantitative profiles of flavonoids has been noted among the analyzed
Eranthis species
[11][31][32].
2.7. Phenolcarboxylic Acids
The investigation of this class of phenolic compounds is represented by a single publication covering only three
Eranthis species and dealing with the identification of phenolcarboxylic acids that are widespread in nature: chlorogenic (
75) (
E. sibirica,
E. stellata, and
E. tanhoensis), caffeic (
76) (
E. sibirica and
E. stellata), salicylic (
77) (
E. sibirica and
E. tanhoensis), and gentisic (
78) (
E. stellata), whereas the concentration of caffeic acid (0.29–0.32 mg/g), chlorogenic acid (0.34–0.96 mg/g), and salicylic acid (0.25 mg/g) has proven to be the highest in
E. sibirica [31]. Thus, there is evidence of variation in the profile and levels of phenolcarboxylic acids among these species
[11][31][32].
2.8. Fatty Acids
To date, fatty acids in the leaves of
E. longistipitata (
82,
84,
88–
98, and
104) and the composition of seed oil from
E. hyemalis (
79–
81,
83,
85–
87,
91,
99–
103, and
105–
111) have been determined
[25][33].
Although the chemical composition of seed oil has been investigated only in
E. hyemalis, this class of compounds deserves special attention because in most other genera of Ranunculaceae it is taxonomically significant. For instance, in most of
Ranunculus L. species, hexadecadienoic acid (16:2n − 6) is dominant and constitutes 2–10% of seed oil. For the genera
Pulsatilla Mill.,
Adonis L., and
Aconitum L. and some
Anemone L. species, the major fatty acid (up to 80% of total) is linoleic acid, whereas the relative abundance of eicosadienoic acid reaches 7–8% in some
Anemone species, and its concentration in the species of
Cimicifuga Wernisch.,
Helleborus L.,
Actaea L., and
Caltha L. is the lowest
[50]. In
E. hyemalis,
cis-13,16-docosadienoic acid (
109) serves as a major fatty acid, constituting up to 57% of seed oil
[33]. In terms of the total set of fatty acids in seed oil,
E. hyemalis is close to the genera
Cimicifuga and
Actaea, but more detailed conclusions require additional investigation.
2.9. Lectins
The name lectin was proposed by W. Boyd in 1954
[51] for proteins that can agglutinate red blood cells and selectively bind to carbohydrates
[52]. So far, more than 500 lectins have been isolated from higher and lower plants
[53] and can accumulate in roots, leaves, fruits, seeds, and wood
[54][55]. It is believed that they provide protection to plants from phytopathogenic microorganisms and phytophages, play a decisive role in the establishment of symbiotic relationships with nitrogen-fixing bacteria, and participate in the transport of hormones and glycoproteins
[55][56][57].
To date, in the family Ranunculaceae, only in
Clematis montana Buch.-Ham and
Eranthis hyemalis have researchers demonstrated the presence of lectins. The
E. hyemalis lectin, called EHL, was first isolated in the second half of the 1980s by B.P. Cammue
[34]. This lectin represents the most widespread type of lectin among plants: ribosome-inactivating proteins
[58][59][60]. EHL, just like other ribosome-inactivating proteins, consists of two chains: chain A is responsible for enzymatic activity, and chain B binds carbohydrates, thereby helping the lectin molecule get inside the cell. In terms of its specificity to carbohydrates, EHL belongs to type II, that is, it can bind to D-galactose and N-acetyl-D-galactosamine. The role of bound carbohydrates is probably to increase the water solubility of a given glycoprotein
[61]. Structurally, EHL resembles the lectin of
Bryonia dioica Jacq. (Cucurbitaceae), but the latter possesses moderate activity, its relative abundance does not exceed 0.4% of the total soluble protein, and this lectin is found in all vegetative organs
[62]. On the contrary, EHL is located in underground organs, its relative abundance reaches 2% of the total amount of soluble proteins, and in terms of activity, EHL exceeds the lectin of
B. dioica 20-fold
[34]. The research of B.P. Cammue was expanded in 1993 by M.A. Kumar and coworkers, who characterized in detail physicochemical properties of the lectin and determined a part of amino acid sequence of chain A
[35].
2.10. Compounds from Other Classes
In a leaf extract of
E. longistipitata, by means of liquid chromatography combined with high-resolution mass spectrometry, the presence of amino acid–related compounds (
112–
117) has been established: D-(+)-pyroglutamic acid, D-(+)-tryptophan, isoleucine, L-phenylalanine, L-tyrosine, and D-(−)-glutamine; the researchers detected organic acids (
118–
120): citric acid, D-α-hydroxyglutaric acid, and gluconic acid; sugars (
121–
123): α-lactose, D-(+)-galactose, and α-trehalose; alcohol D-(−)-mannitol (
124); and phenylpropanoid 6-gingerol (
125)
[25].