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Original article
12 (
8
); 3921-3926
doi:
10.1016/j.arabjc.2016.02.012

A rare bis-rhamnopyranosyl-aromadendrin derivative and other flavonoids from the flowers of Genista cilentina Vals. an endemic species of Southern Italy

, , , ,
a Dipartimento di Chimica, Sapienza Università di Roma, P.le A. Moro 5, 00185 Roma, Italy
b Dipartimento di Biologia Ambientale, Sapienza Università di Roma, P.le A. Moro 5, 00185 Roma, Italy

⁎Corresponding author at: Dipartimento di Biologia Ambientale, Sapienza Università di Roma, P.le A. Moro 5, 00185 Roma, Italy. Tel.: +39 0649913229; fax: +39 0649913841. alessandro.venditti@uniroma1.it (Alessandro Venditti)

Received: , Accepted: ,
© The Author(s) 2016
Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

A rare bis-rhamnopyranoside of aromadendrin, besides isoflavones of chemosystematic importance and peculiar compounds never reported in the genus, was identified from the flowers of Genista cilentina, an Italian endemic species. The structure of the aromadendrin derivative was elucidated by means of 1D and 2D NMR techniques. The exclusivity of the observed molecular pattern in comparison with other Genista species was further discussed from a chemotaxonomic point of view. A huge quantity of flavonoids in aglycone form resulted selectively accumulated in flowers.

Keywords

Genista cilentina Vals.
Aromadendrin bis-rhamnopyranoside
Isoflavonoids
Chemotaxonomy
1

1 Introduction

Genista cilentina Vals. is an Italian endemic species belonging to Fabaceae which is characteristic of Cilento, a territory of Campania region overlooking the Tyrrhenian Sea (Conti et al., 2005). G. cilentina is an upright shrub characterized by striped and rigid branches which are hairy during youth. Its leaves are linear, hairy, thick and trifoliate even if the superior ones are often unifoliate. It owns yellow flowers with dense pedicels which bloom between April and May. The calyx is bell-shaped and pubescent with the upper lip longer than the lower one and presents shorter teeth; its fruit is an elliptic rugged pod prevalently of monospermic kind that appears in June and/or July; its semen is elliptic, small and light green (Valsecchi, 1993). This species can grow both on clay and on rocky grounds (Lucchese and Lattanzi, 1987), even though in recent times it has been also seen proliferating on sandy fields (De Natale and Strumia, 2007).

Genista species have been widely used in the Mediterranean area, not only for medicinal purposes i.e. for treating respiratory diseases (Bouchouka et al., 2012), and as antihyperglycaemic (Edwards et al., 2006), antialgic, anti-inflammatory, oropharyngeal antiseptic, anticholagogue and colipeptic (Agelet and Vallès, 2003), but also for tinctorial uses (G. tinctoria) (Cardon, 2007). Nowadays the use of natural dyes has gained importance due to their eco-compatibility.

Several Genista species have been largely investigated for their phytochemical content, revealing the presence of quinolizidine alkaloids (Wink et al., 1983), isoflavonoids and glycosylated flavonoids (Borges et al., 2001; Serrilli et al., 2010; Noccioli et al., 2012). All these natural products are probably responsible for the observed biological activities: i.e. antimicrobial (Bouchouka et al., 2012), antioxidant (Meriane et al., 2014), antidiabetic (Rauter et al., 2009) antiproliferative (Rigano et al., 2009, 2010) and proapoptotic towards several cancer cell lines (Bontempo et al., 2013).

Isoflavones are a subgroup of flavonoids found almost exclusively in Fabaceae (Leguminosae) and were recognized in a relatively restricted number of families i.e. Phyllanthaceae (Hossain and Mizanur Rahman, 2019), Iridaceae (Al-Qudah et al., 2015) and Solanaceae (L. Li et al., 2015). Isoflavones of phytoestrogenic activity are probably the most important secondary metabolites contained in Genista and recently several studies have been conducted on both hairy roots culture (Luczkiewicz and Kokotkiewicz, 2005a, 2005b) and callus culture (Luczkiewicz and Glod, 2005) and also in order to enhance the production of these metabolites in Fabaceae as well as in species belonging to other botanical families (non-leguminous) through genetic transformation (Rasmussen and Jones, 2013). Moreover, in recent times, several analytic methods have been developed to determine these compounds in the plant extracts (Kiss et al., 2012) as well as in in vivo or in vitro biomasses (Luczkiewicz et al., 2004) as a confirmation of the interest deserved to this kind of flavonoids.

The study of the secondary metabolites composition of endemic Italian species is our ongoing project. Considering that these particular botanical entities are often sources of new or peculiar compounds (Bianco et al., 2016), and also that little data on G. cilentina are available in the literature, we have decided to deepen the phytochemical knowledge about this species, focusing the analysis on the ethanolic extract obtained from the flowers.

2

2 Materials and method

2.1

2.1 General

NMR spectra were recorded on Varian (now Agilent Technologies, Santa Clara, CA, USA) Mercury 300 MHz and/or on Bruker Avance III 400 MHz instruments (Bruker, Billerica, MA, USA) using CDCl3, CD3OD or D2O as deuterated solvents; the chemical shift was expressed in ppm from TMS (the signal of HDO (s) at 4.78 ppm was used as reference for spectra in D2O; and the internal solvent signal (m5) at 3.31 ppm for spectra recorded in CD3OD).

MS spectra were performed on a Q-TOF MICRO spectrometer (Micromass, now Waters, Manchester, UK) equipped with an ESI source, that operated in the negative or positive ion mode. The flow rate of sample infusion was 10 μL/min with 100 acquisitions per spectrum. Data were analyzed by using the MassLynx software developed by Waters. Solvents of RPE grade were purchased from Sigma Aldrich or Carlo Erba Reagenti, silica gel 60 (70-230 mesh ASTM) from Fluka.

2.2

2.2 Bidimensional NMR experiments

Bidimensional spectra were performed on a Bruker Avance III 400 MHz instrument, operating at 9.4 T at 298 K. HSQC experiments were acquired with a spectral width of 12 and 250 ppm for the proton and carbon respectively, an average 1JC–H of 145 Hz, recycle delay of 2 s and a data matrix of 4 K × 256 points. HMBC experiments were acquired with a spectral width of 12 and 250 ppm for the proton and carbon respectively, long range coupling constant of nJC–H of 6 Hz, recycle delay of 2 s and a data matrix of 4 K × 256 points.

2.3

2.3 Plant materials

The aerial parts of G. cilentina were collected from a spontaneous population in the territory of Ascea (SA) (geographic coordinates: 40°12′77″N, 15°18′20″E) and the botanical identification was performed by using available literature (Valsecchi, 1993). A sample of this species was retained for further reference and was catalogued as GC/03082014. The flowers (43.3 g) were manually separated from the plants and were extracted with ethanol 96% (200 ml) three times. The ethanolic extracts were collected and concentrated at reduced pressure until a water suspension was obtained. The suspension was further frozen and then lyophilized in order to preserve eventual temperature sensitive components, obtaining 3.4 g of crude ethanolic extract.

2.4

2.4 Isolation and identification of components

The chromatographic separation was conducted on 2.5 g of the extract on a SiO2 (62.5 g) column using n-butanol saturated with water (82:18, v/v) as eluting mixture. From this step genistein (1) (Boryski and Grynkiewicz, 2001) and biochanin A (2) (Li et al., 2008) in mixture (2:1) were recognized in fraction (Fr.5A) (37.0 mg), while several fractions with more complex mixture of flavonoids (Fr.3-13A) (excluding Fr.5A) and high polarity compounds (Fr.101-125A) were gathered and further purified using different chromatographic systems. In particular, (Fr.3-13A) (700 mg) was re-fractioned on SiO2 (21.0 g) using chloroform/methanol starting from 98:2 and raising gradually the polarity to 8:2. From this step, among several fractions containing waxes and glycerides, were isolated in the order: 4′-O-methyl orobol (3) (Hanawa et al., 1991) (Fr.17-18B) (8.2 mg); genistein (1) and isoliquiritigenin (6) (Sugimoto et al., 2011) in mixture (2:1) (Fr.27-32B) (21.9 mg); apigenin (5) (Miyazawa and Hisama, 2003) (Fr.51-54B) (34.8 mg); luteolin (4) (Burns et al., 2007) (Fr.58-59B) (10.5 mg); biochanin A 7-O-β-d-glucoside (7) (Noccioli et al., 2012) (Fr.68-72B) (53.1 mg); 3,7-bis-α-l-rhamnopyranosyl aromadendrin (8) (Fr.85-86B) (46.3 mg). From (Fr.101-125A) (400 mg), chromatographed on silica gel CC (10.0 g) and eluted with a gradient of chloroform/methanol (85:15 to 6:4), was isolated D-pinitol (9) (DellaGreca et al., 2007) (27.3 mg), followed by glucose and saccharose. All the isolated compounds were identified by comparison of experimental data with those reported in the literature.

3,7-bis-α-l-rhamnopyranosyl aromadendrin (8): 1H NMR (400 MHz, MeOD) δ: 7.37 (2H, d, J = 8.5 Hz, H2′/H-6′), 6.85 (2H, d, J = 8.5 Hz, H-3′/H-5′), 6.22 (1H, d, J = 2.2 Hz, H-6), 6.19 (1H, d, J = 2.2 Hz, H-8), 5.48 (1H, d, J = 1.6 Hz, H-1″), 5.20 (1H, d, J = 11.0 Hz, H-2), 4.68 (1H, d, J = 11.0 Hz, H-3), 4.25 (1H, dd, J = 9.7, 6.4 Hz, H-4″), 4.02 (1H, d, J = 1.5 Hz, H-1″′), 3.97 (1H, dd, J = 3.2, 1.8 Hz, H-4″′), 3.78 (1H, dd, J = 9.5, 3.5 Hz, H-3″), 3.65 (1H, dd, J = 9.7, 3.2 Hz, H-3″′), 3.55 (1H, part. overlapped, H-2″′), 3.48 (1H, m, H-5″), 3.45 (1H, part. overlapped, H-2″), 3.32 (1H, overlapped with solvent signal, H-5″′), 1.24 (3H, d, J = 6.1 Hz, H-6″′), 1.19 (3H, d, J = 6.2 Hz, H-6″).

13C NMR (100 MHz, MeOD) δ: 196.9 (C-4), 166.06 (C-7), 165.06 (C-5), 163.87 (C-9), 159.53 (C-4′), 130.10 (C-2′/C-6′), 128.36 (C-1′), 116.46 (C-3′/C-5′), 103.86 (C-10), 102.30 (C-1″′), 99.61 (C-1″), 98.12 (C-6), 96.87 (C-8), 84.02 (C-2), 78.77 (C-3), 73.74 (C-5″′), 73.57 (C-5″), 72.15 (C3″′), 72.02 (C-3″), 71.74 (C-2″′), 71.62 (C-2″), 71.24 (C-4″′), 70.58 (C-4″), 18.04 (C-6″′), 17.85 (C-6″).

ESI-MS: m/z 603.4 [M+Na]+; m/z 579.5 [M−H]−.

3

3 Results and discussion

The phytochemical analysis of the ethanolic extract obtained from the flowers of G. cilentina revealed the presence in the less polar fractions of several flavonoid and isoflavonoid aglycones yet evidenced in this genus (Harborne, 1969) while the more polar fractions revealed the presence of glycosidic derivatives (Fig. 1). In particular the following were identified: genistein (1) (Boryski and Grynkiewicz, 2001), the main isoflavonoid, together with biochanin A (2) (Li et al., 2008), 4′-O-methylorobol (pratensein) (3) (Hanawa et al., 1991), while luteolin (4) (Burns et al., 2007) and apigenin (5) (Miyazawa and Hisama, 2003) were present in minor amount. In the previous study on this species only compound (4) was already evidenced while compounds (1), (2), (3) and (5) were recognized for the first time as constituents of G. cilentina, even if apigenin (5) is a quite ubiquitous flavonoid. In the present study were evidenced a major variety of flavonoids in aglycone form in respect of what observed before on this species (Noccioli et al., 2012). This result may be due to a selective accumulation of this kind of compounds in the anatomic structures of flowers. This condition has been recently showed in Achillea millefolium, where apigenin and luteolin, as aglycones, resulted mainly located in the flower heads while the apigenin glycosides were concentrated in the roots (Pedneault et al., 2014).

Secondary metabolites isolated from G. cilentina flowers.
Figure 1
Secondary metabolites isolated from G. cilentina flowers.

Compounds (2) and (3) have never been identified before in the genus Genista. In fact, they have been mainly recognized in other Fabaceae i.e. Astragalus membranaceus var. mongholicus (Bunge) P.K. Hsiao (syn. of Astragalus propinquus Schischkin) (Du et al., 2006), Astragalus verrucosus Moris (Pistelli et al., 2003), Glycyrrhiza uralensis Fisch. (Liu et al., 2013), Trifolium pretense L. (Lin et al., 2011), and Andira anthelmia (Vell.) J.F.Macbr. (da Silva et al., 2008) and may represent a molecular aspect of their botanical proximity. To the best of our knowledge, outside Fabaceae, the presence of (3) is reported only in a few families, i.e. Compositae: Eclipta prostrata (L.) L. (Lee et al., 2009); Amaranthaceae: Kochia scoparia (L.) Schrad. (syn. Bassia scoparia (L.) A.J.Scott) (Zhang et al., 2013); Iridaceae: Belamcanda chinensis (L.) DC. (syn. Iris domestica (L.) Goldblatt & Mabb.) (De-Eknamkul et al., 2011). Since compound (3) seems to have a very restricted distribution, its chemotaxonomical implication in the Fabaceae should be taken into account in further chemosystematic study on entities comprised in this family.

Besides isoflavones, it is worth to mention the conspicuous presence of isoliquiritigenin (6), a chalcone not yet recognized in G. cilentina, which was previously evidenced as the only synthesized compound in Genista tinctoria hairy root cultures (Luczkiewicz and Kokotkiewicz, 2005a). Compound (6) is also endowed with interesting biological activities, i.e. gastroprotective (Choi et al., 2015), antiviral (Z. Li et al., 2015) and counteracting the progression of Alzheimer disease (Link et al., 2015).

From the more polar fractions were isolated biochanin A 7-O-β-glucoside (7), already evidenced in the species (Noccioli et al., 2012), and a diglycosidic flavonoid (8), which showed the characteristic NMR signals pattern of a dihydroflavanonol moiety. The diagnostic signals evidenced from proton spectra indicate for (8) the structure of 3,7-bis-α-l-rhamnopyranosyl-aromadendrin (Fig. 1). The key signals are A2B2 system (two doublets at δ 7.38 and 6.87, J 8.6 Hz, both integrating for two protons) evidencing a para-substitution of the B-ring, two doublets at δ 6.19 and 6.22 (J = 2.1 Hz) relative to H-6 and H-8 of A-ring, while H-2 and H-3, both doublets, J = 11.0 Hz (relative trans-configuration), resonate respectively at 5.20 and 4.68 ppm. From the literature it is known that the cis-configuration of protons 2 and 3 in dihydroflavanonols (i.e. epitaxifolin and (+)-(2S,3R)-fustin) shows a coupling constant of about 3 Hz (Kiehlmann and Li, 1995; Park et al., 2000; Kim et al., 2015). This is also in accordance with “The Vicinal Karplus Correlation” which states that coupling between vicinal protons in rigid systems depends on dihedral angle. The protons in cis-configuration present a dihedral angle of about 30° which corresponds in the Karplus’s diagram to a coupling constant of about 3 Hz, while at 180° (which is a value quite near to the trans-configuration dihedral angle) corresponds to the higher value of J, which reaches 10–12 Hz (Silverstein et al., 1980). The relatively high value of the coupling constant between H-2 and H-3 observed for compound (8) is in accordance with this second case. The proton H-3, as well as the respective carbon C-3, is deshielded by the presence of the saccharidic unit at 0.15 and 6.1 ppm respectively in comparison with what observed in free aromadendrin. The observed signals are consistent with the structure of a substituted aromadendrin (Eun et al., 2003; Fossen and Andersen, 2006). The saccharidic portions were evidenced by the presence of two anomeric proton signals (doublets) δH 5.47 and δH 4.01 ppm, and the respective coupling constants (J 1.6 and 1.5 Hz) are consistent with a α-configuration of the two glycosidic linkages. Moreover the two doublets at δH 1.18 and 1.23 ppm (J ∼ 6.0 Hz), integrating both for 3 protons, are characteristics of H-6 of rhamnose units. Experiment on 13-C revealed the presence of signals characteristic of rhamnose units between 73.7 and 70.6, 18.8 and 17.8 ppm (Breitmaier and Voelter, 1990). On HMBC experiment, were showed correlations between the anomeric proton at 5.47 (δC 99.6 ppm) and C-7 (δC 164.6 ppm) of aromadendrin moiety, indicating the first glycosidic bond. The correct assignation of the resonance at 164.6 ppm to C-7 was supported by the presence of additional diagnostic long range correlation between both H-6 (6.22 ppm) and H-8 (6.19 ppm) with C-7 (164.6 ppm); H-6 correlates also with C-4 (196.9 ppm) and C-9 (163.9 ppm), while H-8 showed correlations with C-10 (103.8 ppm) and C-6 (98.1 ppm). In addition, a deshielding effect (∼0.3 ppm) due to glycosidic linkage in C-7 was observed for both H-6 and H-8 in respect of what reported in the literature for aromadendrin aglycone (Zheng et al., 2006; Venditti et al., 2013). The second glycosidic linkage resulted to be in 3 position and was confirmed by a correlation between H-3 (δH 4.68 ppm) and the second anomeric carbon C-1″′ (δC 102.1 ppm), and also between H-1″′ (δH 4.01 ppm) and C-3 (δC 78.8 ppm). The proton in 3 position correlates also with C-2 (84.0 ppm) and C-4 (196.9 ppm); the reverse correlation between H-2 and C-3 was also observed (δH 5.20 and δC 78.8, respectively). From COSY experiments the signals at 5.20 and 4.68 ppm resulted in the same spin system. The deshielding effect of glycosilation has little influence on chemical shift of H-3 (+0.2 ppm in respect to free aromadendrin), but the shift at lower fields was more evident in the carbon spectrum where C-3 resonates at 78.8 ppm (+7 ppm) (Zheng et al., 2006; Venditti et al., 2013). The shift at lower fields due to glycosilation in C-3 was already observed in Engeletin, a 3-O-glucopyranosyl aromadendrin derivative (Huang et al., 2011). The obtained NMR data for compound (8) are consistent with the proposed structure of 3,7-bis-α-l-rhamnopyranosyl aromadendrin.

Aromadendrin is a flavonoid not widespread in nature. It was isolated for the first time in the fruits of Persica vulgaris (Sadykov, 1977) and it was also found (in combination with other compounds) in different plant species such as Chionanthus retusus (Kwak et al., 2009), Citrus spp. (Wesolowska et al., 2012), Populus davidiana (Zhang et al., 2006) and recently from Olea europaea (Venditti et al., 2013). Considering the Fabaceae family, there are only a couple of reports in the literature on the isolation of aromadendrin from Spatholobus suberectus Dunn (Lee et al., 2006) and Akschindlium godefroyanum (Kuntze) H. Ohashi (Chaipukdee et al., 2014), resulting so a quite rare compound also in this family. The presence of two sugar moieties of rhamnopyranoside in the structure of (8) represents an additional peculiarity since compounds with this glycosylation pattern seem to be quite rare in plants and, till now, were evidenced only in Rhamnus disperma (Marzouk et al., 1999), Mimosa spp. (Yusuf et al., 2003), Randonia africana (Berrehal et al., 2010) and Arabidopsis thaliana (Onkokesung et al., 2014). From the more polar fractions was evidenced the presence of D-pinitol (9), which is considered a systematic marker in Fabaceae, in addition to huge amount of glucose and saccharose.

From a chemotaxonomic point of view, the observed flavonoidic pattern of G. cilentina resulted in accordance with that reported by Harborne for Genisteae tribe (Harborne, 1969), with the presence of isoflavones as main components, but, from this botanical entity, was also recognized peculiar compounds, i.e. (2), (3) and (8), which may be characteristic of this endemic species. In particular compound (8), present in conspicuous amount (1.85% on the crude extract), and which has never been evidenced before in other species, may be a useful chemotaxonomic marker at a specific level.

4

4 Conclusions

The phytochemical analysis of G. cilentina flowers revealed the presence of several flavonoid and isoflavonoid aglycones not evidenced before in the species as well as in the genus. These compounds may be selectively accumulated in flowers. Besides the isoflavonoid genistein, the principal taxonomical marker of the genus, it is worth to mention the presence of 4′-O-methylorobol. Since its restricted distribution outside the Fabaceae family, this compound may be an additional chemosystematic marker for the genus. Moreover, from the more polar fraction, was isolated a bis-rhamnosyl derivative of a rare flavanonol, aromadendrin, which resulted to be a new compound and also a characteristic molecular trait of this endemic species. In addition, considering the huge quantity of compound (8) isolated from the studied sample, this aromadendrin derivative may be a good candidate to be a systematic marker at a specific level.

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