Structural characterisation of flavonoids and flavonoid-O-glycosides extracted from Genista tenera by fast-atom bombardment tandem mass spectrometry

The genus Genista L.: A rich source of bioactive flavonoids

The genus Genista L. (family Fabaceae, subfamily Papilionoideae), with its cosmopolitan distribution, has attracted the human interest since ancient times, as it is used in folk medicine and mainly in the Mediterranean area for the treatment of respiratory diseases, rheumatic disorders, diabetes and ulcer, while it is also well known for its yellow pigment. The chemical composition of the Genista species revealed the presence of more than 108 flavonoids. Isoflavones, belonging to the group of phytoestrogens, are important secondary metabolites of the genus. The extracts of the Genista species may act as important source of bioactive phytochemicals for the treatment of many human ailments, mainly inflammation and pain, estrogen related pathology, hyperglycaemia, cancer and microbial infections. Therefore, the present review summarizes and discusses the flavonoid derivatives from the genus Genista, together with their structural features and pharmacological properties, aiming to highlight the recent advances in current knowledge on Genista species as a source of bioactive flavonoids.

Separation of catechin epimers by complexation using ion mobility mass spectrometry

Ion mobility coupled with mass spectrometry provides a fast and repeatable method to separate catechin epimers by previous complexation with selected chiral modifiers and transition metals. Several combinations with chiral ligands such as D- and L-amino acids and/or additional metal cations, chiral crown ethers, tartaric acid and heptakis(2,6-di-O-methyl)-β-cyclodextrin were screened for their ability to affect the separation efficiency. The clusters having the form of [2M + D-amino acid + Cu(2+) - 3H](-) (M stands for (-)-epicatechin or (+)-catechin) showed improvement in stereodifferentiation between two epimeric catechins in comparison to the analysis of pure epimers, where no separation was observed or the separation was hampered by the formation of mixed dimer complexes. Among various examined D-amino acids only those possessing hydrophobic side chains induced the improvement of separation efficiency. The best peak-to-peak resolution (Rp-p) was determined to be 0.71 for [2M + D-Leucine + Cu(2+) - 3H](-) clusters.

Electrospray ionization Fourier transform ion cyclotron resonance mass spectrometric and semi-empirical calculations study of five isoflavone aglycones

Five isoflavones, daidzein, genistein, formononetin, prunetin and biochanin A, known for their biological properties, are investigated by electrospray ionization mass spectrometry in the positive ion mode. The most probable protonation sites are determined taking into account semi-empirical calculations using the PM6 Hamiltonian. Fragmentation mechanisms are proposed based on accurate mass measurements, MS(3) experiments and supported by the semi-empirical calculations. Some of the fragmentation pathways were found to be dependent on the substitution pattern of the B-ring and the ions afforded by these fragmentations can be considered as diagnostic. It was possible to distinguish between prunetin and biochanin A, two isobaric isoflavone aglycones included in this study. Furthermore, a comparison of the fragmentation patterns of genistein and biochanin A, two isoflavones, with those of their flavone counterparts, apigenin and acacetin, enabled us to identify some key ions mainly due to structural features, allowing distinction to be made between these two classes of compounds.

Bioactivity studies and chemical profile of the antidiabetic plant Genista tenera

AIM OF THE STUDY

Genista tenera is a plant endemic to the island of Madeira and is used in folk medicine to control diabetes. In the present work we evaluate the antihyperglycaemic activity of its n-butanol extract and determine its chromatographic profile. In addition, this extract, the ethyl acetate and diethyl ether plant extracts were studied in order to assess the plant antioxidant and acetylcholinesterase inhibitory activities, as well as its cyto- and genotoxicities.

MATERIALS AND METHODS

HPLC-DAD-ESI-MS was used to analyze the flavonoid profile of the n-butanol extract. The antihyperglycaemic activity of this extract was performed over streptozotocin induced diabetic Wistar rats (200 mg/kg, bw/day), for 15 days. Antioxidant activity (DPPH assay) and acetylcholinesterase inhibitory effect (Ellman method) were also performed. Acute cytotoxicity and genotoxicity were assessed by proliferative index quantification and the short-term chromosomal aberration technique, after exposure of lymphocytes to the extracts.

RESULTS AND CONCLUSIONS

The n-butanol extract, where 21 monoglycosyl and 12 diglycosyl flavonoids were detected, significantly lowered blood glucose levels, bringing them to normal values after 15 days of treatment. The best radical scavenging activity was observed for the ethyl acetate extract (48.7% at 139.1 microg/mL), which was also the most effective one at the minimal concentration tested. The highest acetylcholinesterase inhibitory activity (77.0% at 70.0 microg/mL) was also obtained with the ethyl acetate extract. In vitro toxicity studies showed no evidence for acute cytotoxicity or genotoxicity. This is the first report on antidiabetic activity of genus Genista.

Analysis of flavonoids: tandem mass spectrometry, computational methods, and NMR

Due to the increasing understanding of the health benefits and chemopreventive properties of flavonoids, there continues to be significant effort dedicated to improved analytical methods for characterizing the structures of flavonoids and monitoring their levels in fruits and vegetables, as well as developing new approaches for mapping the interactions of flavonoids with biological molecules. Tandem mass spectrometry (MS/MS), particularly in conjunction with liquid chromatography (LC), is the dominant technique that has been pursued for elucidation of flavonoids. Metal complexation strategies have proven to be especially promising for enhancing the ionization of flavonoids and yielding key diagnostic product ions for differentiation of isomers. Of particular value is the addition of a chromophoric ligand to allow the application of infrared (IR) multiphoton dissociation as an alternative to collision-induced dissociation (CID) for the differentiation of isomers. CID, including energy-resolved methods, and nuclear magnetic resonance (NMR) have also been utilized widely for structural characterization of numerous classes of flavonoids and development of structure/activity relationships.The gas-phase ion chemistry of flavonoids is an active area of research particularly when combined with accurate mass measurement for distinguishing between isobaric ions. Applications of a variety of ab initio and chemical computation methods to the study of flavonoids have been reported, and the results of computations of ion and molecular structures have been shown together with computations of atomic charges and ion fragmentation. Unambiguous ion structures are obtained rarely using MS alone. Thus, it is necessary to combine MS with spectroscopic techniques such as ultraviolet (UV) and NMR to achieve this objective. The application of NMR data to the mass spectrometric examination of flavonoids is discussed.

A predictive tool for assessing (13)C NMR chemical shifts of flavonoids

Herein are presented the (1)H and (13)C NMR data for seven monohydroxyflavones (3-, 5-, 6-, 7-, 2'-, 3'-, and 4'-hydroxyflavone), five dihydroxyflavones (3,2'-, 3,3'-, 3,4'-, 3,6-, 2',3'-dihydroxyflavone), a trihydroxyflavone (apigenin; 5,7,4'-trihydroxyflavone), a tetrahydroxyflavone (luteolin; 5,7,3',4'-tetrahydroxyflavone), and three glycosylated hydroxyflavones (orientin; luteolin-6C-beta-D-glucoside, homoorientin; luteolin-8C-beta-D-glucoside, vitexin; apigenin-8C-beta-D-glucoside). When these NMR spectra are compared, it is possible to assess the impact of flavone modification and to elucidate detailed structural and electronic information for these flavonoids. A simple predictive tool for assigning flavonoid (13)C chemical shifts, which is based on the cumulative differences between the monohydroxyflavones and flavone (13)C chemical shifts, is demonstrated. The tool can be used to accurately predict (13)C flavonoid chemical shifts and it is expected to be useful for rapid assessment of flavonoid (13)C NMR spectra and for assigning substitution patterns in newly isolated flavonoids.

LC-MSn methods for saccharide characterization of monoglycosyl flavonoids using postcolumn manganese complexation

A simple tandem mass spectrometry method for differentiating isomeric monoglycosyl flavonols, flavones, and flavanones using manganese complexation is reported. Dissociation of the [Mn(II) (L) (L - H)]+ and [Mn(II) (L)2 (L - H)]+ species provides unique fragment ions that allow the identification of the saccharide moiety as glucose, galactose, arabinose, or xylose. The glycosylation site of the flavonoid can also be determined by the fragmentation pathways of the Mn complexes. The Mn complexation method was adapted for on-line liquid chromatography-mass spectrometry analysis and tested using flavonoid extracts from Fuji apples (Malus domestica Borkh. cv. Fuji) and red onions (Allium cepa L.). Using fragmentation data obtained from collisional activated dissociation of the deprotonated flavonoid glycosides and their Mn complexes, the major flavonoid species in these extracts were identified.

Fragmentation study of a 8-C-glycosyl isoflavone, puerarin, using electrospray ion trap time-of-flight mass spectrometry at high resolution

A mass spectrometry method that combines electrospray with an ion trap time-of-flight mass analyzer has been used to characterize puerarin (7,4'-dihydroxyisoflavone-8-C-beta-D-glucoside). MS(n) spectra (n <or= 6) were obtained in positive and negative ion mode. The combination of accurate mass measurement in MS(2) spectra and sequential MS(n) experiments enabled fragmentation pathways to be elucidated in detail. A novel structure for [M+H-150]+ has been found.

A combined nuclear magnetic resonance and computational study of monohydroxyflavones applied to product ion mass spectra

A method is presented for the estimation of 13C-chemical shifts for carbon atoms in protonated and deprotonated molecules; in principle, this method can be applied to ions in general. Experimental 13C-chemical shifts were found to vary linearly with computed atomic charges using the PM3 method. Pseudo-13C-chemical shifts for atoms in protonated and deprotonated molecules can be estimated from computed atomic charges for such atoms using the above linear relationship. The pseudo-13C-chemical shifts obtained were applied to the rationalization of product ion mass spectra of protonated and deprotonated molecules of flavone and 3-, 5-, 6-, 7-, 2'-, 3'-, and 4'-hydroxyflavones, where product ion formation is due to either cross-ring cleavage of the C-ring (retro-Diels-Alder reaction) or to cleavage of a C-ring bond followed by loss of either a small neutral molecule or a radical. The total product ion abundance ratio of C-ring cross cleavage to C-ring bond cleavage, gamma, varied by a factor of 660 for deprotonated monohydroxyflavones, i.e., from 0.014:1 to 9.27:1. The magnitude of gamma, which is dependent on the relative bond orders within the C-ring of the protonated and deprotonated molecules of monohydroxyflavones, can be rationalized on the basis of the magnitudes of the 13C- and 1H-chemical shifts as determined by nuclear magnetic resonance spectroscopy.

Fragmentation of 3-hydroxyflavone; a computational and mass spectrometric study

In a recent study of the collision-induced dissociation of protonated and deprotonated molecules of 3-, 5-, 6-, 7-, 2'-, 3'- and 4'-hydroxyflavone, it was observed that the ratio, gamma, of the propensities for cross-ring cleavage (CRC) to ring opening (RO) varied by a factor of 660, i.e., from 0.014:1 (for deprotonated 3-hydroxyflavone) to 9.27:1 (for deprotonated 5-hydroxyflavone). An explanation for the variation of gamma was presented in terms of experimental NMR (13)C and (1)H spectra modified by computation. Deprotonated 3-hydroxyflavone exhibited the highest ion signal intensity for fragmentation following RO relative to that for CRC in that gamma = 0.014:1. Because no chemical computations of the fragmentation of protonated and deprotonated monohydroxyflavone molecules have been carried out thus far, the basis on which fragmentation mechanisms for deprotonated 3-hydroxyflavone have been proposed is principally chemical intuition. The energy states of product ions formed in the fragmentation of deprotonated 3-hydroxyflavone molecules were computed by the CBS-4M method implemented in Gaussian 03. The overly demanding calculations needed to handle diradicals reliably and directly were circumvented by a process in which each of the bond-breaking reactions was approximated by a process that gave two monoradicals. Bond energies were calculated, with one exception, from the approximation reactions as the energy of products minus the energy of reactants.

A fragmentation study of isoflavones in negative electrospray ionization by MSn ion trap mass spectrometry and triple quadrupole mass spectrometry

This study has elucidated the fragmentation pathway for deprotonated isoflavones in electrospray ionization using MS(n) ion trap mass spectrometry and triple quadrupole mass spectrometry. Genistein-d(4) and daidzein-d(3) were used as references for the clarification of fragment structures. To confirm the relationship between precursor and product ions, some fragments were traced from MS(2) to MS(5). The previous literature for the structurally related flavones and flavanones located the loss of ketene (C(2)H(2)O) to ring C, whereas the present fragmentation study for isoflavones has shown that the loss of ketene occurs at ring A. In the further fragmentation of the [M-H-CH(3)](-*) radical anion of methoxylated isoflavones, loss of a hydrogen atom was commonly found. [M-H-CH(3)-CO-B-ring](-) is a characteristic fragment ion of glycitein and can be used to differentiate glycitein from its isomers. Neutral losses of CO and CO(2) were prominent in the fragmentation of deprotonated anions in ion trap mass spectrometry, whereas recyclization cleavage accounted for a very small proportion. In comparison with triple quadrupole mass spectrometry, ion trap MS(n) mass spectrometry has the advantage of better elucidation of the relationship between precursor and product ions.

Capillary electrophoresis-mass spectrometry characterisation of secondary metabolites from the antihyperglycaemic plantGenista tenera

Genista tenera is endemic to the Portuguese island of Madeira, where an infusion of the aerial parts of the plant is used in folk medicine as an antidiabetic agent. Consequently the medicinal properties of the secondary metabolites of this plant have been the subject of an ongoing study. A recently reported LC-MS method using a 100 min separation allowed identification of five flavonoid components in an extract of the aerial parts of this plant. In order to obtain additional information on the range and complexity of the plant's secondary metabolite components a CE-MS method has been developed and applied for the analysis of an extract of G. tenera. Twenty-six different components are distinguished in an analysis time of only 10 min. Results demonstrate that CE-MS/MS rapidly generates data complementary to those obtainable by LC-MS/MS and is particularly suited to the analysis of plant metabolites where concentration is not limiting.

Liquid chromatography–diode array detection–electrospray ionisation mass spectrometry/nuclear magnetic resonance analyses of the anti-hyperglycemic flavonoid extract of Genista tenera

The anti-hyperglycemic flavonoid extract obtained from Genista tenera was first studied by liquid chromatography (LC)-diode array detection (DAD) which showed the presence of two major compounds. One of them was identified as genistein-7-O-glucoside. Luteolin-7-O-glucoside was detected as a minor constituent, while luteolin-7,3'-di-O-glucoside and rutin were found in trace amounts. LC-DAD-ESI-MS and NMR were used to confirm the structure of these compounds and allowed the elucidation of the structure of the unknown major compound, which is the flavonoid 5,7,4'-trihydroxyisoflavone-8-C-glucoside.

Quantitative determination and structural characterization of isoflavones in nutrition supplements by liquid chromatography–mass spectrometry

In this paper, an accurate and route method was developed to quantitative determine daidzein, genistein, glycitein, daidzin, glycitin, 6"-O-acetyldaidzin, 6"-O-acetylglycitin and 6"-O-acetylgenistin contents in selected high and low isoflavones in nutrition supplements by on line liquid chromatography-atmospheric pressure chemical ionisation mass spectrometry (LC-APCI-MS). Improved extraction and hydrolysis methods of the isoflavones from three nutrition supplements were also studied and a rapid extraction method was developed. Comparison of different MS2 and MS3 spectra of isoflavones and some unknown compounds were also explored and proposed pathway fragments of nine isoflavones were first systematically suggested.

A phytochemical study of the quinolizidine alkaloids from Genista tenera by gas chromatography-mass spectrometry

Gas chromatography coupled with mass spectrometry has been used to analyse the alkaloids present in the aerial parts of Genista tenera. Anagyrine, cytisine, N-formylcytisine, N-methylcytisine and lupanine were the major compounds, the last two alkaloids being known for their hypoglycaemic activity. Dehydrocytisine, 5,6-dehydrolupanine, rhombifoline, aphylline and thermopsine were the minor alkaloids. The characterisation of the constituents was based on comparison of their Kovats retention indexes and electron impact-mass spectrometric data recorded on-line with those of reference compounds and literature data.

Determination of the glycosylation site of flavonoid monoglucosides by metal complexation and tandem mass spectrometry

Metal complexation and tandem mass spectrometry were used to differentiate C- and O-bonded flavonoid monoglucoside isomers. Electrospray ionization of solutions containing a flavonoid glycoside and a metal salt led to the generation of the key [M(II) (L) (L-H)](+) complexes, where M is the metal ion and L is the flavonoid glycoside. Thirteen flavonoid monoglucosides were examined in combination with Ca(II), Mg(II), Co(II), Ni(II), and Cu(II). Collisional activated dissociation (CAD) of the [M(II) (L) (L-H)](+) complexes resulted in diagnostic mass spectra, in contrast to the CAD mass spectra of the protonated, deprotonated, and sodium-cationized flavonoid glucosides. Five common sites of glycosylation could be predicted based on the fragmentation patterns of the flavonoid glucoside/magnesium complexes, while flavonoid glucoside/calcium complexes also were effective for location of the glycosylation site when MS(3) was employed. Cobalt, nickel and copper complexation had only limited success in this application. The metal complexation methods were also applied for characterization of a flavonoid rhamnoside, and the dissociation pathways of the metal complexes indicate that flavonoid rhamnosides have distinctive dissociation features from flavonoid glucosides.

Flavonoids of an extract of Pterospartum tridentatum showing endothelial protection against oxidative injury

Pterospartum tridentatum is a Leguminosae that grows spontaneously in Portugal. The flowers are used in popular medicine for the treatment of throat irritation conditions and in herbal mixtures for diabetes. Diabetic vascular complications are due, among other reasons, to increased oxidative stress and for that reason antioxidants are believed to be beneficial for the diabetic patient. The flower water extract of this herbal drug showed dose-dependent protective effect of cultured human endothelial cells against oxidative injury induced by H2O2, at concentrations > or =0.3 mg/ml. This water extract, after liquid-liquid and chromatographic fractionation afforded one new isoflavone (5,5'-dihydroxy-3'-metoxi-isoflavone-7-O-beta-glucoside) and three other known isoflavones (prunetin, genistin and sissotrin). The structural characterisation of isolated compounds was achieved by UV, NMR and MS analysis. The flavonol glycoside isoquercitrin was also identified in the extract by HPLC analysis. Isoquercitrin is one of the active antioxidant principles of the extract since it showed dose-dependent protective effect against oxidative injury at concentrations > or =0.3 mM. Isoflavones were inactive at the same concentrations. The results suggest that the water extract of this herbal drug may prevent or reduce the development of diabetic vascular complications.

Structural elucidation of natural 2-hydroxy di- and tricarboxylic acids and esters, phenylpropanoid esters and a flavonoid from Autonoë madeirensis using gas chromatographic/electron ionization, electrospray ionization and tandem mass spectrometric techniques

The chemical composition of Autonoë madeirensis bulbs was characterized as part of a systematic phytochemical study of this species. The compounds reported were mainly identified on the basis of gas chromatography/electron ionization, electrospray ionization and tandem mass spectrometry. The structures of the pure compounds were also characterized by means of other physical and spectroscopic data (m.p., IR, UV, NMR). The compounds identified were 2-hydroxy di- and tricarboxylic acids and esters (malic acid, citric acid and their methyl and ethyl esters), cis- and trans-hydroxycinnamic esters (methyl and ethyl p-coumarate and methyl ferulate) and a new flavone diglucoside, 7-O-[beta-glucosyl-(1-->2)-O-beta-glucosyl]apigenin, the interglucosidic linkage (1-->2) of which is, to the best of our knowledge, reported for the first time in a diglucoside of apigenin. The results may contribute to the chemotaxonomy of the Autonoë genus and lead to a rapid tool for the systematic characterization of these compounds in plant extracts.

Stereospecific fragmentations in the mass spectra of stereoisomeric isoindoloquinazolines

The mass spectrometric behavior of stereo- and regioisomeric, partially saturated isoindoloquinazolines was studied by positive-ion electron ionization (EI) and fast-atom bombardment (FAB/LSIMS) mass spectrometry combined with collision-induced dissociation (CID). A highly stereospecific retro-Diels-Alder process was observed in the cyclohexene-fused isomers under the EI conditions, and a corresponding (although less specific) fragmentation was observed in their FAB spectra. In the absence of RDA fragmentations, regio- and stereoisomers of the cyclohexane-fused heterocycles could be distinguished based on their FAB/CID spectra.

Differentiation of isomeric flavone/isoflavone aglycones by MS2 ion trap mass spectrometry and a double neutral loss of CO

The fragmentation behaviour of seven pairs of isomeric flavone/isoflavone aglycones (solely hydroxylated and/or methoxylated) was studied using ion trap mass spectrometry with atmospheric pressure ionisation (API, both electrospray and APCI) in the positive and negative ion modes. A major difference was found in the neutral loss of 56 u, which was a common feature of all isoflavones in API(+). It was identified as a double loss of CO by accurate mass tandem mass spectrometric (MS/MS) measurements using a hybrid quadrupole time-of-flight (Q-TOF) instrument. Fragmentation of daidzein with (13)C-isotope labelling of the carbon C2 showed that this double loss occurred from the central ring of the molecule. A mechanism for this selective fragmentation is given. Further isoflavone-specific fragmentations were used to develop a guideline for the identification of isoflavone structures. A software-based neutral loss scan of 56 u in the API(+)-MS(2) mode was applied to extracts of leaves of Lupinus albus and to soy flour. The structure elucidation guideline allowed identification of hydroxy and/or methoxy isoflavones. Structures could be confirmed for those available as reference compounds.

Review of the methods used in the determination of phytoestrogens

Interest in analytical methods for plant estrogens (phytoestrogens) has risen sharply in the past 10 years. In this review, we examine the existing analytical methods based on separations by gas-liquid chromatography, high-performance liquid chromatography and capillary electrophoresis in addition to methods of detection by ultraviolet absorption, fluorescence, electrochemical oxidation/reduction and mass spectrometry. These methods are compared with other methods of phytoestrogen analysis utilizing immunoassay approaches. The advantages and disadvantages of each of these methods are highlighted and potential areas for further development identified.