Lipid and Phenolic Constituents from Seeds of Hypericum perforatum L. and Hypericum tetrapterum Fr. and their Antioxidant Activity

Article abstract of DOI:10.1002/cbdv.201700100

Seeds of Hypericum perforatum and H. tetrapterum were extracted with dichloromethane and methanol and investigated by chromatographic and mass spectrometric methods. Both species yielded a fatty oil fraction amounting to 30.5% and 18.0% of the seed weight, respectively. Linoleic acid (C18:2n-6) was shown to be the predominant fatty acid constituent. Moreover, xanthone derivatives, i.e. tetrahydroxyxanthones (THX), xanthone-glycosides and xanthone-sulfonates, were assigned in methanolic extracts. For structure elucidation, one representative xanthone, namely 1,3,6,7-THX, was synthesized and analyzed via HPLC-DAD/MSⁿ and GC/MS. Total THX contents were quantitated applying a validated HPLC-DAD method, resulting in 1.25 g/kg (H. perforatum) and 0.27 g/kg (H. tetrapterum), respectively. Moreover, the free radical scavenging capacity of the methanol extracts was tested using the DPPH antioxidant assay. Both, H. perforatum (IC₅₀ = 8.7 mg/l) and 1,3,6,7-THX (IC₅₀ = 3.0 mg/l), exhibited good DPPH free radical scavenging activity compared to Trolox (IC₅₀ = 6.6 mg/l).

Full text of DOI:10.1002/cbdv.201700100

Article Type: Full Paper
Lipid and Phenolic Constituents from Seeds of Hypericum
perforatum L. and Hypericum tetrapterum Fr. and their
Antioxidant Activity
Miriam Heinricha,b, Peter Lorenza, Rolf Danielsb, Florian C. Stintzinga and Dietmar R.
Kammerer*,a
a WALA Heilmittel GmbH, Department of Analytical Development & Research, Section Phytochemical Research, Dorfstraße 1,
73087 Bad Boll/Eckwälden, Germany, e-mail: dietmar.kammerer@wala.de
b Department of Pharmaceutical Technology, University of Tübingen, Auf der Morgenstelle 8, 72076 Tübingen, Germany
Seeds of Hypericum perforatum and H. tetrapterum were extracted with dichloromethane and methanol and investigated by
chromatographic and mass spectrometric methods. Both species yielded a fatty oil fraction amounting to 30.5 % and 18.0 % of
the seed weight, respectively. Linoleic acid (C18:2n-6) was shown to be the predominant fatty acid constituent. Moreover,
xanthone derivatives, i.e. tetrahydroxyxanthones (THX), xanthone-glycosides and xanthone-sulfonates, were assigned in
methanolic extracts. For structure elucidation, one representative xanthone, namely 1,3,6,7-THX, was synthesized and analyzed
via HPLC-DAD-MSn and GC-MS. Total THX contents were quantitated applying a validated HPLC-DAD method, resulting in 1.25
g/kg (H. perforatum) and 0.27 g/kg (H. tetrapterum), respectively. Moreover, the free radical scavenging capacity of the methanol
extracts was tested using the DPPH antioxidant assay. Both, H. perforatum (IC50 = 8.7 mg/L) and 1,3,6,7-THX (IC50 = 3.0 mg/L),
exhibited good DPPH free radical scavenging activity compared to Trolox (IC50 = 6.6 mg/L).
Keywords: Antioxidants • Fatty acids • St. John’s wort • Seeds• Xanthone derivatives
Introduction
The genus Hypericum includes more than 400 species, which are distributed all over the world [1; 2]. The most intensively studied
representative of this genus, Hypericum perforatum L. (St. John's wort, SJW), is widely used in phytomedicine e.g. for the
symptomatic treatment of mild and moderate depressive episodes [3-6]. Besides its antidepressant activity, several additional
pharmacological effects have been documented for H. perforatum, including among others antiviral, antibacterial and
woundhealing effects [7-10], which have continuingly stimulated investigations into H. perforatum in particular and other
Hypericum species in the past [1]. Belonging to the same genus, the perennial plant Hypericum tetrapterum Fr., also known as St.
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Peter’s wort, has been investigated less thoroughly. However, a broad spectrum of biofunctionalities, including antimicrobial
activity and antioxidant capacity, has been determined for crude methanol extracts prepared thereof [11; 12]
.
The aerial parts of Hypericum plants contain a wide spectrum of secondary metabolites, including acylated phloroglucinol
derivatives (e.g. hyperforin and adhyperforin), naphthodianthrones (e.g. hypericin and pseudohypericin), flavan-3-ols (e.g.
catechin) and further flavonoids (e.g. hyperoside, quercetin, amentoflavone and I3,II8 biapigenin) [13-15]. In contrast, subterraneous
parts of Hypericum received less attention, even though they are known to contain phenolic acids, flavonols, flavan-3-ols and
xanthones [16-18]. As interesting natural plant constituents, xanthones hold a distinct position in the phytochemistry and
pharmacology of Hypericum [19]. Specifically, several structural types, e.g. oxygenated, prenylated and glycosylated xanthones,
which were primarily found in roots of the plant, have been isolated from various Hypericum species [16; 20]
.
Only recently, novel polyprenylated tetraoxygenated xanthones have been detected in roots of H. monogynum. Additionally,
[24]
xanthone derivatives have also been extracted from suspended tissue cultures and calli [21-23]. Furthermore, Dias et al. (2000)
investigated the biosynthesis of oxygenated xanthones in H. androsaemum and obtained high levels of 1,3,5,6- and 1,3,6,7-THX.
[22]
Ferrari et al. (2006)
firstly isolated 1-hydroxy-5,6,7-trimethoxyxanthone and 3-O-methylpaxanthone from calli of H.
perforatum. Various biological properties have been attributed to xanthones in previous studies [25]. Antimicrobial, antioxidant [23]
as well as anti-inflammatory activities [16] have been described, besides other pharmacological effects such as an anticarcinogenic
[26], antitumor and neuroprotective potential [27]
.
Since data for seeds of both H. perforatum and H. tetrapterum are scant in literature [28; 29], the aim of the present study was to
systematically investigate the phytochemical profile of representative samples of seeds from both species, providing a solid basis
for pursuing future routine control testing. Plant seeds are generally known to contain a wide range of substances with
antioxidant activities, e.g. phenolic compounds such as hydroxylated derivatives of benzoic acids, coumarins, flavonoids or
lignans [30]. Consequently, seeds of Hypericum were comprehensively analyzed to complete the knowledge of phytochemical
constituents of Hypericum. In addition, the present study should lay the foundation for evaluating, whether a chemotaxonomic
differentiation of Hypericum species is possible, when focusing on the seed composition. For this purpose, we systematically
compared the seeds of the aforementioned Hypericum species with regard to their lipophilic compounds, their phenolic
secondary metabolites as well as their antioxidant activity.
Results and Discussion
For the comprehensive investigation of H. perforatum and H. tetrapterum seeds, a two-step extraction with solvents of different
polarities und extraction capacities, i.e. CH2Cl2 and MeOH, was performed. The extracts thus obtained were further analyzed via
HPLC-DAD-MSn and GC-MS. In this course, a fragmentation mechanism of 1,3,6,7-THX, both in ESI negative ionization mode and
also in EI mode, when analyzed as TMSi derivative, was proposed. For the comparison of extraction yields, THX were quantitated.
Moreover, the potential of methanolic seed extracts of H. perforatum and H. tetrapterum to scavenge free radicals was tested in
the DPPH antioxidant assay.
Lipophilic constituents of H. perforatum and H. tetrapterum seeds
Overall, seeds of H. perforatum and H. tetrapterum contain a substantial portion of a bright yellow fatty oil. Recovery of the latter
was performed applying a two-step extraction with dichloromethane resulting in yields of 30.5 % and 18.0 %, respectively. The
dichloromethane extracts were further analyzed after derivatization via GC-MS in order to obtain a comparative overview of the
lipophilic seed components. A series of saturated and unsaturated fatty acids were detected as methyl esters, ranging from
palmitic acid (C16:0) to linolenic acid (C18:3), whereas linoleic acid (C18:2) was found to be the predominant fatty acid of both
species. As deduced from relative peak areas, relative proportions of linoleic acid and linolenic acid for H. perforatum were
calculated as 43.2 % and 36.2 % and for H. tetrapterum as 44.1 % and 31.3 %, respectively. Relative peak areas of the detected
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fatty acids are summarized in Figure 1. Additionally, β-sitosterol, behenic acid (C22:0) as well as unidentified monoglycerides
were identified as minor components in seed oils of both investigated species.
Figure 1. Fatty acid profiles of H. perforatum and H. tetrapterum seeds illustrated as relative peak areas. Bars represent means ± standard deviation
(n=3).
Chromatographic analyses of methanolic seed extracts
For the determination of polar constituents Hypericum seeds were further extracted with methanol and subsequently analyzed
via HPLC-DAD-MSn. Figure 2 illustrates the characteristic UV-chromatographic profile of both species, recorded at 260 nm.
Moreover, the methanolic extracts were further evaluated via GC-MS, obtaining a comprehensive view of polar plant secondary
metabolites.
As can be deduced from Figure 3, a series of sugars and xanthones were identified. By comparing the mass spectra of the
respective signals with those of reference standards and with the mass spectra of the NIST database, the disaccharide sucrose
and the trisaccharide raffinose were identified. Hence, S1 was assigned to the sucrose-TMSi derivative and S5 to the raffinose-
TMSi derivative, respectively. The tetrahydroxyxanthones (X1-X3) detected by GC-MS were corroborated by LC-MS analyses
(Figure 2, Figure 3). For structure elucidation and comparison, 1,3,6,7-THX and 1,3,5,6-THX were synthesized according to
1
literature protocols, and, additionally, their structure was confirmed by their H- and 13C-NMR signals. The determination of the
THX configuration is demanding, since their UV data and also mass spectroscopic behavior are highly similar, due to their
identical molecular weight. Surprisingly, 1,3,5,6-THX was detected neither in the seeds of H. perforatum, nor in those of H.
tetrapterum. However, the synthesized 1,3,6,7-THX was congruent with X3 in its chromatographic, UV and mass spectrometric
behavior. Our findings are in contrast to previously published data, concerning the configuration of the two main THX so far
analyzed in Hypericum. One explanation for this differing xanthone profile might be the fact, that herbal and root materials were
analyzed in these previous works, whereas seeds were studied in the present investigations. Additionally, the configuration of
the THX was non-exhaustively proven by NMR experiments or reference standards in the past.
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Figure 2. Chromatographic profiles (HPLC-DAD) of methanolic seed extracts of a) H. perforatum and b) H. tetrapterum recorded at 260 nm. For
peak assignment cf.
Table 1
Moreover, retention indices were calculated for X1, X2 and X3, allowing a valuable comparison of signals under varying
laboratory conditions [31]. In brief, by using the respective homologous n-alkanes, eluting directly prior to and after the target
component, retention indices were determined as follows: 2813.0 (X1), 2887.3 (X2) and 2929.9 (X3). Expectedly, differences in
the fragmentation patterns of X3 in the EI-MS and ESI--MS modes were observed (Figure 4). A fragmentation mechanism of
1,3,6,7-THX, both in ESI negative ionization mode and also in EI mode, when analyzed as TMSi-derivative, was proposed (Scheme
1). As shown in Figure 4a), the molecular ion peak of the TMSi-derivative (m/z 548) was of very low intensity. Release of a methyl
group (loss of 15 Da) yielded a base peak fragment at m/z 533. Subsequently, the loss of a trimethylsilyl moiety (73 Da) followed
by another loss of a methyl group (15 Da) resulted in a minor fragment at m/z 445. Additionally, fragments at m/z 415 and 373
were detected as a result of a dimethyl (30 Da) and a subsequent methylidynesilane (42 Da) cleavage, respectively. In
comparison, in ESI--ionization mode, the molecular ion peak at m/z 259 was observed without any further derivatization, which is
in agreement with literature data [19]. This ion species revealed the successive losses of two CO molecules (28 Da). The resulting
observed mass spectrometric species at m/z 203 is supposed, without any further spectral evidence, to be a substituted
cycloheptatriene anion. Concurrently, being in accordance with a fragmentation pathway described in literature for monomeric
xanthone aglycones [32], the loss of a CO2 molecule (44 Da) followed by the release of a CO molecule (28 Da) occurred.
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Figure 3. Total ion chromatograms (GC-MS) of methanolic seed extracts recovered from a) H. perforatum, b) H. tetrapterum and c) synthetic
1,3,6,7-THX as a reference after silylation with BSTFA. Assignment: S1, sucrose-TMSi; S2-S4, saccharide-TMSi derivatives; S5, raffinose-TMSi; X1
and X2, THX-TMSi isomers; X3, 1,3,6,7-THX-TMSi.
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a)
b)
Figure 4. EI- and ESI negative-MSn mass spectra of compound X3. a) X3-TMSi (C25H40O6Si4; MW 548.92 Da), EI spectrum. The insert exhibits the
molecular ion [M]+ at m/z 548. b) X3 (C13H8O6; MW 260.20 Da), ESI -MSn data. The insert exhibits the UV spectrum.
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Scheme 1. Proposed MS fragmentation mechanism for 1,3,6,7-THX (X3). a) MS fragmentation (EI) for the TMSi derivative, b) MS/MS fragmentation
(ESI) corresponding to the mass spectral data of Figure 4.
Phenolic constituents of H. perforatum and H. tetrapterum seeds
HPLC-DAD-MSn analyses were performed for the determination of phenolic seed constituents. Assignment of individual
compounds was based on their UV/Vis spectral characteristics and mass spectrometric behavior in negative ionization mode.
Besides X1, X2 and X3, which were present as main compounds in both extracts, a range of xanthone derivatives were assigned
Table 1. Peak 1 revealed an [M-H]- ion at m/z 329 and a fragment ion at m/z 167, thus indicating the loss of an O-hexose moiety.
Taking together these findings with the UV maxima at 256 and 294 nm, this compound was identified as vanillic acid hexoside
[33]. This component was the only phenolic acid hexoside detected in both species. Compound 2 exhibited an [M-H]- ion at m/z
261, which may point to the occurrence of maclurin, being known as a precursor in xanthone biosynthesis [34]. However, spiking
experiments with a reference standard proved the extracts to be devoid of maclurin. Consequently, compound 2 was proposed
to be an isomer of 3’,4,4’,6-pentahydroxy-benzophenone (maclurin).
Peaks 3, 5, 8 and 10 revealed an [M-H]- ion at m/z 421 as well as product ions at m/z 331 ([M-H-90]-) and at m/z 301 [M-H-120]-
in MS2 experiments, thus showing the characteristic loss of a C-hexose moiety [32] and a base peak signal at m/z 259 , pointing to
mangiferin (1,3,6,7-THX-C-glucoside). These compounds only differed in their retention behavior and their UV spectra.
Astonishingly, spiking experiments with mangiferin as a reference compound unveiled both Hypericum extracts to be devoid of
mangiferin, since the retention behavior of the latter differed from those of the aforementioned components. Consequently,
these compounds were characterized as mangiferin isomers.
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[17]
Interestingly, although mangiferin was reported to represent the major xanthone-C-glycoside in roots of H. perforatum
and,
although mangiferin was found as a major xanthone in cell cultures of H. perforatum [35], we did not detect this compound in
seeds of both species. However, our findings are in accordance with the studies of Crocket et al. (2011) [16], who did not find
mangiferin in roots of H. perforatum and Dias et al. (2000) [24], who neither detected this compound in in vitro biomass of H.
androsaemum.
Peak 6 also showed an [M-H]- ion at m/z 421, but considerably differed in its fragmentation behavior. It revealed a fragment ion
[32]
at m/z 258 in the MS2 experiment, indicating the loss of a O-hexose moiety and was therefore assigned to THX-O-hexoside
.
Unfortunately, based on the present analytical data it was neither possible to verify the exact position of the glycosylation nor
the definite nature of the hexose moiety, but based on MS2 spectra, a distinction between O-glycosides and C-glycosides could
be made.
An [M-H]- ion at m/z 447 together with the subsequent losses of 90 and 30 Da in the MS2 spectrum as well as a base peak ion at
m/z 284 indicated peak 7 to be a trimethoxyxanthone-C-hexoside. Peak 20 also revealed an [M-H]- ion at m/z 447, however, the
further fragmentation with the loss of 146 Da (loss of a rhamnose moiety) resulted in a base peak fragment ion [M-H-146]- at
m/z 301. The compound was assigned to a hydroxy-trimethoxy-xanthone-O-rhamnoside. Peaks 9 and 12 exhibited [M-H]- ions
at m/z 391 and major fragment ions at m/z 331 [M-H-60]-, m/z 301 [M-H-90]-, m/z 273 [M-H-90-28]- and at m/z 258. It was
therefore assigned to a THX-C-pentoside. Based on their mass spectrometric behavior, compounds 11, 13, 16 and 24 were
tentatively identified as C-pentosides of trimethoxy-xanthone isomers [36]
.
Furthermore, according to their spectral data and fragmentation behavior, compounds 4 and 14 were identified as hydroxy-
trimethoxyxanthone-C-hexoside and hydroxy-trimethoxyxanthone-O-hexoside (molecular ions [M-H]- at m/z 463), respectively.
Detailed MSn data of compounds 31 and 32 are shown in Figure 5. Interestingly, compound 32 produced an [M-H]- ion at m/z
485, which revealed a product ion at m/z 323 (loss of an O-hexose moiety) in the MS2 experiment and an ion at m/z 243 (loss of
SO3) upon further fragmentation [26; 37; 38]. Compound 31 with a molecular ion [M-H]- ion at m/z 323 also showed a characteristic
loss of 80 Da, in this case in the MS2 experiment, resulting in a daughter ion at m/z 243. Hence, the compounds were assigned to
trihydroxyxanthone-sulfonate-O-hexoside and trihydroxyxanthone-sulfonate, respectively. Comparably, peaks 40 and 41 both
exhibited an [M-H]- ion at m/z 323, releasing a fragment ion at m/z 243 in the MS2 experiments. The loss of 80 Da was attributed
to the release of SO3, which is in agreement with the tentative assignment to trihydroxyxanthone-sulfonate isomers.
a)
b)
Figure 5. Fragmentation behavior of compound 31 a) and compound 32 b), both recorded applying the negative ionization
mode.
Moreover, compounds 23 and 39 both revealed molecular ions [M-H]- at m/z 337, releasing fragment ions at m/z 322 and 257,
respectively. The loss of 15 Da in the MS2 spectrum is referred to a methyl radical loss, forming an even product ion at m/z 322.
Additionally, the neutral loss of 80 Da upon fragmentation indicated the presence of a SO3 moiety, resulting in a fragment ion at
m/z 257. The subsequent loss of 15 Da in the MS3 experiment also indicated the loss of a methyl radical, releasing an even
product ion at m/z 242. The two compounds were, therefore, tentatively assigned to dihydroxy-methoxyxanthone-sulfonates.
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Compound 15 gave a deprotonated molecular ion [M-H]- at m/z 499. The MS2 spectrum was characterized by the loss of an O-
hexose moiety (162 Da) resulting in a fragment ion at m/z 337. Since the product ions observed in the MS3 experiment were
identical to those of compound 23, compound 15 was identified as dihydroxy-methoxyxanthone-sulfonate-O-hexoside. Up to
now, only two natural sulfonated xanthones, i.e. 1,3-dihydroxy-5-methoxyxanthone-4-sulfonate and 5-O- -D-glucopyranosyl-
1,3-dihydroxyxanthone-4-sulfonate, have been reported for Hypericum sampsonii [26]. These sulfonated xanthones exhibited
significant cytotoxicity against a mouse lymphocytic leukemia cancer cell line [26]. To the best of our knowledge, we herein report
the occurrence of sulfonated xanthones in Hypericum seeds for the first time. Compounds 26, 27 and 28 exhibited [M-H]- ions at
m/z 517 and fragments at m/z 259 in MS2 experiments, which is characteristic of THX dimers [18; 39]. Compounds 29 and 30
showed almost the same absorbance maxima in a range of 220 to 390 nm. Moreover, they predominantly showed doubly
deprotonated precursor ions [M-2H]2- at m/z 388 and a pseudomolecular ion [M-H]- of significantly lower abundance at m/z 777.
Consequently, these compounds were tentatively identified as trimers of THX. Producing an [M-H]- ion at m/z 273, compound
33 was assigned to a trihydroxymethoxyxanthone. Further compounds (17, 18, 19, 34, 35, 36, 37 and 38) were determined as
xanthone derivatives based on their UV-spectroscopic and mass spectrometric data, but their definite structure could not be
further characterized.
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Table 1. Assignment of characteristic compounds in methanolic seed extracts of H. perforatum and H. tetrapterum: HPLC data and mass spectrometric data, recorded in negative ionization mode: precursor ion shows [M-H]- species.
Peak
no.
tr
[min]
Peak
λmax
[nm]
MSn data [m/z]
Precursor ion
329.0
H. perforatum
H. tetrapterum
assignmenta
MS2
MS3
1
2
9.5
vanilic acid hexoside
214, 256, 294
166.7
230.7
151.7, 122.9, 107.9
212.7, 202.7, 186.7
x
x
x
x
11.8
pentahydroxy-benzophenone
212, 260sh, 324
260.8
3
15.9
16.5
16.6
18.1
18.2
18.5
18.6
18.8
19.6
20.7
20.8
21.5
21.7
mangiferin isomer
210, 256, 316, 364
212, 268, 350
421.0
463.0
421.0
421.0
447.0
421.0
391.0
420.9
417.0
391.0
417.0
463.0
498.9
330.9, 300.9, 258.8
409.0, 373.0, 343.0
330.9, 300.9
257.8
272.8, 257.8
x
x
x
n.d.
x
4
hydroxy-trimethoxyxanthone-C-hexoside
mangiferin isomer
325.0, 315.0, 300.0
272.8, 257.8
5
216, 256, 316, 362
214, 254, 328
n.d.
x
6
tetrahydroxyxanthone-O-hexoside
trimethoxyxanthone-C-hexoside
mangiferin isomer
228.7, 185.7, 173.7, 161.7
298.9, 283.9,
x
7
212, 254, 270sh, 332
218, 252, 278sh, 324
218, 256, 316, 362
222, 254, 278, 384
214, 268, 346
357.0, 326.9
330.9, 300.9
331.0, 300.9
330.9, 300.9, 257.8
381.0, 357.0, 327.0
331.0, 300.9
357.0, 326.9
300.9
x
n.d.
x
8
272.8, 257.8
x
9
tetrahydroxyxanthone-C-pentoside
mangiferin isomer
272.8, 257.8
n.d.
x
x
10
11
12
13
14
15
213.9, 201.7, 185.7, 157.8
298.9, 283.9
x
trimethoxyxanthone-C-pentoside
tetrahydroxyxanthone-C-pentoside
trimethoxyxanthone-C-pentoside
hydroxy-trimethoxyxanthone-O-hexoside
x
n.d.
x
216, 252, 284sh, 326
212, 264, 348
272.8, 257.8
n.d.
x
298.9, 283.9
n.d.
x
216, 256, 360sh
218, 258sh, 312sh
270.8, 178.7, 150.7
321.8, 257.7
n.d.
n.d.
dihydroxy-methoxyxanthone-sulfonate-O-
hexoside
336.9
x
16
17
21.9
22.6
trimethoxyxanthone-C-pentoside
212, 260sh, 346
417.0
401.0
356.9, 326.9
341.0, 310.9
298.9, 283.8
x
x
n.d.
n.d.
xanthone derivative
216, 266, 328
282.9, 267.8
18
19
20
X1
22.7
23.1
24.0
24.8
xanthone derivative
218, 256, 316, 362
218, 254, 276sh, 322
218, 258sh, 360
405.0
565.0
447.0
258.8
330.9, 300.9
272.8, 257.8
257.8
n.d.
x
x
n.d.
x
xanthone derivative
503.0, 463.0, 421.0
373.0, 300.9
hydroxy-trimethoxyxanthone-O-pentoside
tetrahydroxyxanthone
270.9, 178.7
185.7
n.d.
x
204, 222, 256, 282, 320sh,
392
241.8, 228.6
x
X2
25.6
tetrahydroxyxanthone
234, 252, 294, 328sh, 392
258.8
228.7
201.7, 172.7
x
x
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23
24
X3
26
27
28
29
30
31
32
33
34
26.4
26.8
28.3
30.9
31.5
32.8
32.8
33.1
33.2
33.9
35.0
36.1
dihydroxy-methoxyxanthone-sulfonate
trimethoxyxanthone-C-pentoside
1,3,6,7 tetrahydroxyxanthone
tetrahydroxyxanthone-dimer
tetrahydroxyxanthone-dimer
tetrahydroxyxanthone-dimer
tetrahydroxyxanthone-trimer
tetrahydroxyxanthone-trimer
trihydroxyxanthone-sulfonate
trihydroxyxanthone-sulfonate-O-glycoside
trihydroxy-methoxyxanthone
xanthone derivative
222, 248, 302
336.9
417.1
258.9
516.9
516.9
516.9
388.0a
388.0a
323.0
485.0
272.8
327.1
321.9, 256.8
357.0, 327.0
230.8, 214.8, 203.0, 186.9
498.9, 390.9, 258.8,
498.9, 390.9, 258.8
498.9, 258.8
298.9, 202.7
298,8
257.8, 241.7
289.9, 283.8
160.9
n.d.
x
x
n.d.
x
210, 258, 348
220, 254, 314, 362
x
218, 258sh, 282sh, 388sh
220, 260sh, 282sh, 390sh
218, 258, 288, 396sh
220, 260sh, 290, 390sh
220, 258, 290,382sh
220, 258, 292, 388sh
220, 256sh, 292sh, 390sh
220, 256, 292sh, 390sh
220, 258sh, 292sh, 394sh
362.9, 334.9
362.9, 334.9
228.7, 184.7
270.8, 254.8
270.8, 254.8, 226.8
198.9, 142.8
242.9
x
x
x
x
x
n.d.
x
n.d.
n.d.
x
x
242.9
x
323.0
x
x
257.7
229.7, 213.6, 172.7
210.8, 168.7
x
n.d.
x
309.1, 291.0, 273.0, 247.0,
228.9, 170.9
x
35
36
37
38
39
40
41
38.1
38.7
39.8
40.8
41.7
43.0
44.1
xanthone derivative
222, 260sh, 290, 324sh, 396
220, 260sh, 288sh, 394sh
220, 262sh, 288sh, 392sh
218, 260sh, 290sh, 392sh
220, 260sh, 290, 396sh
222, 250, 310
775.0
572.9
329.2
459.1
336.9
322.9
322.9
756.9, 623.0, 474.9
630.9, 604.9
473.0, 456.9, 376.9
124.8
x
x
n.d.
n.d.
n.d.
x
xanthone derivative
528.9, 500.9
293.0, 228.9, 170.8
417.0, 236.8
321.9, 256.8
242.8
xanthone derivative
x
xanthone derivative
176.8, 161.7
257.7, 241.7
196.7, 170.7, 153.8
190
x
dihydroxy-methoxyxanthone-sulfonate
trihydroxyxanthone-sulfonate
trihydroxyxanthone-sulfonate
x
x
n.d.
n.d.
x
222, 310sh
242.8
x
a Precursor ion is due to a [M-2H]2- ion species. Parent ions for the next fragmentation step are presented in italics; sh shoulder; n.d. not detected.
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Antioxidant potential of Hypericum seed extracts and individual phenolic compounds
In the present work, the radical scavenging activity of the methanolic extracts of H. perforatum and H. tetrapterum was evaluated
by means of the DPPH assay. Additionally, the radical scavenging capacity of 1,3,6,7-THX was investigated. The activity of the
synthetic antioxidant Trolox was used as a reference. The 1,1’-diphenyl-2-picrylhydrazyl (DPPH) assay is widely applied for the
determination of the antioxidant activity of either different plant extracts or individual compounds [40]. The assay is based on the
reduction of DPPH forming a colorless reaction product in the presence of a hydrogen donating antioxidant [41]
.
Being in accordance with literature data [42], Trolox revealed an IC50 value of 6.6 mg/L (Table 2). With an IC50 value of 8.7 ± 0.23
mg/L, the methanolic extract of H. perforatum was found to exhibit similar activity. Interestingly, H. perforatum exhibited an
approx. 3.5-fold higher radical scavenging capacity as compared to H. tetrapterum, which revealed an IC50 value of 31.9 ± 0.26
mg/L. These findings can be correlated with the concentrations of THX, which are illustrated in Figure 6, since the total THX
contents of H. perforatum amounted to 1.25 g/kg and of H. tetrapterum to 0.27 g/kg, respectively. With an IC50 value of 3.0 ±
0.14 mg/L, 1,3,6,7-THX showed the highest antioxidant activity. In summary, these outcomes underline that the contents of total
THX are directly proportional to the radical scavenging activity, although other reducing secondary plant metabolites may also
be involved in the effects observed.
Table 2. IC50 values of methanolic seed extracts of H. perforatum and H. tetrapterum, Trolox and 1,3,6,7-THX in the DPPH assay
No
1
Test sample
Trolox
Regression equationa
y = 7.503 x + 0.4132
y = 5.486 x + 2.101
y = 1.536 x + 1.056
y = 16.48 x + 0.3449
R2
IC50 (mg/L)b
6.6 ± 0.13
8.7 ± 0.23
31.9 ± 0.26
3.0 ± 0.14
0.9960
0.9935
0.9956
0.9998
2
H. perforatum
H. tetrapterum
1,3,6,7-THX
3
4
a y refers to the absorbance measured at 516 nm; x refers to the concentration of the test sample [mg/L]
bMeans of three separate sample preparations, each measured in duplicate (CV < 5%).
Figure 6. Contents of individual compounds (X1, X2, X3), calculated in mg THX per kg Hypericum seeds. Bars represent means ±
standard deviation (n=3).
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Table 3. Validation data for the external quantitation of THX derivatives using a standard solution of 1,3,6,7-THX
Precision
intraday [%]
Precision
interday [%]
Recovery
[%]
LODb
[µg/mL]
LOQc
[µg/mL]
Equation of calibration curvea
R2
y = 2.532x + 4.402
0.9991
0.91d/0.23e
0.81d/0.30e
99.88
0.08
0.27
a Detection wavelength λ = 260 nm, b Limit of detection calculated on the basis of a signal-to-noise ratio of 3:1; c Limit of quantitation calculated on
the basis of a signal-to-noise ratio of 10:1; d c = 10 µg/mL; e c = 100 µg/mL.
Conclusions
Altogether, the present study provides novel information about the phytochemical composition of seeds of H. perforatum and H.
tetrapterum. They are rich in fatty oil, consisting of a series of fatty acids, which are primarily unsaturated and with linoleic acid
being the main compound. For the first time, several xanthone derivatives, including sulfonated xanthones, have been reported
for seeds of Hypericum, while 1,3,6,7-THX was unambiguously identified within this compound class. Marked differences were
observed with regard to the profile and contents of xanthone derivatives, however, further analyses applying the methodology
presented in the present study are needed to allow a clear-cut chemotaxonomic differentiation of both species based on their
secondary metabolite profile. Methanolic seed extracts and 1,3,6,7-THX were investigated with regard to their radical scavenging
capacity, showing distinct antioxidant activity in the DPPH assay. The observed endogenous antioxidant activity was attributed to
the remarkable content of naturally occurring xanthone derivatives, which were present in both species, whereas amounts were
considerably higher in H. perforatum. Furthermore, since xanthones were found to constitute an antioxidant principle of
protecting plant cells from reactive oxygen species and also function as phytoalexins, helping host cells to impair pathogens [23]
,
we also propose this role for xanthones in Hypericum seeds. Due to the presence of xanthones, seeds might be protected
against oxidative stress and microbial infestation. Additionally, the results provide a basis for continuative studies in routine
investigations, including a higher number of samples, and also focusing on different varieties of Hypericum as well as covering
diverse cultivation areas, harvest years and dates. Moreover, the outcomes presented in this work could also direct to following
investigations with regard to the structural elucidation of further THX, which are under progress. Conclusively, the lipid fraction
as well as the phenolic fraction of Hypericum seeds might have promising potential in cosmetic and medicinal products.
Experimental Section
General section
Acetonitrile and formic acid of LC-MS grade were purchased from Sigma Aldrich (Steinheim, Germany). Purified water (0.055
µS/cm) was obtained from a Purelab Option-Q system (Elga Berkefeld GmbH, Celle, Germany). The reference standard
mangiferin was obtained from Phytolab GmbH (Vestenbergsgreuth, Germany), sucrose and raffinose pentahydrate were
obtained from Merck (Darmstadt, Germany). For GC-MS analyses, the silylation reagent N,O-bis(trimethylsilyl)trifluoroacetamide
(BSTFA) and the methylation reagent trimethylsulfonium hydroxide (TMSH) were purchased from Fluka (Buchs, Switzerland). For
the identification of fatty acids, a fatty acid methyl ester (FAME) reference mixture C16-C24 (Restek Corporation, Bellefonte, PA,
USA) was purchased and tert-butyl methyl ether (TBME) was obtained from Merck (Darmstadt, Germany). For antioxidant activity
testing, 1,1’-diphenyl-2-picryl-hydrazyl (DPPH) and 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid (Trolox) were
obtained from Sigma Aldrich (Steinheim, Germany) and Merck (Darmstadt, Germany), respectively.
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GC-MS analyses
[43]
GC-MS analyses of fraction I were carried out according to Lorenz et al. (2013)
with a Perkin-Elmer 500 gas chromatograph
coupled to a single quadrupole mass detector. Injection was performed with a volume of 1 µL and a split ratio of 30:1 at a
temperature of 250 °C. A Zebron ZB-5MS cap. column (60 m × 0.25 mm i.d. × 0.25 µm film thickness, Phenomenex, Torrance,
USA) was used. The flow rate of the carrier gas (He) was set at 1 mL/min. The temperature profile started at 100 °C, followed by a
linear temperature increase of 4 °C/min to 320 °C, with a final hold time of 30 min. The mass spectrometer was run in electron
ionization (EI) mode (70 eV). The software Turbomass (v.5.4.2, PerkinElmer Inc., MA, USA) was used for data acquisition and
processing. For determination of the fatty acid composition of H. perforatum and H. tetrapterum seeds, a Varian 450-GC system
equipped with a flame ionization detector (FID) and a mass detector (MS-240) (Agilent Technologies Inc., Palo Alto, CA, USA)
was used. Aliquots of 1 μL were injected at a temperature of 250 °C. The obtained fatty acid methylesters were separated on a
Zebron ZB-WAX-plus capillary column (30 m × 0.25 mm i.d. × 0.25 μm film thickness, Phenomenex, Torrance, CA, USA). The split
ratio was 10:1, and the flow rate of the carrier gas (He) was set at 1.4 mL/min. The temperature program of the column oven
started with 5 min hold time at 170 °C, subsequently an increase from 170 to 230 °C at 3 °C/min and a final hold time of 20 min
at 230 °C was chosen. The mass spectrometer was run in electron ionization (EI) mode (70 eV). The software Varian MS
Workstation (v. 6.9.2, Agilent Technologies) was used for data acquisition and processing. Compounds were identified via
reference standards and by comparison of their mass spectra with those of the NIST spectral database (NIST MS Search, v. 2.0,
2008).
RP-HPLC-(DAD)-ESI-MSn analyses
Chromatographic analyses were carried out on an Agilent 1200 HPLC system (Agilent Technologies Inc., Palo Alto, USA),
equipped with a binary pump, a micro vacuum degasser, an autosampler, a thermostatic column compartment and a UV/VIS
diode array detector. An XSelect CSH C18 reversed-phase column (100 × 2.1 mm i.d., 3.5 μm particle size; Waters, Wexford,
Ireland) was used at 25 °C and a flow rate of 0.34 mL/min. Detection of phenolic compounds was performed at 260 nm. The
mobile phase consisted of formic acid/water (0.1/99.9, v/v; eluent A) and formic acid/acetonitrile (0.1/99.9, v/v; eluent B). The
injection volume was 10 μL, and the gradient used was as follows: 0-3 min, 0-5 % B; 3-18 min, 5-20 % B; 18–35 min, 20-30 % B;
35-50 min, 30-55 % B; 50-60 min, 55-100 % B; 60-65 min, 100 % B; 65-70 min, 100-0 % B; 70–75 min, 0 % B. The LC system was
coupled to an HCTultra ion trap (Bruker Daltonik GmbH, Bremen, Germany) with an ESI source operating in the negative
ionization mode applying the following parameters: capillary voltage: +4000 V, dry gas flow (N2) 9.00 L/min with a capillary
temperature of 365 °C; nebulizer pressure 50 psi. Full scan mass spectra (mass range m/z 50-1000) of HPLC eluates were
recorded during chromatographic separation. The instruments were controlled by AgilentChemStation B.01.03 (Agilent,
Waldbronn, Germany) and esquireControl V 6.1 (Bruker Daltonik GmbH, Bremen, Germany) software. Chromatographic analyses
for quantitation purposes were performed on a Dionex Ultimate 3000 RSLC system (Thermo Fisher Scientific GmbH, Dreieich,
Germany) equipped with a diode array detector. Chromeleon software (v. 7.1, Dionex, Idstein, Germany) was used for system
control and data processing.
NMR spectroscopy
NMR spectra were recorded in DMSO-d6 at 500 (1H) and 125 MHz (13C), respectively, with a Varian Unity Inova NMR
spectrometer (Darmstadt, Germany). Chemical shifts are reported in δ [ppm] and referenced to residual solvent signals of
DMSO-d6 (1H: δ 2.50; 13C: δ 39.51 ppm). 1H- and 13C-NMR signal assignment of compound X3 is based on increment calculations
with a program of T. Vosegaard (Version 1.0, Aarhus University, Denmark). For evaluation of NMR spectra, the program
SpinWorks 3.1.7. (Copyright 2010, K. Marat, University of Manitoba, USA) was used.
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Plant material
Three different batches each of seeds of H. perforatum and H. tetrapterum were purchased from Jelitto perennial seeds GmbH
(Schwarmstedt, Germany). Both species were identified by Prof. Dr. O. Spring and voucher specimens (voucher number: HOH-
016142 (H. perforatum); HOH-016143 (H. tetrapterum)) were deposited at the herbarium of the Department of Botany at
Hohenheim University (Germany).
Extraction of H. perforatum and H. tetrapterum seeds
100 mL of dichloromethane (CH2Cl2) was added to 10 g of Hypericum seeds. The suspension was homogenized with an Ultra-
Turrax (3 min; 15,000 1/min, IKA Werke GmbH & Co. KG, Staufen, Germany). The first extraction step was performed at 7 °C
overnight under the exclusion of light. Afterwards, the plant material was filtered off over Celite by vacuum suction, washed with
CH2Cl2 (50 mL), and subsequently the remaining plant material was re-extracted under the same conditions as described above.
The two extracts were combined, and the solvent was removed in vacuo yielding fraction I, an oily liquid of 3.06 g (H.
perforatum) and 1.81 g (H. tetrapterum), respectively. A second extraction step was performed with methanol (MeOH). For this
purpose, the residue of the plant material was extracted twice with 100 mL of MeOH each at 7 °C overnight. Solvent removal by
vacuum rotovaporation yielded 0.32 g (H. perforatum) and 0.19 g (H. tetrapterum) of fraction II, respectively.
Sample preparation for chromatographic analyses
For comparison with authentic references, mangiferin, 1,3,5,6-THX and 1,3,6,7-THX were dissolved in methanol. For GC-MS
[43]
analyses, silylation of constituents was performed according to methods previously described in literature
to obtain the
corresponding trimethylsilyl derivatives. For this purpose, fraction II (6 mg) or purified compounds (1 mg) were dissolved in 0.5
mL of DMF and treated with 300 µL of BSTFA at 105 °C for 15 min and subsequently injected into the GC-MS system. For the
determination of retention indices, 50 µL of n-alkanes (C28, C29 and C30 in concentrations of 0.2 mg/mL each in chloroform)
were utilized [31]. Fatty acid methyl esters (FAME) were prepared by on-column derivatization with trimethylsulfonium hydroxide
(TMSH, 0.25 M in methanol). Briefly, 20 µL oil sample was diluted in 2000 µL TBME, 10 µL of the test solution were mixed with
170 µL of TBME followed by 60 µL of TMSH. Consecutively, the mixture was directly injected into the GC-MS system.
Quantitation of tetrahydroxyxanthones (THX) and method validation.
1,3,6,7-THX was used as standard compound for the quantitation of THX isomers. A calibration curve was generated from the
signals of six concentration levels (10-100 µg/mL) via linear regression analysis by plotting peak areas against sample
concentrations. For method validation, intraday precision was determined with standard concentrations of 10 and 100 µg/mL,
analyzed six times on the same day. The interday precision was evaluated by analyzing the samples on two different days,
respectively. Recovery was calculated using a spiked methanolic solution of fraction II (H. perforatum). The resulting
concentration was compared with those of the corresponding calibration standard (50 µg/mL) prior to spiking. The HPLC
method was proven to be reproducible, reliable and accurate According to the ICH guideline, limits of detection (LOD) and
quantitation (LOQ) were calculated based on signal-to-noise ratios of 3:1 and 10:1, respectively [44]. For this purpose, the means
of eight blank samples were compared with the signal of the lowest calibration sample concentration (10 µg/mL). Associated
validation data are presented in Table 3.
Stable free radical scavenging activity
The free radical scavenging activities of the methanol seed extracts were determined by the 1,1’-diphenyl-2-picrylhydrazyl
(DPPH) assay according to a protocol previously described [43]. Briefly, fraction II (5.0 mg for H. perforatum and 10.0 mg for H.
tetrapterum, respectively) was dissolved in 20 mL of MeOH for the preparation of stock solutions, which were appropriately
diluted. 200 µL of a range of concentrations were added each to 1800 µl of DPPH (100 µM) and incubated for 30 min at 37 °C.
Subsequently, absorbance (A516 nm) was measured using a UV-Vis spectrophotometer (Lambda 35, Perkin Elmer, Rodgau-
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Jüdesheim, Germany) equipped with a UV-Vis software (UV Winlab ES Version 6.0.2). DPPH scavenging activity was expressed as
IC50 values, which were calculated by plotting A516 nm against the corresponding concentrations and subsequent regression
analyses. Thereby, IC50 was defined as the concentrations of the tested compounds (mg/L) required, bringing about a 50%
reduction in absorbance referred to the blank. For comparison purposes, 1,3,6,7-THX and Trolox were used as a reference
antioxidants.
Synthesis of reference compounds
1,3,6,7-THX (X3) was synthesized according to a modified literature protocol [45]. Thus, K2CO3 (0.421 g, 3.05 mmol) was added to
a suspension of maclurin (2,3’,4,4’,6-pentahydroxy benzophenone, 0.40 g, 1.53 mmol) in 30 mL water under stirring and external
ice cooling. Afterwards, the solution was purged with nitrogen for 15 min and K3[Fe(CN)6] (1.00 g; 3.05 mmol), dissolved in 30 mL
deionized water and de-oxygenated in an analogous manner, was added dropwise, whereby the color turned into dark blue.
Then, the reaction mixture was stirred in an ice bath for 5 h, while the temperature was allowed to rise to room temperature.
After the addition of water (60 mL) the pH value was adjusted to 3.0 by adding glacial acetic acid and the solution extracted
twice with EtOAc (100 mL each). The combined extracts were dried (Na2SO4), and the solvent was distilled off by vacuum
rotovaporation to yield a crude orange residue (0.66 g). The target product was recovered from the latter by vacuum liquid
chromatography (VLC) on silica 60 G (SiO2, 100 g), preconditioned with CH2Cl2. Elution of X3 was performed with a
CH2Cl2/MeOH linear gradient (from 100/0 to 85/15, v/v), and the fractionation was monitored by TLC. The fractions containing
pure X3 were unified and evaporated to dryness in vacuo. Yield: 0.10 g (25.7 % of the theory). Yellowish powder. TLC (SiO2;
CHCl3/AcOH/H2O, 10/8.4/1.6, v/v/v): Rf 0.55 (single spot). GC-MS purity (TMSi derivative): 98.6 %, m/z 548 (1, M+), 533 (100, M-
1
CH3+), 445 (16), 415 (2), 373 (13), 343 (3), 259 (4), 215 (2), 147 (2), 73 (69, OSi(CH3)3+). H-NMR (DMSO-d6, 500 MHz): 13.19 (s,
4
4
1H, OH), 7.35 (s, 1H, H-C(8)), 6.83 (s, 1H, H-C(5)), 6.31 (d, J = 1.6 Hz, 1H, H-C(4)), 6.14 (d, J = 1.7 Hz, 1H, H-C(2)). 13C-NMR
(DMSO-d6, 125 MHz): 178.82 (C=O), 164.67 (C(3’)), 162.59 (C(3)), 157.30 (C(1)), 154.55 (C(6’)), 151.05 (C(6)), 143.90 (C(7)), 111.50
(C(7‘)), 107.78 (C(8)), 102.52 (C(5)), 101.54 (C(2‘)), 97.66 (C(4)), 93.58 (C(2)).
1,3,5,6-THX was synthesized according to Guo et al. (2015) [46]. The NMR signals were consistent with literature data [46]. The mass
spectrum of the TMSi derivative was identical to that of X3-TMSi.
Acknowledgements
The authors are grateful to Dr. Jürgen Conrad (Hohenheim University, Stuttgart) for NMR analyses and wish to thank Prof. Dr.
Otmar Spring (Hohenheim University, Stuttgart) for the taxonomy of plant specimens.
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