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Journal of the Chilean Chemical Society

versión On-line ISSN 0717-9707

J. Chil. Chem. Soc. vol.58 no.2 Concepción  2013

http://dx.doi.org/10.4067/S0717-97072013000200028 

 

ANTIOXIDANT ACTIVITY OF ANTHRAQUINONES ISOLATED FROM LEAVES OF Muehlenbeckia hastulata (J.E. SM.) JOHNST. (POLYGONACEAE)

 

MARCO MELLADOa*, ALEJANDRO MADRIDa, HUGO PEÑA-CORTÉSb, RODOLFO LÓPEZc, CARLOS JARAd, LUIS ESPINOZAa*

aDepartmento de Química, Universidad Técnica Federico Santa María. Av. Espana 1680, Valparaiso, Chile. e-mail: marco.mellado@postgrado.usm.cl
bMax-Planck-Institutfur Molekulare Pflanzenphysiologie WissenschaftsparkPotsdam-Golm. Am Muhlenberg 1 14476 Potsdam, Germany.
cDepartamento de Biología y Química, Facultad de Ciencias Naturales y Exactas, Universidad de Playa Ancha de Ciencias de la Educación.
Av. Leopoldo Carvallo 270, Playa Ancha, Valparaiso, Chile.
dLaboratorio de Radicales Libres, Facultad de Medicina, Universidad de Valparaiso, Av. Brasil 1560, Valparaiso, Chile.


ABSTRACT

Three anthraquinones, the well known emodin (3) and physcion (6) and the new anthraquinone glycoside emodin-8־β־D-idopyranoside (7) were isolated from leaves of Muehlenbeckia hastulata. The antioxidant activity of these compounds was measured using the DPPH assay; all three showed weaker antioxidant activity than gallic acid.

Keywords: Muehlenbeckia hastulata, Anthraquinones, Emodin, Physcion, Emodin-8-β-D-idopyranoside, 2D-NMR, DPPH assay, Gallic acid.


 

1. INTRODUCTION

It is well known that oxygen and nitrogen reactive species are produced in all mammalian cells as the result of normal cellular metabolism, and these species play different roles in normal physiological processes such as protection against pathogens, cellular signaling pathways and the regulation of vascular tone1.

The importance of antioxidant studies resides in the fact that all kinds of foods or medicinal plants containing phenolic compounds usually have high antioxidant activity, which means they may have positive effects on human health. These species play a crucial role in the development of different forms of damage in various human diseases which are connected with oxidative stress as cardiovascular and Alzheimer's diseases, atherosclerosis, and others1-4.

Muehlenbeckia hastulata (J.E. Sm) Johnst., commonly known as quilo, mollaca and voqui, has been used by ancient cultures of Peru, Argentina and Chile as a diuretic, hypotensive, antihemorragic, sedative, as well as for treating rheumatism and in compresses to relieve burns5-9. The chemical composition of the vegetative aerial parts of the plant indicates the usual presence of tannins6, flavonoids5, epicatechin5-6, rutin5 and emodin 8-glycoside5. Substances related to anthraquinones such as chrysophanic acid, rhein, hypericin and protohypericin are detected in root tissues5. The genus Muehlenbeckia accumulates different anthraquinone derivatives for example, Muehlenbeckia tamnifolia Meisn, contains emodin (3) and rhein (4), and Muehlenbeckia vulcanica Meisn. in DC., contains anthraquinone-O-glycosides (5)6.

A chloroform extract of the aerial parts of the plant may act against influenza virus at 0.125 mg/mL. An oxytoxic effect of the hexane root extract (62% of the effectiveness of phenoterol and papaverine) has also been described. Besides, hexane, dichloromethane and ethanol extracts of the aerial parts show an analgesic effect and weak antibacterial activity against Escherichia coli, Klebsiella pneumoniae, Salmonella aviatum, Salmonella aeruginosa, Staphylococcus aureus, Micrococcus flavus and Bacillus subtilis. Finally, recent studies of the antioxidant activity of Muehlenbeckia hastulata extracts report that the ethyl acetate extract from leaves has a high antioxidant activity (IC50 5.79 mg/mL) compared to gallic acid (10.52 mg/mL)5, 14-15.

In this paper, we report the isolation of three anthraquinones (two well know and one new glycoside) from the ethyl acetate (EtOAc) extract of the leaves of Muehlenbeckia hastulata as well as the assay of the DPPH· bleaching activity of these compounds.

Figure 1: Anthraquinone compounds isolated from genus Muehlenbeckia and leaves of Muehlenbeckia hastulata.

2. EXPERIMENTAL PROCEDURES

2.1 General

The melting points were measured on an Electrothermal Melt Temp instrument with a Fluke Digital thermometer. Optical rotations were measured at λ=589 nm (sodium D line) on a Perkin Elmer 241 digital polarimeter equipped with 1 dm cells at the temperature indicated for each case. UV spectra were recorded on an ATI Unicam UV/Vis UV-4 spectrophotometer from 200 to 400 nm, and the measurements were done in quartz cells with a 1 cm optical path using methanol as solvent. IR spectra (KBr) were obtained on a Thermo Scientific Nicolet 6700 FTIR spectrophotometer. 1H NMR, 13C NMR, 13C (DEPT 135), 1D-NOESY, 2D-HSQC and 2D-HMBC spectra were recorded on a Bruker Avance 400 Digital NMR spectrometer, operating at 400.13 MHz for 1H and 100.6 MHz for 13C, respectively. Chemical shifts are reported in δ (ppm downfield from the TMS resonance) and coupling constants (J) are given in Hz. The mass spectra were acquired using an Exactive mass spectrometer (Exactive Orbitrap LC-MS Thermo-Fisher). The spectra were recorded alternating between full-scan and all-ion fragmentation-scan modes, covering a mass range from 150 to 800 m/z. The resolution was set to 10000, with 10 scans per second, restricting the loading time to 100 ms. The capillary voltage was set to 3 kV with a sheath gas flow value of 60 and an auxiliary gas flow of 35 (values are in arbitrary units). The capillary temperature was set to 150 °C, whereas the drying gas in the heated electrospray source was set to 350 °C. The skimmer voltage was set to 25 V, whereas the tube lens was set to a value of 130 V. The spectra were recorded from 1 min to 2 min after the direct injection with a syringe at a flow between 5-10 mL/min. The samples were dissolved in chloroform (samples 6 and 8) or acetone (samples 3 and 7)11.

2.2 Plant material:

The plant material was collected above Valparaiso, Chile, at the geographical coordinates 33° 05' 45'' S - 71° 35' 17'' W at 460 meters above sea level in April 2010. A voucher specimen is kept in the Herbarium of the, "Dr. Herbert Appel A." Natural Products Laboratory, UTFSM, Valparaiso, Chile (MHJ-2011). The plant was classified by Rodrigo Villasenor, professor of Biology and expert in botany, considering the morphology of the material.

2.3 Extraction and isolation

Air dried and ground plant material (0.5 kg of leaves) was extracted twice with EtOAc (2 L) on a shaker for 24 h each time. The obtained extract was concentrated under vacuum to give a residue weighing 56.27 g. This extract was loaded on a silica gel (0.063 to 0.200 mesh) flash column and fractionated with a hexane - EtOAc gradient (0 to 100%), collecting 23 fractions. Fractions 10, 15 and 20 were positive for phenols in the FeCl3 test and for anthraquinone and analogous chromophores with NH3 vapor 12-13.

The isolation of compound 3 was carried out from fraction 15 (5.11 g). The purification was achieved by flash chromatography using silica gel (0.063 to 0.200 mesh) with a hexane - EtOAc gradient (1% to 60%) and in a second fractionation with a narrower hexane-EtOAc gradient (1% to 30%), giving small bright red needles (0.230 g).

Compound 6 was isolated from fraction 10 (0.135 g) which was subjected to flash chromatography on silica gel (0.063 to 0.200 mesh) with a hexane-EtOAc gradient (0.1% to 5%), giving small pale yellow needles (0.030 g).

Compound 7 was obtained from fraction 20 (1.45 g) by flash chromatography on silica gel (0.040 - 0.063 mesh) with an EtOAc-Me2CO gradient (1% to 15%), yielding small bright orange needles (1.03 g).

Compound 8 was prepared from 50 mg of compound 7 which was dissolved in 10 mL of CH2Cl2 with 2 mL of pyridine. A catalytic amount of DMAP and 1.5 mL of Ac2O were added and the solution was stirred for 1 h at room temperature. The work-up was done with a saturated aqueous solution of KHSO4. The organic layer was separated and dried with Mg2SO4 and subsequently filtered, evaporated and purified by flash chromatography on silica gel (0.063 to 0.200 mesh) with a hexane - EtOAc gradient (1% to 50%).

Chemistry Emodin (3).

MP: 259.9 ± 2.0 °C (Me2CO - H2O). [a]D18: -10.0° (c 0.001, Me2CO). IR: 3481, 2923, 2850, 1624, 1577, 1478, 1272, 1227 cm-1. UV/Vis λmax nm (log ε): 220 (4.44), 264 (4.15), 284 (4.20). 1H NMR (acetone d6): 12.17"(1H, s, OH-C1), 12.04 (1H, s, OH-C8), 7.54 (1H, s, H-C4), 7.23 (1H, d, J=2.4 Hz, H-C5), 7.11 (1H, s, H-C2), 6.64 (1H, d, J=2.4 Hz, H-C7), 2.45 (3H, s, H3C-C3). 13C NMR (acetone d6): 191.7 (C10), 182.1 (C9), 166.3 (C8), 166.3 (C6), 163.3 (C1), 149.5 (C3), 136.6 (C5a), 134.2 (C4a), 124.9 (C2), 121.5 (C4), 114.4 (C1a), 110.4 (C8a), 109.6 (C5), 108.8 (C7), 21.9(C3-CH3).MS (m/z (%)= 270 (17) [M]+, 269 (100), 239(3), 137 (15). HREIMS: m/z [M]+ calcd C15H10O5: 270.2369; found: 270.0490.

Physcion (6).

MP: 202.4 ± 1.5 °C (Me2CO - H2O). [a]D18: -45.0 ° (0.001, Me2CO). IR: 3583, 2927, 2846, 1756, 1676, 1596, 1433, 1368, 1230, 1193 cm-1. UV/ Vis Imax nm (log ε): 204 (3.95), 264 (4.02), 284 (3.45). 1H NMR (CDCl3): 12.33™(¾, s, OH-C1), 12.13 (1H, s, OH-C8), 7.63 (1H, s, H-C4), 7.37 (1H, d, J=2.4Hz, H-C5), 7.08 (1H, s, H-C2), 6.69 (1H, d, J=2.3Hz, H-C7), 3.94 (3H, s, CH3O-C6), 2.45 (3H, s, CH3-C3). 13C NMR (CDCl3): 190.6 (C10), 182.1 (C9), 166.6 (C6), 165.2 (C8), 162.5(C1), 148.5 (C3), 135.3(C4a), 133.2 (C5a), 124.5(C2), 121.3 (C4), 113.7(C1a), 110.3 (C8a), 108.2 (C5), 106.8 (C7), 56.1(C6-CH3), 22.2 (C3-CH3). MS (m/z %) =284 (5) [M]+, 283 (36), 223 (88), 197 (46), 196 (100), 181 (92). HREIMS: m/z [M]+ calcd for C16H12O5: 284.2635; found: 284.1256.

Emodin 8-b-D-Idopyranoside (7).

MP: 192.1 ± 2.0 °C (Me2CO - H2O). IR: 3386, 3132 , 2902, 1747, 1746, 1600, 1543, 1508, 1456, 1365, 1263, 1220 cm-1. UV/Vis λmax nm (log ε): 224 (4.43), 272 (4.25), 280 (4.28). MS (m/z %) = 431 (1) [M"+, 269 (13), 240 (16), 137 (100). HREIMS: m/z [M-H]+ calcd for C21H20O10: 432.3775; found:431.0960.

Emodin 8-b-D-Idopyranoside hexaacetate (8).

MP: 212.9 ± 1.8 °C (Me2CO - H2O). [a]D18: -45.0° (c 0.001, CHCl3). IR: 3086, 2925, 2853, 1750, 1674, 1612, 1596, 1433, 1226, 1204 cm-1. UV/Vis λmax nm (log ε): 204 (4.37), 256 (4.47), 264 (4.56), 280 (4.16). 1H NMR (CDCl 3) 7.94 (1H, s, H-C4), 7.75 (1H, d, J=2.3, H-C5), 7.27 (1H, d, J=2.1, H-C2), 7.21 (1H, s, H-C7), 5.51 (1H, dd, J= 8.8 / 8.8, H-C2'), 5.31 (1H, dd, J=9.5 / 9.5, H-C3'), 5.18 (1H, dd, J=9.7 / 9.7, HC5'), 5.13 (1H, d, J=7.9, H-C4'), 4.24 (2H, m, H-C6'), 3.93 (1H, m, H-C1'), 2.53 (3H, s, CH3-C3), 2.49 (3H, s, CH3-COO-C1), 2.35 (3H, s, CH3-COO-C6), 2.12 (3H, s, CH3-COO-C6'), 2.09 (3H, s, CH3-COO-C2'), 2.05 (6H, s, CH3-COO-C3' + CH3-COO-C4').13C NMR (CDCl3): 181.9 (C9), 179.5 (C10), 170.5 + 170.2 + 170.1 + 169.7 + 169.7 + 169.4 + 168.1 (C=O, OAc C6 + C1 + C2' + C3' + C4' + C6'), 158.0 (C1), 154.7 (C1a), 149.8 (C8), 145.6 (C6), 135.8 + 133.8 (C5a + C3), 130.9 (C7), 125.7 (C5), 124.0 (C8a), 121.5 (C4a), 116.2 (C2), 115.0 (C4), 99.8 (C1'), 72.6 + 72.3 (C3' + C4'), 70.3 (C2'), 68.2 (C5'), 61.9 (C6'), 21.6 + 21.1 + 21.0 + 20.8 + 20.6 (CH3-OAc C6 + C1 + C2' + C3' + C4' + C6'). MS (m/z %) = 707(7), 515(30), 467(49), 223(67), 196(31), 181(100). HRMS: m/z [M + Na] calcd for C33H32O16: 707.1610; found: 707.5874

2.4 DPPH ASSAY

The DPPH assay was performed as previously described by Brand-Williams16 with modifications as indicated by Lakic3 and Mellado15. A volume (0.1 mL) of the samples (from 0 to 50 mg/mL of the isolated compound) was mixed with 2.9 mL 50 μΜ DPPH* solution freshly prepared in ethanol. 2.9 mL 50 μΜ DPPH* solution with 0.1 mL ethanol was used as a control. The absorbances of the resulting solutions were recorded after 15 min at room temperature. Each sample was replicated three times. The disappearance of DPPH* was detected spectrophotometrically at 517 nm. Percent radical scavenging capacity (RSC) was calculated using the following equation:

From the obtained RSC (%) values the IC50 value, representing the concentrations of extracts that caused 50% bleaching, was determined by linear regression analysis.

3. RESULTS AND DISCUSSION

3.1 Structural elucidation of compounds 3, 6, 7 and 8 Emodin (3) was isolated as very small, bright red needles. The HR-EIMS exhibited a [M]+ peak at m/z 270.0490 corresponding to the molecular formula C15H10O5 which indicates eleven degrees of unsaturation. Further prominent peaks at m/z 269 and 137 represent the losses of [M-H] and [M-C8H7O2], respectively. The IR spectra showed the characteristic absorption of hydroxyl groups (3481, 1272 and 1227 cm-1), the conjugated carbonyl function (1624 cm-1) and an aromatic system (1577 and 1478 cm-1). The 1H NMR spectrum showed the presence of two chelated hydroxyl protons at dH 12.17 and 12.04 as well as the signature of a tetrasubstituted benzene ring with m-hydrogen atoms 7.23 (1H, d, J=2.4 Hz), 6.64 (1H, d, J=2.4 Hz). This compound exhibits a strong signal at dH 2.45 (s), corresponding to a methyl group bonded directly to an aromatic system. The 13C NMR and DEPT 135 spectra show the presence of two conjugated carbonyl groups at dC 182.1 and 191.7, four aromatic CH groups at dC 108.8, 109.6, 121.5, 124.9, eight aromatic quaternary carbon atoms at dC 110.4, 114.4, 132.2, 136.6, 149.5, 163.3 and 166.3 (2C) and a methyl carbon at dC 21.4. Comparison of the spectroscopic data with literature values confirmed the structure.

Physcion (6) was isolated as tiny, pale yellow needles. The HR-MSEI exhibited a [M-H]+ peak at m/z 283.1180 corresponding to the molecular formula C16H12O5 which indicates eleven degrees of unsaturation. Further pronounced peaks at m/z 223, 197, 196 and 181 represent the losses of [M-2H-CO-CH3O], [M-H-2CO-CH3O], M-2H-2CO-CH3O and [M-H-2CO-CH3O-CH3], respectively. The IR spectra showed characteristic absorption of hydroxyl group (3583, 1230, 1193cm-1), conjugated carbonyl (1676 cm-1), and aromatic system (1596 and 1433 cm-1). The 1H NMR spectrum shows two hydrogen that are forming intramolecular bond to dH 12.33 and 12.1, two doublet signals at dH 7.37 (1H, d, J=2.4Hz) and 6.69 (1H, d, J=2.3Hz). This indicates an aromatic system tetrasubstituted with m-orientation, however, other aromatic signals are displayed as broad singlets as 3. It also shows the existence of a signal from an aromatic methoxyl dH 3.94 and methyl aromatic dH 2.45. According to spectral similarity with substance (or compound) 3, we speculate that this compound may have a similar structure to compound 3. The spectrum of 13C NMR and DEPT 135 showed the existence of two carbonyl conjugated groups dC 190.6 and 182.1, four aromatic CH dC 124.5, 121.3, 108.2, 106.8, eight aromatic C dC 166.6, 165.2, 162.5, 148.5, 135.3, 133.2, 113.7, 110.3, one aromatic methoxyl dC 56.1 and one aromatic dC 22.2, which closely resembles the spectrum of emodin (3). The spectrum 2D-HMBC indicated that the methyl group is located between two m-coupled protons having a dH 7.37 and 6.69. The signals are simple to dH 7.63 and 7.08 and have interactions like the one presented by compound 3. Comparison of the spectroscopic data with literature values confirmed the structure.

Emodin 8-b-D-idopyranoside (7) was isolated as very small orange needles.

The HR-MSEI exhibited a [M-H]+ peak at m/z 431.096 corresponding to the molecular formula C21H20O10, which indicates twelve degrees of unsaturation. Further considerable peaks at m/z 269, 239 and 137 represented the losses of [M-C6H11O5], [M-2H-C6H11O5-CO] and [M-C6H11O5-C8H4O2], respectively.

The IR spectra showed characteristic absorption of hydroxyl group (3386, 1263, 1220 cm-1), carbonyl group (1747 and 1746 cm-1) and aromatic system (1600, 1543, 1508 cm-1). Because of the high polarity of compound 7 its acetylation was necessary for total structural elucidation.

Emodin 8-b-D-idopyranose hexaacetate (8) was isolated as a yellow amorphous solid. The HR-MSEI exhibited a [M+Na]+ peak at m/z 707.1610 corresponding to the molecular formula C33H32O16 which indicates 18 degrees of unsaturation, eleven corresponding to the anthraquinone system, one to the glycoside and six carbonyl groups attached to the hydroxyl groups of anthraquinone and carbohydrate. Further considerable peaks at m/z 515, 467, 223, 196 and 181 represented the losses of [M-4Ac+3H], [M-4Ac-H2O-CO+H], [M-5Ac-H2O-CO-C6H8O4-OAc+3H], [M-5Ac-CO-H2O-C6H8O4-OAc-CH2=CH+3H] and [M-5Ac-CO-H2O-C6H8O4-OAc-CH2=CH-CH3+3H], respectively. The IR spectra showed characteristic absorptions of C=C (3086 cm-1), carbonyl groups (1750 and 1674 cm-1), aromatic system (1612, 1596, 1433 cm-1) and C-O bond (1226 and 1204 cm-1). The 1H NMR spectrum shows four signals for the two aromatic anthraquinone subsystems. Unlike 3 and 6, the signals appeared as broad singlets, now redefined as doublets, partially overlapped (dH 7.27) by the solvent signal (CDCl3, dH 7.26). On the other hand, we can see typical sugar resonances between dH 5.51 and dH 3.93, indicating that there is an O-glycoside bond. The presence of a signal at dC 2.53 is also observed, corresponding to a methyl group bonded to an aromatic system, followed by six signals of acetate groups. The 13C spectrum and DEPT 135 clearly show two conjugated ketone carbonyl groups at dC 118 and 179.5, followed by six signals corresponding to the ester carbonyls from dC 170.5 to 168.1. Four of these signals are assigned to the carbohydrate and the other two to the anthraquinone portion. On going upfield, anthraquinone signals at dC 158.0 to 115.0 are detected. They are adjacent to the carbohydrate resonances at dC 99.8 to 61.9, corresponding to the anomeric carbon and a carbinol carbon. It ends with six signals of the acetate methyl groups. The definitive structure was determined by 2D-HMBC experiment (see Figure 2).

Figure 2: HMBC Correlations in emodin 8-β-D-idopyranoside hexaacetate (8).

The relative stereochemistry of the glycoside portion of 8 was determined by a NOESY experiment (See Figure 3). The H-1' proton δH 3.93) interacts with the protons resonating at δΙΙ 5.18 (positive phase) and at δΙΙ 4.24 (negative phase), H-5' H-6' respectively, indicating their axial spatial arrangement. The H-2' proton δH 5.51) has a reversed-phase interaction with H-3', which places them in an equatorial orientation. Finally, H-3' δH 5.13) is equatorial, because its NOE with H-5' is in negative phase.


Figure 3: NOESY experiment with spatial correlations of emodin 8-β-D-idopyranoside hexaacetate (8).

3.2 Antioxidant activity

The antioxidant activity of the compounds isolated from the EtOAc extract of this plant was measured by bleaching of the DPPH radical. Of the compounds tested (3, 6, 7 and 8), compound 6 (IC50 = 56.05 mg/mL) was the most active (see Figure 4).


Figure 4: Antioxidant activity of 3, 6, 7, 8 and GA (Gallic acid).

Comparing compound 7 (IC50 = 75.65 mg/mL) with compound 8 (IC50 = 104.63 mg/mL) the presence of the acetate groups is seen to have a negative effect in this assay, losing 28% of DPPH radical scavenging activity. An explanation to this phenomenon is the elimination hydrogen of hydroxyl groups. Besides, both compounds have a hydrogen at the anomeric position that explains their ability to bleach DPPH solution.

Moreover, comparing the activity of compounds 3 and 7, the glycoside is more active by 48.5%. An explanation of this phenomenon may be the donation of H- by the glycoside portion (anomeric hydrogen and phenolic hydrogen), leading to an increase of the DPPH radical bleaching.

Comparing all the results, they demonstrate that although compound 3 (IC50 = 112.32 mg/mL) contains more phenolic OH groups, it is less active than compound 6 (IC50 = 56.05 mg/mL). On the other hand, compound 7 (IC50 = 75.65 mg/mL) has more hydroxyl groups and shows stronger activity that compound 3. However activities of compound 3 and 8 (IC50 = 104.62 mg/mL) are similar. A possible explanation is that due to the formation of intermolecular hydrogen bonds between the hydroxyl group at C-6 and the carbonyl group at C -9 (see figure 5) the compound loses the ability to donate H-, causing activity to decrease by 49.9%.

It is noteworthy that catecholic anthraquinones such as alizarin have a high antioxidant capacity compared with emodin, (40% and 95% lipid peroxidation inhibition, respectively). Besides, alizarin and purpurin have a high DPPH bleaching capacity (20.55 and 72.96 μg/mL, respectively) 17, 18.


Figure 5: Postulated intermolecular hydrogen bonding of compound 3

However, all compounds tested displayed lower activity (4.5 to 9.1-fold) than gallic acid (IC50 = 12.28 mg/mL). This is due to the distribution of the OH groups, which are all at neighboring positions on the aromatic ring, allowing gallic acid to be oxidized to the corresponding quinone.

4. CONCLUSIONS

Three anthraquinones, emodin, physcion and emodin-8-β-D-idopyranoside from the leaves of Muehlenbeckia hastulata have been isolated and characterized by spectroscopic techniques (IR, NMR and MS). In order to determine the final structure of compounds 3 and 6 it was indispensable to perform 2D-HSQC and 2D-HMBC experiments. Due to the high polarity of compound 7 the acetylated derivative 8 was prepared, showing that it is an O-glycosylated anthraquinone. The anthraquinone skeleton of compound 8 was demonstrated by 2D-HSQC and 2D-HMBC experiments and the stereochemistry of the sugar was determined by a 1D-NOESY test.

Concerning the antioxidant activity assays, they show that compound 6 is the most active compared to the other analyzed compounds. Compound 3 may have weaker activity because of the formation of intermolecular hydrogen bridges which may inactivate its antioxidant property. Moreover, it has been established that 7 is more active than 3. The presence of the glycoside moiety increases x radical scavenging activity by 48.9% due to the capability osidic portion's ability to donate H- and form the oxidized compound.

 

ACKNOWLEDGMENTS

The authors thank the Dirección de Investigación y Postgrado (DGIP) of the Universidad Técnica Federico Santa Maria (grant N° 13.11.36 (2011-2012), PIIC QUI 2010 (M.M.) and PAC QUI 2011), Ursula Martinez Ortiz, Guillermo Díaz Fleming and Rodrigo Villasenor of Universidad de Playa Ancha de Ciencias de la Educación for technical support in Infrared spectrophotometry (CESPAM) and Alvaro Cuadros Inostroza of the Max-Planck Institute for Plant Molecular Physiology, Potsdam-Golm, Germany, for technical support in HR-MSEI.

 

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(Received: December 12, 2012 - Accepted: March 26, 2013).

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