RevistaBrasileiradeFarmacognosia25(2015)462–464
w w w. s b f g n o s i a . o r g . b r / r e v i s t a
Original
Article
Cucumin
S,
a
new
phenylethyl
chromone
from
Cucumis
melo
var.
reticulatus
seeds
Sabrin
R.M.
Ibrahim
a,b,∗,
Gamal
A.
Mohamed
c,daDepartmentofPharmacognosyandMedicinalChemistry,CollegeofPharmacy,TaibahUniversity,AlMadinahAlMunawwarah,SaudiArabia bDepartmentofPharmacognosy,FacultyofPharmacy,AssiutUniversity,Assiut,Egypt
cDepartmentofNaturalProducts,FacultyofPharmacy,KingAbdulazizUniversity,Jeddah,SaudiArabia dDepartmentofPharmacognosy,FacultyofPharmacy,Al-AzharUniversity,AssiutBranch,Assiut,Egypt
a
r
t
i
c
l
e
i
n
f
o
Articlehistory:
Received29May2015 Accepted30June2015 Availableonline21July2015
Keywords: Cucumismelo
Cucurbitaceae Chromone Flavonoid Antioxidant
a
b
s
t
r
a
c
t
A new phenylethyl chromenone, cucumin S
[(R)-5,7-dihydroxy-2-[1-hydroxy-2-(4-hydroxy-3-methoxyphenyl)ethyl]chromone] (1), along with five known compounds:
5,7-dihydroxy-2-[2-(4-hydroxyphenyl)ethyl]chromone (2), 5,7-dihydroxy-2-[2-(3,4-dihydroxyphenyl)ethyl]chromone (3),
luteolin(4),quercetin(5),and7-glucosyloxy-5-hydroxy-2-[2-(4-hydroxyphenyl)ethyl]chromone(6)
wereisolatedfromtheEtOAcfractionofCucumismelovar.reticulatusSer.,Cucurbitaceae,seeds.Their
structuresweredeterminedbyspectroscopicmeans(1Dand2DNMR),aswellasHRESIMS,optical
rotationmeasurement,andcomparisonwithliteraturedata.Theisolatedcompounds1–6wereassessed
fortheir antioxidant activity using DPPHassay. Compounds 3, 4, and 5showed potent activities
comparedtopropylgallateatconcentration100M.
©2015SociedadeBrasileiradeFarmacognosia.PublishedbyElsevierEditoraLtda.Allrightsreserved.
Introduction
FamilyCucurbitaceaeincludespumpkins, squash,musk mel-ons,watermelons,cucumbers,andgrounds,whichareknownas vinecrops.Cucurbitfruitsaresignificantsourceofdietaryfibers, beta-carotene(pro-vitaminA),minerals(potassium),andvitamin C(Fleshmanetal.,2011;AdamsandRichardson,1981).Naturally, theyarelowinsodium,fat,andcholesterol(Lester,1997).Cucumis melo var. reticulatus Ser., Cucurbitaceae (netted muskmelon or cantaloupe)isoneofthemostcultivatedcucurbits.Itisan orange-fleshed,sweet,andaromaticmelonthatishighlypopularforits nutritiveandmedicinalproperties.Itisaworm-seasoncropgrows inalltropicaland subtropicalregions oftheworld.C.melohas potentialinthetreatmentofpain,inflammation,cough,dysuria, diabetes, liver diseases, and cardiovascular disorders (Ibrahim, 2010, 2014). It exhibited antioxidant, antimicrobial, and anti-inflammatory activities(Ibrahim, 2010,2014;Vouldoukiset al., 2004).PreviousstudiesonC.meloL.ledtoisolationof hydrocar-bons(VelchevaandDonchev,1997),peptides(Ribeiroetal.,2007), fattyacids,volatilesesquiterpenes(Lewinsohnetal.,2008;Portnoy etal.,2008),phenylethylchromonederivatives,triterpenes,sterols, and one triacylglyceride (Ibrahim, 2010, 2014). In the present
∗ Correspondingauthor.
E-mail:sribrahim@taibahu.edu.sa(S.R.M.Ibrahim).
work,we reportedthe isolation and structuralcharacterization ofanewphenylethylchromonederivative:(R )-5,7-dihydroxy-2-[1-hydroxy-2-(4-hydroxy-3-methoxyphenyl)ethyl]chromone (1), alongwithfiveknowncompoundsfromtheEtOAcfractionofC.
meloseeds.Theantioxidantactivityofcompounds1–6was evalu-atedusingDPPHassay.
Materialsandmethods
Generalexperimentalprocedures
MeltingpointswerecarriedoutusinganElectrothermal9100 DigitalMeltingPointapparatus(ElectrothermalEngineeringLtd., Essex,England).OpticalrotationwasrecordedonaPerkin-Elmer Model341LCPolarimeter.Shimadzu1601UV/VISandShimadzu Infrared-400(Shimadzu,Kyoto,Japan)spectrophotometerswere usedto measure theUltraviolet (UV) and Infrared(IR) spectra, respectively.HRESIMSspectraweremeasuredwithaMicromass Qtof2massspectrometer(ThermoFinnigan,Bremen,Germany). BrukerAvanceDRX500and700(BrukerBioSpin, Billerica,MA, USA) were used torecord NMR spectra. Chromatographic sep-arationwas achievedusing RP18 (0.04–0.063mm), silicagel 60 (0.04–0.063mm), and Sephadex LH-20 (0.25–0.1mm) (Merck, Darmstadt,Germany).TLCanalysiswasperformedonpre-coated silicagel60F254TLCplates(0.2mm,Merck,Darmstadt,Germany).
http://dx.doi.org/10.1016/j.bjp.2015.06.006
S.R.M.Ibrahim,G.A.Mohamed/RevistaBrasileiradeFarmacognosia25(2015)462–464 463
ThecompoundsweredetectedbyUVabsorptionatmax255and 366nmfollowedbysprayingwithp-anisaldehyde/H2SO4reagent andheatingat110◦Cfor1–2min.2,2-Diphenyl-1-picrylhydrazyl (DPPH)andpropylgallatewerepurchasedfromSigmaChemical Co.(Taufkirchen,Germany).
Plantmaterial
SeedsofCucumismelovar.reticulatusSer.,Cucurbitaceae,were obtainedfromcultivatedplantsatMankabad,Assiut,Egypt.Plant material was identified and authenticated (voucher specimen 2013-5)byProf.Dr.MohamedA.Farghali,ProfessorofHorticulture (VegetableCrops),FacultyofAgriculture,AssiutUniversity.
Extractionandisolation
The dried seeds (263g) were defatted in a Soxhlet appara-tususingn-hexane(3×1l),thenextracted withMeOH (4×2l). The MeOH extract was concentrated under reduced pressure to afford a dark brown residue (18.9g). The latter was sub-jected tosilica gel vacuumliquid chromatography (VLC) using
n-hexane and EtOAc toafford 12.6 and 2.9g, respectively. The EtOAcfractionwassubjectedtosilicagelVLCusingn-hexane:EtOAc gradient to give five subfractions: C-1 (0.93g, n-hexane:EtOAc 8:2),C-2(0.41g,n-hexane:EtOAc6:4),C-3(0.48g,n-hexane:EtOAc 4:6), C-4 (0.24g,n-hexane:EtOAc2:8), and C-5(0.51g, EtOAc). SubfractionC-2(0.41g)waschromatographedoversilicagel col-umn(80g×50×3cm) usingn-hexane:EtOAcgradienttoobtain 1 (4.9mg, yellow crystals). Silica gel column chromatography (70g×50×3cm)ofsubfractionC-3(0.48g)usingn-hexane:EtOAc gradientafforded2(15mg,yellowcrystals)and3(9mg,yellow crystals).SubfractionC-4(0.24g)waschromatographedover sil-icagelcolumn(50g×30×2cm)usingn-hexane:EtOAcgradient toyield4(6.2mg,yellowpowder)and5(11mg,yellowpowder). Compound6(8.1mg,yellowpowder)wasisolatedfrom subfrac-tionC-5(0.51g)throughsilicagelcolumn(50g×50×2cm)using CHCl3:MeOHgradientelution.
Antioxidantactivity
Theantioxidantactivityofcompounds1–6wasevaluatedusing 2,2′-diphenylpicrylhydrazyl(DPPH) assayaspreviouslyoutlined (Mohamedetal.,2013,2014).Thedecreaseintheabsorptionofeach compound(100M)inDPPHsolutionwasmonitoredat517nm usingaspectrophotometer.TheabsorbanceofDPPHinMeOH(with orwithoutcompounds)wasmeasuredafter2min.Theantioxidant activityofeachcompoundwasmeasuredinrelationtopropyl gal-late(astandardantioxidant).Determinationswereperformedin triplicate.The%activitywascalculatedbythefollowingequation:
Antioxidantactivity=100×1−Compoundabsorbance
Blankabsorbance
Spectraldata
CucuminS(1).Yellow crystals,mp 115–116◦C;[␣]25 D+51.6 (c0.5,CHCl3),+49.7(c0.05,MeOH);UV(MeOH)max(logε)255 (3.89),352(3.15)nm;IR(KBr)max3445,2978,1655,1620,1584, 1065cm−1; NMR data(CD
3OD,500and 125MHz),seeTable 1; HRESIMSm/z345.0976[M+H]+(calc.forC
18H17O7,345.0974).
Resultsanddiscussion
Compound 1 wasobtained as yellowcrystals. Its molecular formula C18H16O7 wasdetermined fromtheHRESIMS(positive
Table1
NMRspectraldataofcompound1(CD3OD).
No. ␦H[mult.,J(Hz)]a ␦C(mult.)a HMBCb
2 – 168.6(C) –
3 6.47s 110.6(CH) 2,4,10,8′
4 – 181.7(C) –
5 – 161.5(C) –
6 6.06d(2.5) 98.2(CH) 5,7,8,10
7 – 164.2(C) –
8 6.24d(2.5) 95.8(CH) 7,9,10
9 – 158.6(C) –
10 – 102.3(C) –
1′ – 129.6(C) –
2′ 7.28d(2.5) 115.6(CH) 1′,4′,6′,7′
3′ – 149.0(C) –
4′ – 145.9(C) –
5′ 6.78d(7.5) 117.8(CH) 1′,3′,4′,6′
6′ 7.23dd(7.5,2.5) 119.8(CH) 1′,4′,7′
7′ 3.26dd(14.5,6.0) 40.6(CH2) 2,1′,2′,6′,8′
2.73dd(14.5,3.5)
8′ 5.91dd(6.0,3.5) 85.8(CH) 2,3,1′
3′-OCH3 3.87s 57.3(CH3) 3′ aMeasuredat500and125MHz.
bMeasuredat700MHz.
ion-mode):m/z345.0976[M+H]+(calc.forC
18H17O7,345.0974). TheUVabsorptionmaximaat255and352nmandtheIR absorp-tionbandsat1655,1620,and1584cm−1suggestedthepresence ofachromonemoietyin1(Ibrahim,2010,2014).TheIRspectrum showedabandat3445cm−1characteristicforhydroxylgroups.The 1HNMRspectrum(Table1)showedsignalsforanolefinicproton
at␦H6.47(1H,s,H-3)andtwometa-coupledprotonsfor 1,2,3,5-tetrasubstitutedphenylmoietyat␦H 6.24(1H,d,J=2.5Hz,H-8) and6.06(1H,d,J=2.5Hz,H-6).Theycorrelatedtothecarbon sig-nalsat␦C110.6,95.8,and98.2,respectivelyintheHSQCspectrum. Furthermore,the1Hand13CNMRspectradisplayeddiastereotopic methyleneprotonsat␦H3.26(dd,J=14.5,6.0Hz,H-7′a)and2.73 (dd,J=14.5,3.5Hz,H-7′b)/␦C40.6(C-7′)andanoxymethineat␦H 5.91(dd,J=6.0,3.5Hz,H-8′)/␦
C 85.8(C-8′),indicating the pres-enceofanAMXspinsystem.Itwasconfirmedbytheobserved 1H-1HCOSYandHMBCcorrelations(Fig.1).Theoxymethinewas
locatedatC-8′basedontheHMBCcrosspeaksofH-8′toC-2,C-3, andC-1′ andabsenceofcrosspeakswithC-2′ andC-6′.Also,the presenceofatrisubstitutedphenylethylmoietywasevidentbythe signalsat␦H7.23(dd,J=7.5,2.5Hz,H-6′)/␦C119.8(C-6′),7.28(d, J=2.5Hz,H-2′)/115.6(C-2′),and6.78(d,J=7.5Hz,H-5′)/117.8 (C-5′).Thismoietywasconfirmedbythe1H-1HCOSYandHMBCcross peaks.Moreover,thesingletsignalat␦H3.87,correlatingwiththe carbonsignalat␦C57.3intheHSQCexperimentwasassignedto amethoxylgroup.The13CNMRandHSQCspectraof1revealed thepresenceof18carbons:onemethoxyl,onemethylene,seven methines,andninequaternarycarbonsincludingcarbonylcarbon at␦C181.7(C-4).TheconnectivityofthephenylethylmoietyatC-2 ofthechromoneringwassecuredthroughtheHMBCcorrelations
464 S.R.M.Ibrahim,G.A.Mohamed/RevistaBrasileiradeFarmacognosia25(2015)462–464
Table2
TheDPPHradicalscavengingactivityresults.
Sample* Inhibition%
PG 97.39±1.8
1 63.12±1.6
2 67.09±1.8
3 86.20±0.9
4 87.16±0.7
5 91.24±0.8
6 56.07±1.2
* Conc.(100M);Eachvaluerepresentsthemean
±S.D.,n=3.
ofH-7′toC-2andH-8′toC-2andC-3(Fig.1).TheHMBCcrosspeaks ofH-8toC-7(␦C164.2),C-9(␦C158.6),andC-10(␦C102.3),H-6to C-5(␦C161.5),C-7(␦C164.2),C-8(␦C95.8),andC-10(␦C102.3), H-5′ toC-3′(␦
C149.0),andH-2′andH-6′ toC-4′ (␦C145.9) sup-portedthepresenceofhydroxylgroupsatC-5,C-7,andC-4′.The HMBCcorrelationofthemethoxygroupat␦H3.87tothecarbon at␦C149.0establisheditsattachmentatC-3′.Duetothescarcity ofthecompound,therelativeconfigurationatC-8′ wasassigned tobeR-formbasedonthecomparison1Hchemicalshiftand cou-plingconstantofH-8′aswellasopticalrotationof1withthoseof seriesofanalogouscompounds(Bernhardtetal.,2000;Kimetal., 2010;Parketal.,2010;Ibrahimetal.,2014).Onthebasisofthese findings,1wasidentifiedas(R )-5,7-dihydroxy-2-[1-hydroxy-2-(4-hydroxy-3-methoxyphenyl)ethyl]chromoneandnamedcucumin S.
The known compounds were identified by analysis of the spectroscopic data(1H and 13C NMR) and comparison of their data with those in the literature, in addition to co- chro-matography with authentic samples to be: 5,7-dihydroxy-2-[2-(4-hydroxyphenyl)ethyl]chromone (2) (Ibrahim, 2010; Ibrahim and Mohamed, 2014), 5,7-dihydroxy-2-[2-(3,4-dihydroxyphenyl)ethyl]chromone (3) (Ibrahim, 2010; Ibrahim and Mohamed, 2014), luteolin (4) (Harborne, 1988; Mohamed, 2008), quercetin (5) (Harborne, 1988; Mohamed,2008), and 7-glucosyloxy-5-hydroxy-2-[2-(4-hydroxyphenyl)ethyl]chromone (6)(Ibrahim,2010).
DPPH radical is widely used as a model systemto investi-gatethescavengingactivityofseveralnaturalcompounds.DPPH isscavengedbytheantioxidantsthroughthedonationofproton formingthereducedDPPHwhichcanbequantifiedbyitsdecrease ofabsorbance.
Compounds1–6showedaconcentrationdependent scaveng-ingactivitybyquenchingDPPHradical.Theyexhibitedmaximum inhibitionofDPPHfreeradicalthatrangedfrom91.24to56.07% (Table2).Theirantioxidanteffectswererelatedtothenumberof freehydroxyl groups especiallythat locatedatC-3′ andC-4′ in theirstructures.Compounds3,4,and5showedpotentactivities comparedtopropylgallateatthesameconcentration.However,
absenceorblockingofthehydroxylgroupbyamethylorglucose moietyleadstoadecreaseintheactivityasin1,2,and6(Dugas etal.,2000;Al-Musayeibetal.,2014).
Authors’contributions
SRMIandGAMcontributedincollectingtheplantsample, run-ningthelaboratorywork,analysisofthespectroscopicdata,and writingthemanuscript.GAMcarriedouttheantioxidantactivity measurement.SRMIhasrevisedandapprovedthesubmission.
Conflictsofinterest
Theauthorsdeclarenoconflictsofinterest.
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