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Cu(II) complexes of 2-(diphenylmethylene)hydrazinecarboxamide derivatives: Synthesis, characterization and antifungal activity against Fusarium oxysporum f. sp. cubense tropical race 4
⁎Corresponding author. Tel.: +60 4 6533888; fax: +60 4 6574854. abdussalam@usm.my (Abdussalam Salhin Mohamed Ali)
-
Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Abstract
The semi- and thiosemicarbazones of benzophenone derivatives (L1–L6) had been synthesized and their respective Cu(II) complexes (1–6) were obtained as precipitates by interaction with CuCl2 in ethanolic solution. All these compounds were characterized by 1H and 13C NMR, IR, UV–Vis and elemental analysis (CHN). The structures of L1 and L4 were further elucidated by X-ray crystallography. Antifungal activities of the ligands and Cu(II) complexes were carried out to ascertain their effectiveness in the inhibition of Fusarium oxysporum f. sp. cubense tropical race 4 (Foc TR4), a highly virulent pathogen against Cavendish banana in tropical region. A few of the complexes showed noteworthy inhibition on the growth of Foc TR4 with complex 6 was the most potent antifungal agent.
Keywords
Thiosemicarbazone
Benzophenone
Antifungal
Fusarium oxysporum f. sp. cubense tropical race 4
1 Introduction
Semicarbazones and their sulphur analogues, thiosemicarbazones form an important class of nitrogen and oxygen or sulphur donor ligands that show coordination to a variety of metal ions (Patel and Shah, 2010; Rodriguez-Argüelles et al., 1997). Advancement in bioinorganic chemistry had increased the interest of researches in semi- and thiosemicarabzone complexes. It had been recognized that the remarkable biological properties of semi- and thiosemicarbazones were often related to their metal complexing ability (Beraldo and Gambino, 2004; Prathima et al., 2010; Suvarapu et al., 2012). Among the transition metal complexes, Cu(II) complexes had received much attention due to their enhanced biological activities, notably in antimicrobial (Bacchi et al., 1999; Rodríguez-Argüelles et al., 2009).
Banana (Musa spp.) is among the world’s most popular fruits. In Malaysia, it is the second most widely cultivated, covering about 26,000 ha which constitutes approximately 11.0% of the total fruit area in the country (Agriculture Statistical Handbook, 2011). Like other plantations, the crop faces several diseases during its development. Fusarium wilt, also known as Panama disease caused by a fungal pathogen, Fusarium oxysporum f. sp. cubense (Foc) is considered as one of the most devastating threats to the banana production worldwide (Heslop-Harrison and Schwarzacher, 2007; Hwang and Ko, 2004). Lately, the presence of a new variant of the Foc, namely tropical race 4 (TR4) had imposed additional impact on the production of Cavendish cultivar in Asian countries (Dita et al., 2010; Dong and Wang, 2011; Leong et al., 2009; Li et al., 2012). To date, there is still no effective control measure in combating the disease (Ploetz, 2006). Herein, we report the synthesis and characterization of semi- and thiosemicarbazones of 3-aminobenzophenone and 2-amino-5-chlorobenzophenone and their respective Cu(II) complexes. These compounds were subsequently tested for their in vitro antifungal activities against Fusarium oxysporum f. sp. cubense tropical race 4 (Foc TR4).
2 Experimental
Melting points were determined by Stuart Scientific (UK) apparatus. Elemental analyses (CHN) were carried out on a Perkin Elmer Series II, 2400 analyser. NMR spectra were obtained on a Bruker Avance III 500 MHz spectrometer in DMSO-d6 using tetramethylsilane as an internal standard. IR spectra were recorded as KBr pellets on a Perkin Elmer System 2000 FT-IR spectrophotometer in the wave number range of 4000–400 cm−1. UV–visible spectra were recorded on a Perkin Elmer Lambda 25 spectrophotometer. ESI-MS analyses were carried out using Thermo Finnigan LCQ mass spectrometer with spray voltage of 4.5 KV. Magnetic measurements were carried out by the Gouy technique with Magnetic Susceptibility Balance AUTO manufactured by Sherwood Scientific. Thermogravimetric analysis (TGA) data were recorded by Mettler Toledo TS0801RO Sample Robot (TGA/SDTA851e), heating rate of 20 °C/min in the range of 30–900 °C.
2.1 Synthesis of ligands
As stated in our previous reports (Chan et al., 2013a,b), semi- and thiosemicarbazones were synthesized following the reaction scheme below (Scheme 1):
Synthetic route of the ligands. (a) NH2NHCONH2·HCl, CH3COONa, distilled H2O (b) NH2NHCSNH2/ NH2NHCSNHCH2CH3, conc. H2SO4, EtOH.
2.1.1 3-Aminobenzophenone semicarbazone (L1)
This compound was isolated from an ethanolic solution as a brown crystal (Yield: 79%. m.p. 196–197 °C) and was fully characterized by CNH, FT-IR, 1H and 13C NMR analysis. Details about the structure conformation were described elsewhere (Chan et al., 2013a).
2.1.2 3-Aminobenzophenone thiosemicarbazone (L2)
Pale brown solid. Yield: 69%. m.p. 152–154 °C. Anal. Calc. for C14H14N4S: C: 62.22; H: 5.19; N: 20.74%. Found: C: 62.27; H: 5.11; N: 20.76%. 1H NMR (DMSO-d6, ppm): δ 5.49 (2H, s, Ar-NH2), 6.37–6.39 (1H, dt, Ar-H), 6.41 (1H, t, Ar-H), 6.74–6.77 (1H, d, Ar-H), 7.28 (1H, t, Ar-H), 7.36–7.40 (3H, m, Ar-H), 7.70–7.72 (2H, m, Ar-H), 8.33 (1H, s, CSNH2), 8.37 (1H, s, NH), 8.61 (1H, s, CSNH2). 13C NMR (DMSO-d6, ppm): δ 149.95 (C—NH2), 150.01 (C⚌N), 177.61 (C⚌S).
2.1.3 3-Aminobenzophenone 4-ethyl-3-thiosemicarbazone (L3)
White solid. Yield: 70%. m.p. 144–145 °C. Anal. Calc. for C16H18N4S: C: 64.43; H: 6.04; N: 18.79%. Found: C: 64.46; H: 5.97; N: 18.74%. 1H NMR (DMSO-d6, ppm): δ 1.18 (3H, t, CH3), 3.59–3.65 (2H, m, CH2), 5.49 (2H, s, Ar-NH2), 6.38–6.40 (1H, dt, Ar-H), 6.42 (1H, t, Ar-H), 6.76–6.78 (1H, ddd, Ar-H), 7.29 (1H, t, Ar-H), 7.39–7.43 (3H, m, Ar-H), 7.70–7.72 (2H, m, Ar-H), 8.44 (1H, s, NH), 8.89 (1H, s, CSNH). 13C NMR (DMSO-d6, ppm): δ 149.52 (C—NH2), 149.93 (C⚌N), 176.19 (C⚌S).
2.1.4 2-Amino-5-chlorobenzophenone semicarbazone (L4)
Colourless crystal. Yield: 77%. m.p. 194–196 °C. Anal. Calc. for C14H13ClN4O: C: 58.23; H: 4.51; N: 19.41%. Found: C: 58.20; H: 4.39; N: 19.46%. 1H NMR (DMSO-d6, ppm): δ 5.10 (2H, s, Ar-NH2), 6.67 (2H, s, CONH2), 6.88 (1H, d, Ar-H), 6.88 (1H, d, Ar-H), 7.24–7.26 (1H, dd, Ar-H), 7.35–7.38 (3H, m, Ar-H), 7.59–7.61 (2H, m, Ar-H), 8.03 (1H, s, NH). 13C NMR (DMSO-d6, ppm): δ 119.61 (C—Cl), 143.67 (C⚌N), 144.56 (C—NH2), 155.92 (C⚌O).
2.1.5 2-Amino-5-chlorobenzophenone thiosemicarbazone (L5)
Pale yellow solid. Yield: 78%. m.p. 200–202 °C. Anal. Calc. for C14H13ClN4S: C: 55.17; H: 4.27; N: 18.39%. Found: C: 55.13; H: 4.18; N: 18.45%. 1H NMR (DMSO-d6, ppm): δ 5.29 (2H, s, Ar-NH2), 6.91 (1H, d, Ar-H), 6.92 (1H, d, Ar-H), 7.27–7.30 (1H, dd, Ar-H), 7.36–7.41 (3H, m, Ar-H), 7.70–7.71 (2H, m, Ar-H), 8.28 (1H, s, CSNH2), 8.61 (1H, s, CSNH2), 8.61 (1H, s, NH). 13C NMR (DMSO-d6, ppm): δ 119.68 (C—Cl), 144.62 (C—NH2), 146.43 (C⚌N), 178.07 (C⚌S).
2.1.6 2-Amino-5-chlorobenzophenone 4-ethyl-3-thiosemicarbazone (L6)
Pale yellow solid. Yield: 77%. m.p. 161–162 °C. Anal. Calc. for C16H17ClN4S: C: 57.74; H: 5.11; N: 16.84%. Found: C: 57.69; H: 5.04; N: 16.91%. 1H NMR (DMSO-d6, ppm): δ 1.18 (3H, t, CH3), 3.60–3.65 (2H, m, CH2), 5.30 (2H, s, Ar-NH2), 6.91 (1H, d, Ar-H), 6.92 (1H, d, Ar-H), 7.28–7.30 (1H, dd, Ar-H), 7.39–7.42 (3H, m, Ar-H), 7.69–7.71 (2H, m, Ar-H), 8.63 (1H, s, NH), 8.85 (1H, s, CSNH). 13C NMR (DMSO-d6, ppm): δ 119.74 (C—Cl), 144.60 (C—NH2), 146.10 (C⚌N), 176.76 (C⚌S).
2.2 Synthesis of Cu(II) complexes
General procedure: Cupric chloride (0.5 mmol) dissolved in a minimum quantity of ethanol was added dropwise to a hot ethanolic solution of the respective ligand (1 mmol). The mixture was refluxed for 2–3 h. After keeping the mixture at room temperature for overnight, the coloured solid separated out. This was filtered off and washed with small quantity of ethanol then it was air dried. Analytical data of the complexes are summarized in Table 1.
Compound
Colour
Yield (%)
m.p. (°C)
Found (Calculated) (%)
m/z (+c ESI)
μeff (B.M.)
C
H
N
[Cu(L1)Cl2]·2H2O (1)
Black
72
>300
43.27
3.53
14.36
357.8 (M+-2Cl, 41%)
1.67
(43.24)
(3.60)
(14.41)
[Cu(L2)2Cl2]·2H2O (2)
Brown
63
220–222
47.30
4.45
15.69
603.0 (M+-2Cl-2H2O, 57%)
1.80
(47.29)
(4.50)
(15.76)
[Cu(L3)Cl2]·2H2O (3)
Dark brown
67
147–148
40.89
4.64
12.01
361.1 (M+-2Cl-2H2O, 53.5%)
1.78
(40.98)
(4.70)
(11.95)
[Cu(L4)2]Cl2 (4)
Green
67
180–181
47.26
3.59
15.85
640.9 (M+-2Cl, 100%)
1.75
(47.22)
(3.65)
(15.74)
[Cu(L5)Cl]Cl·H2O (5)
Army green
60
186–187
36.84
3.22
12.18
367.8 (M+-2Cl-H2O, 62%)
1.78
(36.76)
(3.28)
(12.25)
[Cu(L6)Cl]Cl (6)
Dark green
70
197–199
41.16
3.57
12.03
395.9 (M+-2Cl, 83%)
1.77
(41.11)
(3.64)
(11.99)
2.3 X-ray Crystallography
Crystals were placed in the cold stream of an Oxford Cyrosystems Cobra open-flow nitrogen cryostat (Cosier and Glazer, 1986) operating at 100.0 (1) K. Crystallographic data were collected using a Bruker SMART APEX II DUO CCD diffractometer (Bruker, 2009). The data were then reduced using SAINT (Bruker, 2009) software. SADABS and SAINT software (Bruker, 2009) were used for absorption correction and data reduction, respectively. The structure was refined by full-matrix least squares on F2 and solved by direct methods using the SHELXTL (Sheldrick, 2008) software package. N-bound H atoms were located in a difference Fourier map and refined freely [N—H = 0.79 (2)–1.08 (3) Å]. The remaining hydrogen atoms were positioned geometrically [C—H = 0.95 Å] and were refined using a riding model, with Uiso ∼ (H) = 1.2 Ueq(C). The crystal L1 was an inversion twin with a 0.180 (1):0.820 (1) domain ratio.
2.4 Antifungal assay
The in vitro antifungal activities of the synthesized compounds were tested against Foc TR4 by disc-diffusion method as described by Murray et al. (1995) with slight modifications. A total of 1 mL of 1 × 106 spore/mL suspension was streaked on a PDA plate. The plate was allowed to air dry in laminar flow before four sterilized Whatman® filter paper discs (diameter 6 mm) were placed on it. For every test compound, a total of 20 μL of each concentration (1000, 500 and 250 μg/mL) was spread on the discs accordingly by using micropipette. DMSO which showed no inhibition on Foc TR4 was used as the negative control. Each test was conducted in triplicate. The inoculated plate was then allowed to air dry and incubated under the standard incubation condition for seven days before the inhibition zone was recorded in percentage using the formula below.
3 Results and discussion
Ligands (L1–L6) were synthesized in satisfactory yield via acid-catalysed condensation while the Cu(II) complexes (1–6) were obtained through the coordination of the ligands with CuCl2 in 1:1 or 2:1 M ratio. The complexes possess magnetic moment (μeff) within the range of 1.67–1.80 B.M., showing that the Cu(II) ions are in d9 system, corresponding to one unpaired electron. Thermogravimetric analysis (TGA) was carried out to further characterize the Cu(II) complexes. The correlations between the different decomposition steps of the complexes with the corresponding weight loss were studied. The first decomposition step of complexes 1–3 and 5 occurred in the temperature range of 29–134 °C in which the complexes are expected for the loss of water molecules, while 4 and 6 are stable up to around 150 °C. The possible structures of the complexes were established by comparing their analytical and spectroscopic data with those of the free ligands.
3.1 Spectroscopic studies
3.1.1 1H NMR
The two singlets at 8.33, 8.61 and 8.28, 8.61 ppm in the spectra of L2 and L5, respectively are attributed to the protons of the terminal amino group. This shows that the C-N bond of the thioamide cannot rotate freely which is due to its partial double bond character (Bakkar et al., 2003). On the other hand, in the spectra of semicarbazones-L1 and L4 the NH2 protons of the amide group are distinguishable as a singlet at 6.70 and 6.67 ppm, respectively. This indicates that the two protons of the NH2 group are equivalent (Souza et al., 1992). Due to the paramagnetic properties of the Cu(II) complexes, their 1H NMR spectra could not be obtained.
3.1.2 FT-IR
The significant IR bands that are important in determination of the coordination mode of the ligands are mainly due to C⚌N and C⚌O or C⚌S groups. An absorption in 1569–1598 cm−1 region is assigned to the C⚌N stretching vibration. Coordination of the azomethine nitrogen to the Cu(II) centre has resulted in moving the frequency of C⚌N band to lower values (1561–1589 cm−1) (El-Asmy and Al-Hazmi, 2009; Gulya et al., 2009). For semicarbazones, a band at 1686 cm−1 is attributed to the C⚌O stretching of an amide, which is observed at 1656 cm−1 and 1674 cm−1 for 1 and 4, respectively. For thiosemicarbazones, the C⚌S stretching is ascribed to a band in the region of 811–830 cm−1. Upon chelation, this band is shifted to lower wave numbers (776–801 cm−1) representing the displacement of electrons towards the Cu(II) centre and hence weakening of the C⚌S bond (Agarwal et al., 2006; Rosu et al., 2007). In addition, in the IR spectra of L4–L6, the δ(N–H)aromatic which appears at around 1620 cm−1, is shifted to a higher frequency (1635 cm−1) upon complexation. It indicates that aromatic amine at the ortho position is participated in coordination to the metal centre (Kurt et al., 2007).
3.1.3 UV–Vis
In the UV–Vis spectrum of complex 1, a band at 380 nm can be due to the combination of O → Cu and N → Cu charge transfers (Kala et al., 2007), while a weak d–d transition expected for square planar geometry of the Cu(II) complex is observed at 628 nm. The expected weak d–d band of complex 4 is not detected even with concentrated solution. However, on the basis of IR, CHN and magnetic moment, it is considered to possess 6-coordinate geometry. On the other hand, in the UV–Vis spectra of thiosemicarbazone complexes (3, 5 and 6), bands in the regions of 433–457 and 604–639 nm are assigned to the azomethine N → Cu(II) charge transfer and the weak d–d transition, respectively. Such a transition is expected for square planar complexes (Kurt et al., 2007). Lastly, complex 2 is proposed to have a tetragonal geometry due to the presence of a charge transfer band observed at 527 nm (Chandra et al., 2007). As representative the proposed structures of complexes 3 and 6 are displayed in Fig. 1.
Proposed structures of complexes 3 and 6.
3.2 X-ray crystallography
Compounds L1 and L4 crystallized in orthorhombic and monoclinic crystal system, respectively. Besides that, L1 has the space group of Pccn while L4 of P21 (Table 2). Fig. 2 shows the molecular views of the compounds. L1 contains four molecules of similar geometries in the asymmetric unit. In each molecule, the benzene rings make a dihedral angle of 88.13 (11)°, 86.22 (10)°, 82.65 (11)° and 78.22 (11)°, respectively, indicating that both rings are essentially perpendicular to each other. The corresponding dihedral angle for L4 is 76.55 (6)°. The crystal structure of L4 is further stabilized by an intramolecular N—H···N hydrogen bond, forming a S(6) ring motif whereas in the crystal structure of L1 no intramolecular hydrogen bond is found. In the crystal packing of L1 (Fig. 3), molecules are linked via intermolecular N—H···O and N—H···N hydrogen bonds into the extended one-dimensional chains along [1 0 0]. Adjacent chains are cross-linked via further N—H···O inter actions into two-dimensional networks parallel to (0 0 1) plane. The crystal structure is further consolidated by R22 (6) and R22 (10) ring motifs (Bernstein et al., 1995). Meanwhile, in the crystal packing of L4, the molecules are linked into chains along [0 1 0] via pairs of bifurcated intermolecular N—H···O and N—H···N hydrogen bonds. Further stabilization of the crystal structure is provided by R12 (5) and R21 (6) ring motifs (Bernstein et al., 1995) which are formed via these intermolecular interactions.
Compound
L1 (CCDC 823247)
L4 (CCDC 864946)
Formula
C14H14N4O
C14H13ClN4O
Formula weight
254.29
288.73
Colour, shape
Brown; block
Colourless, block
Crystal system
Orthorhombic
Monoclinic
Space group
Pccn
P21
Z
32
2
Lattice constants
a = 12.3855 (1) Å,
a = 6.8865 (4) Å,
b = 34.5746 (5) Å,
b = 7.5842 (5) Å,
c = 24.2283 (3) Å,
c = 13.4476 (8) Å,
γ = β = α = 90°
β = 100.782 (1)°, γ = α = 90°
Volume (Å3)
10375.1 (2)
689.95 (7)
Dx (Mg m−3)
1.302
1.390
μ (mm−1)
0.09
0.28
F (000)
4288
300
θ range (°)
2.4–29.0
3.6–30.0
h, k, l
−14/17, −48/42, −31/34
−9/9, −7/10, −18/18
Reflections collected
86,543
7809
Reflections unique
15,195
3263
Tmin/Tmax
0.965/0.980
0.852/0.979
R (int)
0.057
0.022
Number of parameters
765
202
Goodness of Fit
1.07
1.03
Final R index [I > 2σ(I)]
0.073
0.029
The molecular structures of L1 and L4, showing 30% probability displacement ellipsoids for non-H atoms and the atom-numbering scheme. Intramolecular hydrogen bond is shown as dashed line.
The crystal structures of L1 and L4, viewed along the b and a axes, respectively. H atoms not involved in hydrogen bonds (dashed lines) have been omitted for clarity.
3.3 Antifungal activities
In the present study, the in vitro antifungal effects of the synthesized compounds were evaluated against Foc TR4 isolates obtained from different geographical areas in Malaysia (Table 3). The specific gene of Foc TR4 (VCG 01213) was detected in eight isolates according to the specific primer developed by Lin et al. (2009), except for isolate B2472N (Fig. 4). The size of the amplified gene for the eight isolates was approximately 250 bp.
Isolatea
Location
A2279N
Changkat Jering, Perak
A2282N
Titi Gantung, Perak
B2461N
Serdang, Selangor
B2472N
Serdang, Selangor
J2330N
Johor Bahru, Johor
J2331N
Johor Bahru, Johor
P2305N
Kubang Semang, Penang
T9888N
Kuala Terengganu, Terengganu
T9889N
Kuala Terengganu, Terengganu
PCR products of specific Foc TR4 gene from nine Foc isolates. (L) 100 bp ladder (1) J2330 N (2) A2282 N (3) T9888 N (4) P2305 N (5) J2331 N (6) A2279 N (7) T9889 N (8) B2461 N (9) B2472 N (10) Control.
The inhibitory assays were carried out by disc-diffusion method employing 1000, 500 and 250 μg/mL concentrations of the test compounds dissolved in DMSO. The effectiveness of an antifungal agent in sensitivity testing is based on the size of inhibition zone. Results obtained (Table 4) indicate that Cu(II) complexes exhibit higher inhibition on the growth of Foc TR4 than the parent ligands. This might be due to the combined activity effect of the ligand and the metal ion. Upon chelation, the polarity of the metal ion will be reduced due to the overlapping of the ligand orbital and partial sharing of the positive charge of the metal ion with donor groups. The lipophilicity of the chelates is enhanced thus facilitating the penetration of the complexes through the cell membrane of the microorganism and causing the normal cellular processes to be impaired (Prasad et al., 2011). Therein, Cu(II) complexes of thiosemicarbazones (2–3 and 5–6) show more enhanced antifungal activity than those of semicarbazones (1 and 4). This is in agreement with the finding by Nomiya et al. (2004) and Kasuga et al. (2001). In addition, Cu(II) complexes derived from 2-amino-5-chlorobenzophenone show higher inhibition on Foc TR4 than those derived from 3-aminobenzophenone. The presence of a halogen atom is assumed to help in regulating the electron density of the whole molecule and results in the fast penetration through the cell membrane of the test organism (Hania, 2009).
Compound
A2279N
A2282N
B2461N
J2330N
1000
500
250
1000
500
250
1000
500
250
1000
500
250
L1
–
–
–
–
–
–
44
33
–
–
–
–
L2
50
–
–
–
–
–
50
39
–
67
44
33
L3
–
–
–
44
–
–
44
–
–
67
39
–
L4
50
–
–
–
–
–
50
–
–
72
–
–
L5
61
–
–
56
33
–
61
50
39
39
–
–
L6
67
39
–
50
44
33
78
50
39
61
–
–
1
–
–
–
–
–
–
56
39
–
–
–
–
2
72
–
–
56
–
–
50
39
–
39
–
–
3
39
–
–
28
–
–
61
–
–
72
–
–
4
56
33
–
56
–
–
56
–
–
67
39
33
5
78
50
33
72
39
–
100
61
33
61
44
33
6
61
44
33
94
56
44
94
56
33
67
56
39
J2331N
P2305N
T9888N
T9889N
1000
500
250
1000
500
250
1000
500
250
1000
500
250
L1
22
11
–
50
–
–
–
–
–
28
–
–
L2
28
11
–
67
–
–
61
33
–
33
17
11
L3
17
11
–
–
–
–
–
–
–
22
11
–
L4
–
–
–
–
–
–
67
–
–
22
–
–
L5
33
–
–
50
39
33
–
–
–
28
–
–
L6
44
28
17
50
39
33
39
–
–
61
33
17
1
28
22
–
–
–
–
–
–
–
39
–
–
2
33
17
–
50
–
–
–
–
–
33
28
–
3
33
17
–
50
–
–
39
–
–
44
22
11
4
33
17
–
61
–
–
56
39
–
33
11
–
5
39
11
–
72
61
44
78
–
–
44
11
–
6
67
33
22
72
56
39
56
33
–
67
28
22
4 Conclusion
Ligands derived from 3-aminobenzophenone and 2-amino-5-chlorobenzophenone exhibit bidentate and tridentate behaviours, respectively with the former are through azomethine nitrogen and amide oxygen or thioamide sulphur while the latter with the addition of aromatic amine nitrogen. In current research, the in vitro antifungal activities of the synthesized compounds against Foc TR4 were evaluated. Complexation with Cu(II) induces significant alternation in the biological activity of the ligands. Cu(II) complexes show enhanced inhibition of Foc TR4 where complex 6 exhibits the most promising antifungal activity.
Acknowledgements
We thank Universiti Sains Malaysia-RU Grant No. 1001/PKIMIA/811196 and Science Fund Grant No. 1001/PKIMIA/823003 for the financial support of this work.
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Appendix A
Supplementary data
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.arabjc.2014.02.014.
Appendix A
Supplementary data
Supplementary data 1
Supplementary data 1
This article contains supplementary data.