Synthesis, structural characterization and biological activity of a trinuclear zinc(II) complex: DNA interaction study and antimicrobial activity

Article abstract of DOI:10.1007/s12039-013-0528-1

A trinuclear zinc(II) complex [Zn3L2(μ-O2CCH3)2(H2O)2]H2O2CH3OH (1) was synthesized from an in situ reaction between zinc acetate and a Schiff base ligand (H2L = 2-((2-hydroxyphenylimino) methyl)-6-methoxyphenol). The ligand was prepared by (1:1) condensa

Full text of DOI:10.1007/s12039-013-0528-1

c
J. Chem. Sci. Vol. 125, No. 6, November 2013, pp. 1445–1453. ꢀ Indian Academy of Sciences.
Synthesis, structural characterization and biological activity
of a trinuclear zinc(II) complex: DNA interaction study
and antimicrobial activity
BHASKAR BISWASa,∗, NIRANJAN KOLEa, MOUMITA PATRAb, SHAMPA DUTTAc
and MOUSUMI GANGULYc
aDepartment of Chemistry, bDepartment of Physics, Raghunathpur College, Purulia 723 133, India
cDepartment of Environmental Science, The University of Burdwan, Burdwan 713 104, India
e-mail: mr.bbiswas@rediffmail.com
MS received 9 April 2013; revised 6 September 2013; accepted 11 September 2013
Abstract. A trinuclear zinc(II) complex [Zn3L2(μ-O2CCH3)2(H2O)2]·H2O·2CH3OH (1) was synthesized
from an in situ reaction between zinc acetate and a Schiff base ligand (H2L = 2-((2-hydroxyphenylimino)
methyl)-6-methoxyphenol). The ligand was prepared by (1:1) condensation of ortho-vanillin and ortho-
aminophenol. The ligand and zinc(II) complex were characterized by elemental analysis, Fourier Transform
1
Infrared (FTIR), H-Nuclear Magnetic Resonance (NMR), UV-Vis spectroscopy, Powder X-ray Diffraction
(PXRD) and thermogravimetric analysis. 1 crystallizes in P-1 space group with a = 11.9241(3) Å, b =
12.19746 Å, c = 20.47784 Å with unit cell volume is 2674.440 (Å)3. Binding property of the complex with calf
thymus DNA (CT-DNA) has been investigated using absorption and emission studies. Thermal melting and
viscosity experiments were further performed to determine the mode of binding of 1 with CT-DNA. Spectro-
scopic and viscosity investigations revealed an intercalative binding mode of 1 with CT-DNA. The ligand and
its zinc complex were screened for their biological activity against bacterial species and fungi. Activity data
show that the metal complex has more antibacterial and antifungal activity than the parent Schiff base ligand
and against those bacterial or fungi species.
Keywords. Zinc(II); Schiff base; DNA binding study; antimicrobial investigation.
1. Introduction
that DNA is the primary intracellular target of anti-
cancer drugs; interaction between small molecules and
DNA can cause damage in cancer cells, blocking the
division and resulting in cell death.9 This inspires syn-
thetic chemists to search for new metal complexes for
bioactive compounds.
The discovery and development of antibiotics are
among the most powerful and successful achievements
of modern science and technology for the control of
infectious diseases. Metal-based drugs represent a novel
group of antibacterial and antifungal agents with poten-
tial applications for the control of bacterial and fungal
infections.1–3 Transition metal complexes have power-
ful antimicrobial activities and are already in common
day-to-day use in medicinal field such as silver ban-
dages for treatment of burns, zinc antiseptic creams,
bismuth drugs for the treatment of ulcers and metal
clusters as anti-HIV agents.4–6
The synthesis of a new ligand is the most important
step in the development of metal complexes regarding
its unique properties and novel reactivity. Schiff bases
are very attractive class of ligands because of their ease
of preparation and simple modification of both stearic
and electronic properties. Schiff bases are compounds
with imine functional groups and were first prepared by
Hugo Schiff in 1864.10 The chemistry of Schiff base
and its zinc complexes have occupied a place of con-
siderable importance as they have a variety of appli-
cations including biological, clinical, analytical and
industrial, in addition to their important roles in cata-
lysis and organic synthesis.11–13 In the present study,
we report the results on the synthesis, spectroscopic
and structural characterization, DNA-binding property
and antimicrobial activities of a Zn(II) Schiff base
complex.
Transition metal complexes that can bind the DNA
under physiological conditions are of current interest
in the development of metal-based anticancer agents.7–9
Numerous biological experiments have demonstrated
∗For correspondence
1445

1446
Bhaskar Biswas et al.
CHO
OH
H2N
HO
H
C
N
HO
OH
O
O
Scheme 1. Synthetic route for the ligand.
2. Experimental
yellow solution was filtered and the supernatant liquid
was kept in air for slow evaporation. Yield: 0.183 g
(83% based on metal salt). Anal. Calc. for C34H42N2
O15Zn3 (1): C, 44.63; H, 4.63; N, 3.06. Found: C, 44.49;
H, 4.53; N, 3.01%. IR (KBr, cm−1): 3421 (vOH),1608,
2.1 Preparation of the complex
2.1a Chemicals, solvents and starting materials: High
purity O-vanillin (E Merck, India), O-aminophenol
(E Merck, India), zinc(II) acetate and all other sol-
vents were purchased from the respective concerns and
used as received. The calf thymus (CT) DNA and ethi-
dium bromide (EB) were obtained from Sigma Aldrich
Corporation (St. Louis, MO, USA). DNA concentra-
tions were determined spectrophotometrically using the
reported data for a molar absorption coefficient of
6600 M−1 cm−1 at 260 nm. For viscosity studies, the
DNA sample was dissolved in the experimental buffer
1587 (vC=N), 1475, 1442, 1389 (vOAc); UV-Vis (λmax
,
1
nm): 249, 303, 367, 422; H NMR (δ ppm, 400 Mz,
DMSO-d6) δ = 8.90 (s, 1H), 7.58 (s, 1H), 6.99–6.40
(Ar-H, 5H), 4.13–4.09 (μ-OAc, 3H), 3.78 (-OCH3, 3H),
ppm (figure 1).
2.2 Physical measurements
Elemental analyses (carbon, hydrogen and nitrogen) were
performed on a Perkin–Elmer 2400 CHNS/O elemen-
tal analyser. IR spectra (KBr discs, 4000–300 cm−1)
were recorded using a Perkin–Elmer FT-IR model RX1
spectrometer. Ground-state absorption and steady-state
and sonicated to uniform size of about 280
50
base pairs using a Labsonic 2000 sonicator (B Braun,
Germany) using a needle probe of 4 mm diameter.
The sonicated DNA sample was dialysed several times
against the experimental buffer under sterile conditions
in a cold room and stored at 5◦C till use. All other chemi-
cals and solvents were of analytical grade and were used
as received without further purification.
N
CH
H
H
O
2.1b Preparation of H2L and 1: The Schiff base H2L
was prepared following a reported method14 with a little
modification. The details are given below:
Zn
OH2
C
OH
H
O
O
O-aminophenol (0.1092 g, 1 mmol) was refluxed with
O-vanillin (0.1523 g, 1 mmol) in 20 ml dehydrated
alcohol. After 10 h, the reaction solution was kept in
open atmosphere and red coloured crystalline com-
pound was separated out from solution, which was
dried and stored in vaccuo over CaCl2 for subsequent
use. Yield 0.213 g (87.2%). Anal calc. for C14H13NO3
(H2L): C, 69.12; H, 5.39; N, 5.76; Found: C, 69.03; H,
5.30; N, 5.68 IR (KBr pellet, cm−1): 3365 (s) (vOH),
1618 (s), 1595 (s) (vC=N), 1509 (s), 1462 (s), 1333 (s),
1245 (s) (vOAr); UV-Vis (λmax, nm): 233, 275, 347, 461;
1H NMR (δ ppm, 400 Mz, DMSO-d6) δ = 12.58 (s,
1H), 8.70 (s, 1H), 7.26–6.90 (Ar-H, 7H), 5.94 (s, 1H),
3.93 (s, 3H) ppm (scheme 1).
C
CH3
H3CO
O
Zn
O
O
OCH3
H3C
C
H
O
H2O
O
O
H
N
CH
H
H
C
OH
H
A methanolic solution (5 cm3) of H2L (0.244 g, 1 mmol)
was added dropwise to a solution of Zn(OAc)2.2H2O
(0.219 g, 1 mmol) in the same solvent (10 cm3). The
Figure 1. Proposed structure of the trinuclear zinc(II)
complex.

Synthesis and bioactivity of a trinuclear zinc(II) complex
1447
fluorescence measurements were made with a Jasco 2.5 In vitro antimicrobial activity
model V-530 UV-Vis spectrophotometer and Hitachi
1
Antibacterial activity of the ligand and the com-
plex were tested against the bacterial species Bacil-
lus subtilis, B. megaterium, Staphylococcus aureus,
Escherichia coli, Salmonella typhimurium, Pantoea
ananetis, Pseudomonas aeruginosa and Pseudomonas
fluoresence by Kirby Bauer Disc diffusion method.15
The ligand and complex were also tested against the
fungal species Penicillium, Cladosporium, Aspergillus
parasiticus, Rhizopus, fusarium, Alternaria, Helmin-
thosporium and Colletotrichum, cultured on malt
extract agar medium and also performed by the disc dif-
fusion method. The test organisms were grown on nutri-
ent agar medium in petri plates. The disks were soaked
with test samples, drained and then placed on the nutri-
ent agar plate using sterilized forceps. The discs were
placed on previously seeded plates and incubated at
37◦C (for bacteria) and 28◦C (for fungi) and the dia-
meter of inhibition zone15b around each disc was mea-
sured after 24 h for bacteria and 72 h for fungi. At the
end of the incubation period, the zones of inhibition
around the disc were measured in mm. On the basis
of preliminary tests, the compounds effecting signifi-
cant zones of inhibition were then selected and used
for determination of minimum inhibitory concentration
(MIC).
model F-4010 spectrofluorimeter, respectively. The H
NMR spectra were recorded on a Bruker AC300 spec-
trometer. Thermal analysis was done on Perkin–Elmer
Diamond TG/DTA system. The crystalline phase was
identified by powder X-ray diffraction patterns (XRD)
of the materials by using a BRUKER AXS (Model:
8D-ADVANCE) diffractometer with a Cu Kα radiation
source under continuous scanning mode in the range
of 8◦–60◦. Electrospray ionization (ESI) mass spec-
trum was recorded using a Waters Micro Q-TOF Mass
Spectrometer.
2.3 Thermal melting studies
Absorbance versus temperature profiles (optical melt-
ing curves) of DNA and zinc(II) Schiff base complex-
DNA were measured on the Shimadzu Pharmaspec
1700 unit (Shimadzu Corporation, Kyoto, Japan)
equipped with the peltier-controlled TMSPC-8 model
accessory in eight-chambered quartz cuvette of 1 cm
path length. The temperature was ramped from 40–
110◦C at a scan rate of 0.5◦C/min. monitoring the
absorbance changes at 260 nm. Tm is taken as the mid-
point of the melting transition as determined by the
maxima of the first derivative plots.
Linear growth of the fungus was obtained by measur-
ing the diameter of the colony on a Petri plate after 72 h
and the percentage of inhibition was calculated from the
following relationship:
2.4 Viscosity measurements
ꢄ
ꢅ
For viscosity studies, sonicated DNA of (280 40 base
pairs) was used. Viscosity of the DNA-metal com-
plex was determined by measuring the time needed
to flow through a Cannon–Manning semi micro size
75 capillary viscometer (Cannon Instruments Company,
State College, PA, USA) that was submerged in a ther-
mostated water bath (20 1◦C). Small volumes of the
solution of 1 were added to sonicated CT DNA solution
placed in the viscometer. Mixing was effected by slowly
bubbling dry nitrogen gas. Flow times were measured
in triplicate to an accuracy of 0.01 s with an electronic
stopwatch Casio Model HS-30W (Casio Computer Co.
Ltd., Tokyo, Japan). Relative viscosities for DNA either
in the presence or absence of 1 were calculated from the
relation:
% inhibition = (C − T ) /C × 100,
where C = diameter (in mm)2 of the fungus colony in
the control plates after 72 h and T = diameter of the
fungus colony in the treated plates.
3. Results and discussion
3.1 Synthesis and formulation
Schiff base ligand H2L was synthesized by 1:1 conden-
sation of O-aminophenol and O-vanillin in dehydrated
alcohol. 1 was prepared using reaction among Zn(II)
salt and the ligand in methanol. Coordination geo-
metry of 1 was determined by different spectroscopic
characterization.
ꢀꢁ
ꢂ
ꢃ
ηsꢁp/ηsp = tcomplex − to /to / {(tcontrol − to) /to} , (1)
where ηsꢁp and ηsp are specific viscosities of the 1-DNA
complex and the DNA, respectively; tcomplex, tcontrol, and
3.2 Spectroscopic characterization
to are the average flow times for the DNA-1 complex, 3.2a IR characterization: Coordination sites of the
free DNA and buffer, respectively. Schiff base ligand involved in coordination with the

1448
Bhaskar Biswas et al.
metal ion has been examined by careful comparison of of imine-CH and all aromatic-CH proton signals pro-
the IR spectra of the free ligand and complex. The IR duce in the range of 6.907–7.261 ppm. The proton sig-
spectrum of the free ligand is characterized mainly by nal at 3.939 ppm assigns methoxy protons. The sig-
the strong peaks at 3365 and 1618, 1595 cm−1 which nals found in the spectrum of the zinc(II) complex are
are attributed to the stretching frequencies of -OH (aro- in the range of 2.086–8.874 ppm. There are no signals
matic) and C=N (imine), respectively. The strong bands at 12.581 and 5.941 ppm in the Zn(II) complex which
at 1608 and 1587 cm−1 due to C=N stretching in the assigns the deprotonated phenolic-OH groups during
spectrum of the ligand shifts to lower wave numbers chelation with Zn(II) ion (figure S3). New character-
in the zinc complex by 8–15 cm−1 indicating that the istic signals at 4.09–4.13 are assignable to involve-
imine nitrogen atoms of the Schiff base are coordi- ment of the COOH group in chelation with Zn(II) ions.
nated to the metal ion. The band at 3365 cm−1 due to The proton signal at 3.76 ppm assigns the participation
O-H (aromatic) in ligand disappears in the complex, of methoxy-O in connection with metal ion. The sig-
thus clearly indicating the coordination of the ligand nals at ∼2.5 indicate the presence of coordinated water
through the deprotonated O-H group. The strong peak molecules.
at 3420 cm−1 is due to presence of water molecules.
Mass spectral study of this Zn complex is in good
The peaks at 1475, 1442 and 1389 cm−1 are assignable agreement with the theoretical molecular mass of the
to symmetrical stretching frequencies of carboxylate complex and also the data confirm the trinuclear nature
(-COOH) moiety. Further, the IR spectra of the complex of the Zn(II) complex. The ESI mass spectrum of
of Zn(II) exhibit new peaks at 1293, 1230 cm−1, indi- [Zn3L2(μ-O2CCH3)2(H2O)2]·H2O·2CH3OH exhibited
cate the chelation between phenolate-O and zinc(II) ion three triplets of peak envelopes (figure S4) at m/z
(figure S1). Characteristic bands in the range of 514– 306.0, 308.0, 310.0 and 610.9, 612.9, 614.9 and 915.9,
876 cm−1 may be assigned to M-N and M-O stretching 918.0 and 919.9. The molecular ion peak is found at
modes, respectively.16
m/z 915.99. The m/z values were very consistent with
assignment to the ions. ESI-MS (MeOH) m/z 306.2
[ZnL+H]+ (base peak, calcd. 306.9), 612.9 [Zn2L2+H]+
(calcd. 612.01), 915.99 [{Zn3L2(μ-O2CCH3)2(H2O)2]·
H2O·2CH3OH}+H]+ (calc. 916.03).
3.2b UV-visible spectroscopy: The electronic spec-
tra provided good information regarding the arrange-
ments of the ligands around the metal ions. The UV-
Vis spectra for ligand and metal complex show high
intensity transitions in the range of 200 to 470 nm. The
Schiff base ligand H2L shows the characteristic absorp-
tion bands at 233, 275, 347 and 461. The bands at
233 and 275 nm are assigned to π-π* transition of the
C=N chromophore, whereas the band at 347 nm is due
to n-π* transition and 461 nm is for intraligand charge
transfer17 (figure S2). On complexation, these bands
were shifted to lower wavelength region, suggesting the
coordination of imine nitrogen and phenolate oxygen
with Zn(II) ion (figure S2). The complex is soluble in
common organic solvents.
3.3 PXRD characterization
The powder X-ray diffraction (PXRD) pattern of
[Zn3L2(μ-O2CCH3)2(H2O)2]·H2O·2CH3OH is shown
in figure 2. All the peaks in the diffraction pattern can
be indexed with the triclinic structure with P-1 space
group using WINPLOTR programme. The extracting
cell parameters are a = 10.71613 Å, b = 12.19746 Å,
3.2c 1H NMR and mass spectrometric characteriza-
tion: The NMR spectra of Schiff base and its Zn(II)
complex are recorded in dimethylsulphoxide (DMSO-
d6) solution using tetramethylsilane (TMS) as an inter-
nal standard. The spectrum of the complex is exam-
ined in comparison with the parent Schiff base. Upon
examination, it is found that two types of phenolic-
OH signal appear in the spectrum of H2L ligand at
12.581 and 5.941 ppm. The first one indicates aromatic
OH proton in vanillin and the second one indicates the
aromatic OH proton in aminophenol, respectively. The
characteristic signal at 8.704 ppm indicates the proton
Figure 2. Powder X-ray diffraction patterns along with h,
k, l values for Zn(II) complex.

Synthesis and bioactivity of a trinuclear zinc(II) complex
1449
c = 20.47784 Å, α = 91.43◦, β = 91.80◦, γ = 90.28◦ complex (figure 3). After addition of CT-DNA to the
and the unit cell volume is 2674.440 (Å)3. From PXRD, solution of Zn(II) complex in tris-buffer, it is clearly
it is revealed that the complex is in crystalline phase.
observed that the absorption peak at 417 nm under-
goes a significant decrease in molecular absorption
(hypochromic effect) with no detectable shift in the
absorption wave length. The gradual decrease in the
absorption wavelength indicates strong interaction of
1 with DNA double strand. The binding constant, Kb
for the complex has been determined from the plot of
[DNA]/(εA − εF) vs. [DNA] and found to be 3.11 ×
104 M−1 (R = 0.9670 for four points) (figure 4).
3.4 Thermogravimetric analysis
Thermal behaviour of the Zinc(II) cluster was fol-
lowed up to 800◦C in a static nitrogen atmosphere
with a heating rate of 10◦C per minute. [Zn3L2(μ-
O2CCH3)2(H2O)2]·H2O·2CH3OH (1) decomposed in
four steps (figure S5). The first two steps from 40◦
to 297◦C corresponds to the endothermic process with
the loss of solvated methanol, solvated aqua molecule,
two coordinated water molecules and bridging acetate
molecules with the mass loss of 25.7% (calcd. 23.8%).
In the next two steps, the coordination environment is
decomposed and the experimental mass loss agrees well
with calculated mass loss.
3.5b Spectrofluorimetric titration: The fluorescence
spectral method using standard intercalator ethidium
bromide (EB) as a reference was used to determine the
relative DNA-binding properties of the complex to the
CT-DNA in tris-buffer (5 mM, pH 8.0). Fluorescence
intensities of EB in DNA were measured at different
complex concentrations. Addition of the complex to the
DNA pretreated with EB causes an appreciable reduc-
tion in the fluorescence intensity (figure 5) indicating
that 1 competes with EB to bind with DNA. Reduction
of the emission intensity gives a measure of the DNA-
binding propensity of the complex and stacking interac-
tion (intercalation) between adjacent DNA base pairs.19
Relative binding tendency of the complex with the CT-
DNA was determined from the comparison of the slope
of the lines in the fluorescence intensity versus complex
concentration plot.
3.5 DNA binding studies
3.5a Spectrophotometric titration: The DNA-binding
experiments were performed in Tris–HCl buffer
(50 mM Tris–HCl, pH 8) using a methanol solution of
the complex 1. Concentration of CT DNA was deter-
mined from the absorption intensity at 260 nm with an ε
value18 of 6600 M−1 cm−1. Absorption titration experi-
ments were made using different concentrations of CT
DNA, while keeping the complex concentration con-
stant. Due correction was made for the absorbance of
the CT DNA itself. Samples were equilibrated before
recording each spectrum. A broad spectrum in the range
of 250–500 nm is shown in the UV-Vis spectrum of the
Quenching of EB bound to DNA by the title com-
plex is in agreement with the linear Stern–Volmer
equation:
ꢄ
ꢅ
I0/I = 1 + K
complex ,
(2)
sv
0.5
a
14
12
10
8
0.4
a
e
0.3
e
0.2
0.1
0.0
6
2
3
4
5
6
7
8
250
300
350
400
450
500
[DNA]×106
λ, nm
Figure 3. Electronic spectrum of the title complex through Figure 4. Plot of [DNA]/(εA − εF) vs. [DNA] for the titra-
titration with CT-DNA in tris–HCl buffer; the increase of tion of CT-DNA with 1 in tris–HCl buffer, binding constant
DNA concentration is indicated by an arrow.
Kb = 3.11 × 104 M−1 (R = 0.9670 for four points).

1450
Bhaskar Biswas et al.
8000
a
e
7000
6000
5000
4000
3000
2000
1000
0
560 580 600 620 640 660 680 700 720 740
λ (nm)
Figure 5. Emission spectra of the CT-DNA-EB system in
tris–HCl buffer based on the titration of 1. kex = 524 nm; The
arrow indicates the increase of the complex concentration.
Figure 7. Thermal melting profile (relative absorbance
change at 260 nm versus temperature) of CT DNA (20 μM)
and its association with Zn(II) complex.
where I0 and I represent fluorescence intensities in the
absence and presence of quencher, respectively. Ksv is
the linear Stern–Volmer quenching constant and [com-
plex], the concentration of the quencher. From the slope
of the regression line in the derived plot of I0/I versus
[complex] (figure 6), the Ksv value for the complex was
found to be 3.08 × 104; (R = 0.9917 for four points)
indicating a strong affinity of the complex to CT-DNA.
experiment had a melting temperature (Tm) of 78◦C
(figure 7). Melting temperature of the native DNA
enhanced in the presence of the complex. The ꢀTm
value is 4◦C when the D/P ratio is 0.8. Such high stabi-
lization of the DNA helix is essentially due to the strong
binding of 1 either in the grooves or by intercalation or
by both modes.
3.5c Thermal melting studies: The binding was
further tested from optical thermal melting studies.20
Double stranded CT-DNA under the conditions of the
3.5d Hydrodynamic studies: To further probe and
distinguish between groove-binding, intercalative or
partial intercalative and groove-binding modes, visco-
sity of the DNA solution was measured in the presence
1.9
1.8
1.7
1.6
1.5
1.4
1.3
1.2
1.1
10
15
20
25
30
35
40
[complex] x106
Figure 6. Plot of I0/I vs. [complex] for the titration of
CT-DNA-EB system with 1 using spectrofluorimeter; linear
Stern–Volmer quenching constant (Ksv) for 1 = 3.08 × 104
(R = 0.9917 for four points).
Figure 8. Plot of ηsꢁp/ηsp versus D/P [DNA/EtBr (•),
DNA/Zn-complex (ꢀ)].

Synthesis and bioactivity of a trinuclear zinc(II) complex
1451
Table 1. Antibacterial activity of Schiff base ligand and its zinc complex.
Zone of inhibition (mm)
Compound
1 mg/0.01 ml
Escherichia
coli
Salmonella
typhimurium
Pseudomonas
aeruginosa
Staphylococcus
aureus
Bacillus
subtilis
Methanol (control)*
H2L
Zn(II)-L complex
–
7
14
–
6
13
–
6
15
–
5
10
–
3
7
of increasing concentrations of 1 and the change in of the partial sharing of its positive charge with donor
relative viscosities with varying inputs of 1 were esti- groups and possible π-electron delocalization on the
mated.21 Hydrodynamic method provides unequivocal whole chelate ring. The lipid and polysaccharides are
evidence for the mode of binding.22,23 Relative specific some important constituents of cell walls and mem-
viscosity of the CT-DNA–1 complex increased steadily branes, which are preferred for metal ion interaction. In
as the D/P (complex/DNA molar ratio) increased and addition to this, the cell wall also contains amino phos-
ultimately attained saturation at D/P ≥ 1.0 (figure 8). A phates, carbonyl and cysteinyl ligands, which main-
control experiment was also performed with the classi- tain the integrity of the membrane acting as a diffusion
cal intercalator EB. A comparative study was performed barrier and also provides a suitable site for bonding.
with the classical intercalator EB. Results favour a Chelation can reduce not only the polarity of the metal
intercalation mode of binding of the iron complex into ion, but also increases the lipophilic character of the
the double helical organization of the CT DNA.
chelate, and the interaction between the metal ion and
the lipid is favoured. This may lead to breakdown of the
permeability barrier of the cell, resulting in interference
with the normal cell process.
3.6 Bio-assay investigations
Schiff base ligands with multifunctionality have a
greater chance of interaction either with nucleoside
bases (even after complexation with metal ion) or with
biologically essential metal ions present in the bio-
system can be promising candidates as bactericides
since they always tend to interact especially with some
enzymatic functional groups, in order to achieve higher
coordination numbers.25,26 Our zinc(II)-Schiff base
complex shows more promising antimicrobial activi-
ties (figure S6) than the substituted phenol-based (N,O)
donor macrocyclic Schiff base zinc(II) complex.27
However, Pushpanathan and Suresh Kumar,27 have
shown great effect in antimicrobial study by investiga-
tion of substituent effect on the macro-cyclic system.
The substituent group was strongly deactivating the
macro-cyclic system and depleting the electron density
inside the ring. As a result, the metal ion became more
In testing the antibacterial and antifungal activity of
these compounds, we used more than one test organ-
ism to increase the chance of detecting antibiotic
principles in tested materials. A volume of 100 μL of
microbial suspension was spread onto agar plates cor-
responding to the broth in which they were maintained.
Isolated colonies of each organism that might be play-
ing a pathogenic role should be selected from primary
agar plates and tested for susceptibility by disc diffusion
method. The ligand and its metal complex have been
screened for their antimicrobial activity and the results
are presented in tables 1 and 2.
Enhanced activity of the complex over the Schiff base
can be explained on the basis of mechanistic aspects
of Tweedy’s Chelation theory.24 Chelation reduces the
polarity of the metal ion considerably, mainly because
Table 2. Antifungal activity of Schiff base ligand and its zinc complex.
Zone of inhibition (mm)
Compound
1 mg/0.01 ml
Aspergillus
parasiticus
Penicillium
Cladosporium
Rhizopus
Helminthosporium
Methanol (control)*
H2L
Zn(II)-L complex
–
5
9
–
5
7
–
6
9
–
6
10
–
4
7

1452
Bhaskar Biswas et al.
electrophilic to be bound strongly with metal-binding
sites in the enzymes of the bacteria. In our case, metal–
ligand coordination reduces the polarity of the metal ion
significantly which makes this trinuclear Zn(II) cluster
almost non-polar and it is also reflected from the sol-
ubility of this complex in various non-polar solvents.
Due to enhancement of non-polar character in this zinc
cluster, it is probably adsorbed on the surface of the
cell wall of microorganisms and disturbs the respira-
tion process of the cell and thus blocks the synthesis of
the proteins that restricts further growth of the organ-
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4. Conclusion
In the present investigation, we have reported synthe-
sis, structural characterization and biological activity of
a zinc(II) complex with a Schiff base ligand. Binding
constant for the complex is found to be 3.11 × 104 M−1
(R = 0.9670 for four points). The linear Stern–Volmer
quenching constant was determined as 3.08 × 104;
(R = 0.9917 for four points). The complex 1 enhanced
the thermal stabilization of native DNA by 4◦C clearly
suggesting strong stabilization effects. Viscosity mea-
surements provided evidence for intercalation of 1 into
the DNA double helix. Overall studies indicate an
intercalative mode of binding with the CT-DNA. Bio-
logical activities of the Schiff base and the zinc(II)
complex against bacterial and fungal organisms are
promising which need further extensive studies on ani-
mals and humans.
10. Schiff H 1864 Justus Liebigs Ann. Chem. 131
118
11. Biswas B, Mitra M, Adhikary J, Krishna G R, Bag P P,
Reddy C M, A-Alcalde N, Chattopadhyay T, Das D and
Ghosh R 2013 Polyhedron 53 264
12. Biswas B, Hunaiti A A, Räisänen M T, Ansalone S,
Leskelä M, Repo T, Chen Y-T, Tsai H-L, Naik A D,
Railliet A P, Garcia Y, Ghosh R and Kole N 2012 Eur. J.
Inorg. Chem. 4479
Supplementary information
13. Biswas B, Pal A, Krishna G R, Reddy C M, Tuna F and
The electronic supporting information can be seen in
www.ias.ac.in/chemsci.
Ghosh R 2012 Polyhedron 31 671
14. Constable E C, Housecroft C E, Zampese J A and Zhang
G 2012 Polyhedron 44 150
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Acknowledgements
We thank Dr. C Malla Reddy, IISER, Kolkata, for pro-
viding mass spectral studies. Bhaskar Biswas thanks the
University Grant Commission, New Delhi, India for
financial support.
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