CHAPTER-5
Oxovanadium Complexes with bidentate N, O ligands: Synthesis,
Characterization, DNA binding, Nuclease activity and Antimicrobial studies
INTRODUCTION
Vanadium has atomic number 23. It is a soft, silvery grey, ductile transition metal.
Vanadium was originally discovered by Andrés Manuel del Río, a Mexican mineralogist, in
1801. He first named it panchromium, because of the varied colors of its salts, but changed the
name later on to erythronium (‘red’) as a reference to the red color of its salts when treated with
acids [1, 2]. However, soon he withdrew his discovery, since a French chemist incorrectly
declared that this new element was only impure chromium. Vanadium was rediscovered in 1831
by the Swedish chemist Neils Gabriel Sefström (1787-1845) in remnants of iron ore quarried at
the Taberg in Småland. He named the element vanadin, after the goddess of beauty, youth and
love, Vanadis, referring to the beautiful multicoloured compounds [3]. After Sefström announced
the discovery of vanadium, the brown lead ore from Mexico was reanalyzed and it was shown
that it really contained vanadium instead of chromium. Vanadium is corrosion resistant and is
sometimes used to make special tubes and pipes for the chemical industry.
Vanadium occurs with an abundance of 0.014% in the earth’s crust and is widespread.
The element is the second most abundant transition metal in the oceans [4]. Vanadium has an
electronic configuration of [Ar]3d34s2 and can exist in eight oxidation states ranging from –3 to
+5, but with the exception of –2 [5]. Only the three highest, +3, +4 and +5 are important in
biological systems [6-8]. Under ordinary conditions, the +4 and +5 oxidation states are the most
stable. The coordination chemistry of vanadium is strongly influenced by the oxidation/reduction
properties of the metal center and the chemistry of vanadium ions in aqueous solution is limited
to oxidation states of +2, +3, +4 and +5. Vanadium compounds of oxidation state of +2 and +3
are unstable to air and their compounds are predominantly octahedral. Many oxovanadium(V)
complexes contain the VO2+ entity and the cis geometry in dioxo complexes have been
confirmed by structural determination [9]. The oxo complexes of the halides, alkoxides,
peroxides, hydroxymates and amino carboxylates have been characterized [10]. The oxidation of
ligands by vanadium(V) prevents the isolation of a larger number of complexes. On the other
hand, the oxidizing properties of vanadium(V) compounds are useful for many preparative
reactions, namely for the catalysis of oxidations.
Vanadium(IV) is the most stable oxidation state under ordinary conditions and majority
of vanadium(IV) compounds contain the VO2+ unit (oxovanadium(IV) or vanadyl ion) which can
persist through a variety of reactions and in all physical states. The VO2+ ion forms stable
anionic, cationic and neutral complexes with several types of ligands and has one coordination
position occupied by the vanadyl oxygen. A wide variety of oxovanaduim(IV) complexes have
been prepared and characterized [11]. A square pyramidal geometry has been well established
with the oxovanadium(IV) oxygen apical and the vanadium atom lying above the plane defined
by the donor atoms of the equatorial ligands. These square pyramidal complexes generally
exhibit strong tendency to remain five coordinate. However, orange polynuclear linear chain
structures (···V=O···V=O···) [12, 13] and orange octahedral structures with a weak coordination
of a solvent molecule are observed in the solid state for the Schiff base-oxovanadium(IV)
complexes which have a six-membered N-N chelate ring. These complexes take a distorted
octahedral coordination. The absorption band due to V=O stretching vibration of
oxovanadium(IV) complexes is usually observed at a higher wavenumber compared to those of
vanadate(V) complexes. The V=O stretching vibration, however, is susceptible to a number of
factors including electron donation from basal plane ligand atoms, solid-state effects, and
coordination of additional molecules. Therefore, there has been considerable work done to assign
the V=O stretching frequencies in oxovanadium(IV) compounds [14-16]. Electronic absorption
spectra of oxovanadium(IV) complexes are normally interpreted in terms of the energy level
scheme derived from a molecular orbital 18 treatment for a square-pyramidal structure with C4v
symmetry at the metal center [17, 18]. Due to the dπ configuration of V(IV) ions, vanadium(IV)
species are easily identified by EPR spectroscopy. Typical eight-line patterns are observed due to
hyperfine interaction of the 51V nucleus (I = 7/2).
The chemistry of vanadium metal is becoming significant due to its biological [19] and
industrial [20] outcomes like antimicrobial [21, 22] spermicidal [23], anti-leukemia [24],
antitumor [25] and recently as insulin mimetic [26, 27]. Fluconazole and Itraconazole are
triazole-derived class of known antifungal agents [28-30], potentially used against various fungal
species. 1,2,4-Triazole compounds also possess a variety of interesting biological activities such
as antibacterial [31-33], antitumor [34, 35], antitubercular [36-40], anticonvulsant [41-44],
anticancer [45, 46], analgesic [47], cytotoxic [48], antiproliferative [49] and plant growth
regulatory [50, 51].
Since vanadium has been found to have insulin-like properties in biological systems,
there is a significant activity in studying the properties and potential applications of new
synthetic vanadium compounds which could replace insulin in diabetes treatment [52-54].
LITERATURE REVIEW
Vanadium coordination complexes have shown increased potency over inorganic
vanadium. Vanadium complexation with organic ligands may decrease the toxicity by lowering
the vanadium dose required for effectiveness and improved GI absorption. Structure and
mechanism of the formation of the Schiff base complexes and stereochemistry of four coordinate
chelate complexes formed from Schiff bases and their analogues have been discussed in several
reviews [55, 56].
Kolawole and Patel [57] have synthesized a series of oxovanadium complexes. The
ν(V=O) stretching frequencies fall in the range 861–994 cm−l and the effective magnetic
moments at room temperature of the complexes are between 1.64 and 1.81 BM. The complexes
were green, and their spectroscopic and magnetic properties suggest that they have tetragonal
pyramidal structures.
Schiff base of 4-aminoantipyrine and its complexes have a variety of applications in
biological, clinical, analytical and pharmacological areas. Raman et al. [58] prepared a new
series of transition metal complexes of Cu(II), Ni(II), Co(II), Mn(II), Zn(II), VO(IV), Hg(II) and
Cd(II) from the Schiff bases derived from 4-aminoantipyrine, 3-hydroxy-4-nitrobenzaldehyde
and o-phenylenediamine. Structural features were obtained from their elemental analysis,
magnetic susceptibility, molar conductance, IR, UV–Vis, 1H NMR and EPR spectral studies. The
data show that these complexes have composition of ML type. The UV–Vis, magnetic
susceptibility and EPR spectral data of the complexes suggest square–planar geometry around
the central metal ion except VO(IV) complex which has square–pyramidal geometry. The redox
behaviour of the copper and oxovanadium(IV) complexes was studied by cyclic voltammetry.
Antimicrobial screening tests gave good results for the metal complexes. The nuclease activity of
the above metal complexes shows that Cu, Ni and Co complexes cleave DNA through redox
chemistry whereas other complexes were not effective.
Costa Pessoa et al. [59] have reported the preparation and characterization of new
oxovanadium(IV)-Schiff base complexes using amino acids and aromatic hydroxy aldehydes.
X-ray crystallographic studies were done in order to confirm the molecular refinement.
More recently, a comprehensive review by Mannar Maurya [60] have reported the
synthesis,
reactivity
and
structural
aspects
of
vanadium
through
bis(acetylacetonato)oxovanadium(IV) with 137 references.
Sumita Rao et al. [61] have described the subnormal magnetic moments of
oxovanadium(IV) Schiff base complexes derived from aroylhydrazones. The results revealed
that the tridentate ligand which on reaction with VO2+ form stable binuclear oxovanadium(IV)
complexes bridging through enolic oxygen and leading to the spin-spin exchange interaction
leading to subnormal magnetic moments. The EPR and magnetic susceptibility data of the
complexes suggest square-pyramidal geometry around the central metal ion.
Umesh et al. [62] have synthesized and characterized cationic mononuclear
oxovanadium(IV) complexes with tetradentate Schiff base derived from 2,4-dihydroxy-5-acetyl
acetophenone and substituted diamines. The electronic spectral data reveals the C4v symmetry to
the complex. Cyclic voltammetry assigns quasi-reversibility to the electron transfer step. The
FAB mass spectral results suggest the monomeric nature of the complex. The NMR of
vanadium-51 have been reviewed by Rheder et al. [63].
The antimicrobial and cytotoxic properties of oxovanadium(IV) complexes of triazolederived Schiff bases have been reported by Zahid Chohan et al. [64]. In-vitro antimicrobial
evaluation shown that the Schiff bases are weaker than their oxovanadium(IV) complexes.
The DNA binding and DNA cleavage activity of oxovanadium-Schiff base complex have
been reported by Swastik Mondal et al. [65]. The X-ray diffraction analysis indicates oxobridged octahedral vanadium(V) centers, which was in good agreement with spectral results. The
DNA interaction results revealed that the complexes of oxovanadium(IV) bind through
intercalative mode and also acts as very good cleaving agent even in the absence of oxidant.
Adeola Nejo et al. [66] have described the preliminary insulin-enhancing activity of
oxovanadium(IV) complexes. In vitro studies showed that the complexes significantly increased
glucose uptake when compare to the basal glucose uptake in the transformed and sensitized cells.
Cyclic voltammetry data revealed that the complexes are electrochemically pseudo reversible.
EXPERIMENTAL
Materials and methods
The solvents and chemicals used in this work were of Analar grade. Vanadium sulfate
trihydrate obtained from Sigma-Aldrich (USA). All materials used were of highest purity
available and without further purification. Solvents employed were of 99% purity or purified by
the known laboratory procedures. The CT-DNA was purchased from Bangalore Genie, the Tris
buffer, sodium chloride, 1,10-phenanthroline and hydrochloric acid (AR) were purchased from
Merck.
PROCEDURES
Synthesis of oxovanadium(IV) complexes
All oxovanadium(IV) complexes were prepared from vanadyl sulfate salt in 1:2 and 1:1:1
ratio using synthesized Schiff base ligands (L1-L4) and phenanthroline.
Synthesis of 1:2 ratio complexes (1 to 4)
To a 20 mL aqueous methanolic solution of vanadyl sulfate (0.163 g, 1 mmol), a
methanolic solution of respective Schiff base ligands (L1-L4) (2 mmol) was added drop wise and
the mixture was stirred for 30 min at room temperature and it was refluxed for 4 hours. The
resulting precipitates were filtered off, washed with hot water, methanol and dried under vacuum
over CaCl2.
Synthesis of 1:1:1 ratio complexes (5 to 8)
A methanolic solution of respective Schiff base ligand (L1-L4) (1 mmol) and
phenanthroline (0.198 g, 1 mmol) in methanol was added drop wise to a aqueous methanolic
solution of vanadyl sulfate (0.163 g, 1 mmol). The reaction mixture was stirred for 30 min and
refluxed for 4 h on a water bath. The volume of the solution was reduced to half of its original
volume. The solid compound obtained was filtered off, washed with hot water, methanol, then
with ether and dried in vacuum over CaCl2.
CHARACTERIZATION
The synthesized VO(IV) complexes were characterized by analytical, physical and
various spectral techniques. Infrared spectra were recorded in the range 4000-200 cm-1 on a
JASCO FTIR-8400 spectrophotometer using Nujol mull. The electronic spectra of the complexes
were recorded in DMSO using HITACHI-3900 spectrophotometer. The 1H-NMR spectra of the
VO(IV) complexes were recorded in DMSO-d6 solution. The mass spectra of the VO(IV)
complexes were determined on Varian 1200L model mass spectrometer. All these spectral
techniques are discussed in results and discussion section. The antimicrobial activities were done
by disc-diffusion method. The DNA binding studies were carried out in Tris-buffer solution with
CT-DNA by absorption spectroscopy.
BIOLOGY
Antimicrobial activity
The in vitro antimicrobial screening effects of the synthesized compounds were tested
against five bacterial strains namely Bacillus Subtilis, E.coli, Staphylococcus aureus, Ralstonia
solanacearum and Xanthomonas vesicatoria and four fungal strains namely Aspergillus niger,
Aspergillus flavus, Fusarium oxysporum and Alternaria solani by disc diffusion method using
nutrient agar medium for antibacterial activity and potato dextrose agar medium for antifungal
activity [66, 67].
The bacteria and fungi were sub-cultured in the agar and potato dextrose agar medium
and were incubated for 24 h for bacteria and 48 h for fungi at 37 ºC. Standard antibacterial drug
(Gentamycine) and antifungal drug (Fluconazole) were used for comparison. The discs having a
diameter of 4 mm were soaked in the test solutions and were placed on an appropriate medium
previously seeded with organisms in Petri plates and stored in an incubator at the above
mentioned period of time. The inhibition zone around each disc was measured and the results
have been recorded in the form of inhibition zones (in percentage). In order to clarify any effect
of DMF on the biological screening, separate studies were carried out with solutions alone of
DMF and they showed no activity against any microbial strains. The stock solution (1 mg/ml-1)
of the test compounds was prepared in DMF. Each test was performed in triplicate in individual
experiments and the average is reported.
DNA interaction experiments
The interaction of the oxovanadium complexes with CT-DNA was examined by UV-vis
and viscosity experiments. The detailed procedure was discussed in Chapter III.
RESULTS AND DISCUSSION
All the complexes are stable at room temperature, non-hygroscopic, insoluble in water
but slightly soluble in methanol and ethanol and completely soluble in DMF and DMSO. The
analytical and physical data are summarized in Table 1.
Table 1: Analytical data and physical properties of the VO(IV) complexes.
Found (calcd) %
Compound
Molecular
formula
1
2
3
4
5
6
7
8
C32H30N6O7V
C30H25N5O5V
C34H28N6O7V
C32H30N6S2O3V
C30H28N8O5V
C29H27N5O6V
C32H34N6O5V
C24H27N5SO4V
Yield
Molar
conductance
C
H
N
M
58.09
4.53
12.70
7.71
(57.94)
(4.22)
(12.66)
(7.59)
61.43
4.26
11.94
8.70
(61.05)
(4.15)
(11.26)
(8.34)
59.73
4.09
12.29
7.46
(59.37)
(3.94)
(11.99)
(7.27)
58.01
4.29
12.58
7.43
(57.89)
(4.22)
(12.24)
(7.29)
57.05
4.43
16.94
8.08
(56.91)
(4.34)
(16.73)
(7.87)
58.78
4.60
11.94
8.70
(57.95)
(4.32)
(11.61)
(8.56)
60.66
5.37
13.27
8.05
(59.74)
(5.16)
(13.11)
(7.81)
53.96
5.07
13.15
9.58
(53.34)
(4.96)
(13.02)
(9.23)
(%)
51
59
61
53
43
56
54
58
(Ω-1cm2mol-1)
18.4
15.1
22.3
17.6
19.9
14.3
16.7
17.1
Molar conductance measurements
The molar conductance values at the 10-3M concentration are too low to account for any
dissociation of the complex in DMF. Hence, the synthesized VO(IV) complexes may be
regarded as non-electrolytes.
IR spectra
The Schiff bases shows prominent peaks at ca 1675-1657 cm-1region corresponding to
ν(C=O) and another band at 1598-1540 cm-1corresponding to ν(C=N). In addition, a broad band
with fine structure was observed at 3441 and 3434 cm-1 for L1 and L3, respectively, that can be
attributed to ν(OH) in both L1 and L3. The IR spectra of all the ligands (L1-L4) were presented in
Chapter II.
The important IR spectral frequencies of oxovanadium(IV) complexes are listed in Table
2. In the spectra of all the complexes, the ν(C=O) band intensity decreases significantly as
compared to the free ligands and new bands appear in the region 1669-1637 cm-1. A sharp peak
observed at 965-988 cm-1 region due to ν(V→O).The ν(C=N) frequency is found in the complex
around 1564-1517 cm-1 (which is lower than that observed in free ligand) and lends support to
the coordination of azomethine nitrogen. Involvement of azomethine nitrogen in the
complexation is also supported by the presence of a new band at 438-416 cm -1, assignable to
ν(V→N) for these VO(IV) complexes. The representative IR spectra of complexes 2 and 4 with
their respective ligands are showed in Figures 1 and 2.
110
100
%T
50
0
4000
3000
2000
Wavenumber [cm-1]
1000
600
(a)
60
40
%T
20
0
4000
3000
2000
Wavenumber [cm-1]
(b)
Figure 1: IR spectra of (a) L2 and (b) its complex 2.
1000
400
110
100
%T 50
0
-10
4000
3000
2000
Wavenumber [cm-1]
1000
600
(a)
60
40
%T
20
0
4000
3000
2000
W avenumber [cm-1]
(b)
Figure 2: IR spectra of (a) L4 and (b) its complex 4.
1000
400
Table 2: Important infrared frequencies (cm-1) of VO(IV) complexes.
Compound
ν (C=N)
ν (C=O)
ν (M-N)
ν (M-O)
L1
1603
1603
-
-
L2
1597
1684
-
-
L3
1586
1650
-
-
L4
1574
1656
-
-
1
1546
1645
421
965
2
1534
1663
434
977
3
1520
1653
420
971
4
1547
1657
423
969
5
1564
1669
416
988
6
1537
1637
429
985
7
1533
1661
436
974
8
1557
1642
431
981
Electronic spectra and magnetic susceptibility measurements
The UV-Visible spectra of VO(IV) complexes were recorded in DMF at room
temperature and the characteristic bands are given in Table 3. All the complexes showed intense
band in the range 298-415 nm, which could be associated with metal to ligand charge transfer
transition (n-π*). Figure 3 shows the representative electronic spectra of complexes 1 and 5
which exhibit two bands having λmax at 298 and 349 nm. These can be assigned to the π → π*
transition and the ligand-to-metal-ion charge-transfer band. This result suggest that the square
pyramidal geometry for VO(IV) complexes. The weak d-d transitions were noticed in the range
of 571-585 nm.
All the oxovanadium(IV) complexes prepared are paramagnetic in the solid state and
ambient temperature paramagnetic susceptibilities were obtained. The effective magnetic
moments obtained were listed in the Table 3. With an electronic configuration of [Ar]3d1,
vanadium(IV) has one unpaired electron for which the spin-only formula predicts a magnetic
moment of 1.73 BM. The experimental values are in the range of 1.81-1.60 BM for the VO(IV)
complexes suggesting the square pyramidal geometries for the oxovanadium(IV) complexes.
Figure 3: Electronic spectra of complexes 1 and 5.
Table 3: Electronic absorption bands of Schiff base ligands and their VO(IV) complexes.
Electronic absorption bands and their
Magnetic moment
assignments (nm)
µeff BM
complexes
1
π-π*
n-π*
d-d transition
L1
317
402
-
-
L2
311
413
-
-
L3
309
411
-
-
L4
305
407
-
-
1
267
356
585
1.61
2
271
358
582
1.79
3
254
355
577
1.75
4
258
361
573
1.77
5
262
359
586
1.72
6
264
364
583
1.60
7
258
362
571
1.69
8
259
367
565
1.81
H-NMR
The Figure 4 is a representative 1H-NMR spectrum of complex 4. A signal due to
azomethine proton of Schiff base ligand (L4) (ca 8.25 ppm) shows a down field shift ca 9.07
ppm [68] in the spectrum of complex 4 indicates the coordination of the azomethine group to the
metal ion. This down field shift is due to deshielding of =CH proton. This was further supported
by IR inferences. The signal due to a multiplet ca 7.42-8.36 ppm due to aromatic protons and
signal around 2.74 ppm due to methyl protons are unaffected in the case of VO(IV) complexes.
Figure 4:1H-NMR spectrum of complex 4.
Mass spectra
The mass spectrum of complex 4 (representative spectrum) is shown in Figure 5. The
peak observed at m/z = 686 corresponds to the mass of mononuclear VO(IV) complex 4 (base
peak). The fragmentation peaks were also observed.
Figure 5: Mass spectrum of complex 3.
Thermal studies
The thermal behavior of the free ligands and the VO(IV) (2, 3, 6 and 7) complexes was
investigated by means of TG and DTG measurements under nitrogen atmosphere up to 700 ºC
(Figure 6). Thermograms of the complexes indicate that their decomposition occurs in one (1-4)
and two steps (5-8). The degradation starts at 180-260 ºC with a loss of a part of ligand molecule,
while an exothermic weight loss of ca.88.7 % (in complex 2), 90.2 % (in complex 3), 44.3 % (in
complex 6) and 45.2 % (in complex 7) associated with removal of ligand moiety occurs in the
temperature range of 180-500 ºC. Further decomposition occurs in the temperature range of 580680 ºC corresponds to the final residue estimated as free vanadium oxide with weight loss of
11.2 % (in complex 2), 18.2 % (in complex 3), 9.7 % (in complex 6) and 12.0 % (in complex 7).
The experimental results were in good agreement with the theoretical calculations and it is
presented in Table 4.
Figure 6: TG curves of VO(IV) complexes.
Table 4: Stepwise Thermal Degradation Data obtained from TGA Curves and their
Composition.
% Residue
% Weight loss
Complex
Process
Temp.
Products
Calcd
Expt
range
No. of Calcd
moles
Expt
Nature
(°C)
C32H30N6O7V
I
410-530
C32H30N6O6
89.30
89.05
1
10.68
10.15
VO
C30H25N5O5V
I
430-520
C30H25N5O4
88.73
88.51
1
11.26
11.15
VO
C34H28N6O7V
I
280-560
C34H28N6O6
90.24
90.08
1
9.75
9.66
VO
C32H30N6S2O3V
I
360-490
C32H30N6O2S2
89.83
89.64
1
10.16
10.08
VO
C30H28N8O5V
I
210-490
C16H15N3O3
44.65
44.53
1
12.27
12.06
VO
II
500-580
C12H8N2
I
200-380
C15H12N4O2
12.66
12.42
VO
II
500-580
C12H8N2
I
220-330
C17H15N3O3C1
11.26
11.03
VO
II
510-580
I
210-410
C16H15N3OS
11.94
11.79
VO
II
430-560
C12H8N2
C29H27N5O6V
C32H34N6O5V
C24H27N5SO4V
1
49.90
49.63
1
1
53.91
53.74
2H8N2
1
1
52.76
52.65
1
1
BIOLOGICAL RESULTS
Antimicrobial activity
The in vitro antimicrobial screening results are given in Table 5. On the basis of observed
zones of inhibition, it was found that the VO(IV) complexes are more active than their respective
Schiff bases. It was found that all the complexes exhibit good antibacterial activity against
S.aureus. Complexes 1, 3, 5 and 7 show potent antibacterial activities against all the
microorganisms and rest of the complexes show moderate activity when compared to the
standard. The antifungal activity of all the four complexes was moderate and that of ligands were
weak. It was observed that complexes5 and 7 were particularly active against all the tested fungi.
The higher activity of the metal complexes may be owing to the effect of metal ions on chelation.
The polarity of the metal ion will be reduced to a greater extent due to the overlap of the ligand
orbital and partial sharing of the positive charge of the metal ion with donor groups. Thus, it
enhances the penetration of the complexes into lipid membranes and blocking of the metal
binding sites in the enzymes of microorganisms.
Table 5: In vitro antimicrobial activity of the Schiff base ligands and their metal complexes.
Compound
Concentrati
on(µg mL-1)
Antibacterial activity (Zone of inhibition in
Antifungal activity (Zone of inhibition in
%)*
%)*
B.subtilis
E.coli
S.aureus
X.vesicatoria
A.niger A flavus
F.oxyspor
A.solani
um
L1
100
45
48
44
49
35
43
37
39
L2
100
33
39
37
34
29
32
31
30
L3
100
54
51
58
39
45
41
42
L4
100
35
40
38
36
31
33
35
32
1
100
78
81
78
83
51
55
57
53
2
100
67
72
77
63
47
53
55
48
3
100
80
83
81
79
53
55
59
56
4
100
67
66
71
69
41
52
47
51
5
100
79
82
80
85
73
74
76
71
6
100
63
65
73
65
46
54
61
63
7
100
82
87
85
89
75
77
81
73
8
100
68
70
75
72
53
51
59
60
Gentamycine
100
100
100
100
100
-
-
-
-
Fluconazole
100
-
-
-
-
100
100
100
100
*average of three replicates
52
DNA binding properties
The binding propensity of the complexes 1 and 5 with CT-DNA has been explored by
different techniques. The absorption spectra of the complex in absence and presence of CT-DNA
are monitored keeping the complex concentration as constant. With increase in the concentration
of CT-DNA, the absorption bands of the complexes are affected, resulting in hyperchromism or
hypochromism with a hypsochromic shift. In general, hypsochromism and red-shift are
associated with the intercalative binding of the complex to the double helix, due to strong
intercalation between the complex and the base pairs of DNA. The extent of hypochromism is
commonly consistent with the strength of the intercalative interaction [69, 70]. The absorption
spectra of complexes 1 and 5 at different concentrations of CT-DNA are given in Figure 6. In the
UV region, both complexes exhibit two absorption bands. Complex 1 shows absorption bands at
205 and 259 nm and complex 5 exhibits at 229 and 259 nm. This hypochromism can be
attributed to the π-π* transition. With increase in the concentration of CT-DNA, complexes 1
shows hypochromism with a 4 and 9 nm red-shift at 205 and 229 nm, while a complex 5 shows
hypochromism with a 1 and 2 nm red-shift at 259 and 258 nm. This results suggesting that the
interaction between the complexes (1 and 5) and CT-DNA is by intercalative mode. The results
indicated that the binding strength of the complex 5 is stronger than that of 1. This may be due to
the presence of phenanthroline ligand in 5 which may provide an aromatic moiety extending
from the metal center through which overlapping would occur with the base pairs of the DNA by
intercalation.
The binding of complexes 1 and 5 to duplex DNA led to a decrease in the absorption
intensities (Figure 7) with a small amount of red shift in the UV–Vis absorption spectra. For the
complexes 1 and 5, the intrinsic binding constant Kb has been determined from the spectral
titration data (Figure 8) using the equation (1):
(a)
(b)
Figure 7: Absorption spectra of (a) complex 1 and (b) complex 5, in Tris-HCl buffer upon
addition of DNA = 0.5 µM, 0-100 µL. Arrow shows the absorbance changing
upon increasing the concentration of DNA.
(a)
(b)
Figure 8: Plot of [DNA]/ (εa– εf) vs. [DNA] for the titration of DNA (10, 20, 30, 40, 50 and 60μM) with (a) 1
and (b) 5 complexes.
Viscosity measurements
The nature of binding of the complexes 1 and 5 to the CT-DNA was further investigated
by viscometric studies. Viscosity measurements were carried out using an Ubbelodhe viscometer
at room temperature. Flow time was measured by hand with digital stopwatch, each sample was
measured three times and the average flow time was calculated. A significant increase in the
viscosity of DNA on addition of complex results due to the intercalation which leads to the
separation among the DNA bases to the increase in the effective size in DNA which could be the
reason for the increase in the viscosity [71]. Plot of (η/ηo) 1/3 versus [complex]/[DNA] gives a
measure of the viscosity changes (Figure 9). A gradual increase in the relative viscosity was
observed on addition of the complexes 1 and 5 to DNA solution suggesting mainly intercalation
mode of binding nature of the complexes.
Figure 9: Effect of increasing amounts of VO(IV) complexes and ligands on the
relative viscosities of CT-DNA at room temperature.
Nuclease activity
Gel electrophoresis experiments using pUC19 plasmid DNA were performed with free
ligands (L1 and L3) and their complexes (1 and 3) in the presence and absence of H2O2 as an
oxidant. When super coiled DNA is conducted by electrophoresis, faster migration will be
observed for DNA of closed circular confirmation (Form I). If, one strand is cleaved, the
supercoiled DNA will relax to produce a slower moving nicked circular form (Form II). If, both
strands are cleaved, a linear confirmation (Form III) that migrates between Form I and Form II
will be generated [72]. This method using supercoiled pUC19 DNA in the presence of the
ligands (L1 and L3) and their metal complexes (1 and 3) was carried out in a medium of 50 µM
Tris-HCl/NaCl buffer (pH 7.2). Both complexes 1 and 3 showed remarkable cleavage in the
presence of H2O2 (30 µM) as an oxidizing agent.
Figure 10 shows the gel electrophoretic results of the interaction of compounds with
plasmid pUC19 DNA. From the figure 10 it is observed that, DNA cleavage by ligands (lane 3
and
lane 5) does not occur, indicating the importance of the metal for nuclease activity. Both
the complexes 1 and 3 can cleave DNA in the presence and absence of H2O2 (lane 6-10). The
cleavage is more efficient in the presence of H2O2 which may be due to the reaction of hydroxyl
radical with DNA [73]. Even in the absence of oxidant, the complexes exhibit significant DNA
nuclease activity (lane 7 and lane 9). This may be due to hydrolytic cleavage of the DNA
catalyzed by oxovanadium(IV) complexes.
Figure 10: Cleavage of supercoiled pUC19 DNA (0.5 µg) by the ligands and complexes 1 and 3
in a buffer containing 50 mMTris-HCl at 37 °C (30 min): lane M: marker; lane 1:
DNA control; lane 2: DNA + H2O2; lane 3: L1 (10-3 M) + DNA; lane 4: L1 + DNA
+ H2O2; lane 5: L3 + DNA; lane 6: L3 + DNA + H2O2; lane 7: complex 1 (10-3 M) + DNA;
lane 8: complex 1 + DNA+ H2O2; lane 9: complex 3+ DNA; lane 10: complex 3+ DNA
+ H2O2.
CONCLUSION
In conclusion, the author have synthesized eight novel oxovanadium(IV) complexes (1-8)
with the prepared Schiff base ligands (L1-L4). The synthesized VO(IV) were characterized by IR,
1
H-NMR, mass spectra and thermal studies. The DNA-binding propensity of complexes 1 and 5
were examined by absorption spectroscopy and viscosity measurements. Experimental results
indicate that both the complexes can bind to CT-DNA in intercalative mode and complex 5have
stronger binding affinity than 1. Both the complexes 1 and 3 can promote cleavage of plasmid
DNA in the presence and absence of oxidizing agent (H2O2) and thus, they may be regarded as
hydrolytic cleaving agents. The antimicrobial activity results revealed that oxovanadium
complexes are more potent than the free ligands. Based on the data obtained from the analytical,
physical and spectral studies, the following structures (Figure 11) are proposed for the VO(IV)
complexes (1-8).
S
O
HC
N
N
N
V
O
N
O
N
N
CH
S
4
Figure 11: Structure of VO(IV) complexes (1-8).
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