N. Raman, M. Selvaganapathy / Inorganic Chemistry Communications 37 (2013) 114–120
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Many biologically active compounds used as drugs possess modified
pharmacological and toxicological potentials when administered in the
form of metal-based compounds. Since the novel ligand and its transi-
tion metal complexes exhibit good DNA binding affinity, it is considered
worthwhile to investigate their other biological activities, such as
antibacterial and antifungal activity. The in vitro antimicrobial activity
of the ligand and its complexes was investigated against the sensitive
two Gram-negative (Escherichia coli and Pseudomonas aeruginosa) and
two Gram-positive (Bacillus subtilis and Staphylococcus aureus) bacterial
strains and for in vitro antifungal activity against Aspergillus niger,
Rhizoctonia bataticola, Candida albicans and Aspergillus flavus, by disc dif-
fusion method. The minimum inhibitory concentration (MIC) values of
the compounds are listed in Tables 3 and 4. The DMF control showed
no activity against any microbial strain. The activity of the complexes
has been compared with the activity of common standard antibiotics,
Kanamycin and Clotrimazole which are shown in Fig. 4a and b
(Tables 3 and 4).
Inspection of these data reveals that the newly prepared Schiff base
and its metal complexes showed an amazing effect against these micro-
bial strains. Such increased activity of the complexes can be explained
on the basis of Overtone's concept [30] and Tweedy's Chelation theory
[31]. According to Overtone's concept of cell permeability, the lipid
membrane that surrounds the cell favors the passage of only the lipid
soluble materials due to which liposolubility is an important factor,
which controls the antimicrobial activity. 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. Further, it increases the delocalization of
π-electrons over the whole chelate ring and enhances the lipophilicity
of the complexes. This increased lipophilicity enhances the penetration
of the complexes into lipid membranes and blocking of the metal bind-
ing sites in the enzymes of microorganisms. These complexes also dis-
turb the respiration process of the cell and thus block the synthesis of
the proteins that restricts further growth of the organisms.
Furthermore, the mode of action of the compound may involve for-
mation of a hydrogen bond through the azomethine group with the ac-
tive center of cell constituents, resulting in interference with the normal
cell process. Also, the increase in lipophilicity enhances the penetration
of Schiff base and its metal complexes into the lipid membrane and thus
confines further growth of the organism [32].
A comparative study of the ligand and its complexes (MIC values)
indicates that complexes exhibit higher antimicrobial activity than the
free ligand (Fig. 4a and b). From the MIC values, it was found that the
[CuL(Met)Cl2] was more potent among the other investigated com-
plexes. It may be attributed to the atomic radius and the electronegativ-
ity of Cu(II) ions. Current studies reveal that the high atomic radius and
electronegative metal ions in their metal complexes exhibit high anti-
microbial activity. Higher electronegativity and large atomic radius de-
crease the effective positive charges on the metal complex molecules
which facilitates their interaction with the highly sensitive cellular
membranes towards the charged particle.
Fig. 3. Cyclic voltammogram of [CuL(Met)Cl2] in buffer pH = 7.2 at 25 °C in the presence
of increasing amount of DNA. Arrow indicates the changes in voltammetric currents upon
increasing the DNA concentration.
Cu(III) → Cu(II) (Epa = 0.426 V, Epc = −0.156 V, ΔEp = 0.57 V, and
E1/2 = 0.135 V); in the second redox couple, the cathodic peak appeared
at −0.408 V for Cu(II) → Cu(I) (Epa = 0.154 V, Epc = −0.408 V,
ΔEp = 0.54 V, and E1/2 = −0.126 V); and in the third redox couple,
the cathodic peak appeared at −0.836 V for Cu(I) → Cu(0) (Epa =
0.008 V, Epc = −0.836 V, ΔEp = 0.83 V, and E1/2 = −0.404 V). The
Ipa/Ipc ratios for these three redox couples were 1.56, 1.27, and 1.20, re-
spectively, which indicate that reaction of the complex on the glassy car-
bon electrode surface is a quasi-reversible redox process. During the
incremental addition of CT DNA to the complex, the redox couples
caused a less negative shift in E1/2 and decrease of ΔEp (Table 2). The
Ipa/Ipc values also decreased in the presence of DNA (Table 2).
For Ni(II) → Ni(I) the redox couple cathodic peak appeared at
−0.539 V in the absence of CT DNA (Epa = −0.064 V, Epc = −0.539 V,
ΔEp = 0.47 V, and E1/2 = −0.298 V). The Ipa/Ipc ratio was approximately
unity. This indicates the quasi-reversible redox process of the metal com-
plex. During incremental addition of CT DNA to the complex, the redox
couple caused a negative shift in E1/2 and a decrease in ΔEp. Finally a
quasi-reversible transfer process with the redox couple [Zn(II) → Zn(0)]
was observed for the Zn(II) complex. The cathodic peak appeared at
−0.532 V in the absence of DNA (Epa = −0.065 V, Epc = −0.532 V,
ΔEp = 0.464 V, and E1/2 = −0.295 V). The Ipa/Ipc ratio was 1.03. This
indicates the quasi-reversible redox process of the metal complex. In-
cremental addition of DNA to the Zn(II) complex resulted in a slight de-
crease in the current intensity and negative shift of the oxidation peak
potential. The resulting minor changes in the current and potential are
indicative of diffusion of the metal complexes bound to the large, slowly
diffusing DNA molecule [29].
DNA is the pharmacologic target of many drugs currently in clinical
use or in advanced clinical trials. Small molecules that bind genomic
DNA have proven to be effective anticancer, antibiotic, and antiviral
therapeutic agents [33–35]. Agarose gel electrophoresis assay is a useful
method to investigate various binding modes of small molecules to
supercoiled DNA. Thus, suitably designed metal complexes, after bind-
ing to DNA, can induce several changes in the DNA conformation, such
as bending, ‘local denaturation’ (overwinding and underwinding), in-
tercalation, micro loop formation and subsequent DNA shortening,
leading to a decrease in the molecular weight of the DNA. The photo-
graph, as shown in Fig. 5, shows bands with different bandwidths com-
pared to the control and this is the differentiating criteria for binding
and cleavage abilities of the complexes with pUC19 DNA in this study.
DNA cleavage was controlled by relaxation of the super coiled circular
form of pUC19 into the open circular form and linear form. The general
Table 2
Electrochemical parameters for interaction of DNA with Cu(II), Ni(II) and Zn(II)
complexes.
Compound
Redox couple
E
1/2(V)a
Free
0.135
ΔEp(V)b
Ipa/Ipc
Bound
Free
Bound
[CuL(Met)Cl2]
Cu(III) → Cu(II)
Cu(II) → Cu(I)
Cu(I) → Cu(0)
Ni(II) → Ni(I)
Zn(II) → Zn(0)
0.123
−0.144
−0.416
−0.341
−0.358
0.57
0.54
0.83
0.472
0.464
0.59
0.56
0.85
0.523
0.376
1.46
1.27
1.20
0.95
1.03
−0.126
−0.404
−0.298
−0.295
[NiL(Met)Cl2]
[ZnL(Met)Cl2]
Data from cyclic voltammetric measurements:
a
E1/2 is calculated as the average of anodic (EPa) and cathodic (Epc) peak potentials;
Ea1/2 = EPa + Epc/2.
b
ΔEp = Epa − Epc
.