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Synthesis, characterization, DNA interaction and antibacterial activities of two tetranuclear cobalt(II) and nickel(II) complexes with salicylaldehyde 2-phenylquinoline-4-carboylhydrazone
Inorganic Chemistry Communications 14 (2011) 1569–1573
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Inorganic Chemistry Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i n o c h e
Synthesis, characterization, DNA interaction and antibacterial activities of two tetranuclear cobalt(II) and nickel(II) complexes with salicylaldehyde 2-phenylquinoline-4-carboylhydrazone Zhi-hong Xu a,⁎, Xiao-wei Zhang a, Wan-qiang Zhang a, Yuan-hao Gao a, Zheng-zhi Zeng b, 1 a b
College of Chemistry and Chemical Engineering, Xuchang University, Xuchang, 461000, PR China College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, 730000, PR China
a r t i c l e
i n f o
Article history: Received 28 March 2011 Accepted 7 June 2011 Available online 14 June 2011 Keywords: Salicylaldehyde 2-phenylquinoline-4carboylhydrazone Tetranuclear complex Groove binding Antibacterial activity
a b s t r a c t Salicylaldehyde 2-phenylquinoline-4-carboylhydrazone (H2L), and its novel tetranuclear cobalt(II), nickel(II) complexes have been synthesized and characterized. The crystal structure of (NiL)2·[NiL(H2O)(DMF)]2· H2O·DMF obtained from DMF solutions of compound 2 was determined by X-ray diffraction analysis. The DNA-binding investigation suggests that the two complexes bind to DNA via groove binding mode. The in vitro antibacterial activity of complexes against Escherichia coli, Staphylococcus aureus, Bacillus subtilis was screened and compared to the activity of the free ligand, the antibacterial activity of complex 1 is active than complex 2 which is in consistent with their DNA-binding behaviors. © 2011 Elsevier B.V. All rights reserved.
Interaction of DNA with transition metal complexes has attracted considerable interests due to its various applications in cancer therapy and molecular biology [1–7]. Among them, Schiff base metal complex is a kind of attractive reagent due to their special activities in pharmacology and physiology [8–12]. Previous works had demonstrated that 4-quinolinecarboxylic acid amides and hydrazides, substituted at position 2, exhibit pronounced antiinflammatory and analgesic activity at a quite low toxicity [13]. However, to the best of our knowledge, less attention was paid on the interaction of DNA with Schiff base metal complexes derived from 2-phenylquinoline-4carboxylic acid. In our previous works [14–18], mononuclear complexes were obtained and exhibited different DNA-binding mode while the H2L interacted with different salts (nitrate, chloride), where intercalation mode was obtained when the salts were chloride and groove mode was obtained when the salts were nitrate. However, it was more interesting in the present work, tetranuclear cobalt(II) and nickel(II) complexes of the salicylaldehyde 2-phenylquioline-4-carboylhydrazone (H2L, Scheme 1) were obtained and exhibited groove binding mode when the salts were substituted by acetate. In addition, the antibacterial properties have been studied with a view to evaluate their pharmaceutical activities.
⁎ Corresponding author. Tel./fax: + 86 3744369257. E-mail addresses: [email protected] (Z. Xu), [email protected] (Z. Zeng). 1 Tel./fax: + 86 9318912582. 1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2011.06.005
The ligand H2L was prepared according to a method in the literature [14,19], and complexes were synthesized by reacting the ligand H2L and Co(AcO)2·4H2O and Ni(AcO)2·4H2O in ethanol [20]. The complex 2 crystallized in the triclinic lattice with a space group Pī (X-ray diffraction data shown in Table S1 and Table S2). The structure of the compound 2 is shown in Fig. 1. The structure of the compound is similar to the literature [21]. The compound consists of a tetranuclear planar neutral molecular unit. The molecule lies in the conversion center with the Ni…Ni separation of 3.098 and 7.626 Å for the phenoxide oxygen bridged pair, Ni1…Ni1(#1), and the N―N bridged pair, Ni1…Ni2 (symmetry code (#1) − x + 2,−y, − z + 2), respectively. The Ni1―N3―N2―Ni2 torsion angle is ca. 178.3(3) Å. The Ni1 is coordinated in a distorted octahedral environment to the deprotonated phenolic oxygen (O2), azomethine nitrogen (N3), and deprotonated enolimide oxygen (O1) of a ligand forming respectively five- and six-membered chelate rings, and a water O (O6) donor and an O (O5) of DMF in the opposite axial sites, the sixth coordination site is occupied by the symmetry related phenoxy oxygen of another ligand. The Ni2 bonds in the square plane to an ONO tridentate donors of a ligand and a N (N2) from another ligand. The five- and sixmembered chelate ring Ni1O1C1N3C18O2 is planar with the mean deviation of 0.018 Å. The bond distances of Ni2―O3, Ni2―N6 and Ni2―O4 agree with the bond lengths of the Ni complex of the analogous dinegative tridentate aroylhydrazone [22]. The other Ni―O and Ni―N bond distancesconsist with the Ni complex of the aroylhydrazone [23–27]. Even though the DMF is coordinated to nickel(II) in crystalline form of complex 2, the DMF will be replaced by water in aqueous solution
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Z. Xu et al. / Inorganic Chemistry Communications 14 (2011) 1569–1573
Scheme 1. 2-phenylquinoline-4-carboylhydrazone, H2L.
(See Fig. S1). The complexes will therefore exist as (NiL)2·[NiL (H2O)2]2·4H2O in our experimental section. The absorption spectra of complex 1 and 2 in the absence and presence of CT-DNA (at a constant concentration of complexes) are given in Fig. 2. In the presence of DNA, the absorption bands of 1 and 2 at about 267 nm exhibited hypochromism of about 21 and 43.2% and bathochromism of about 5 and 8 nm, respectively. The spectroscopic changes suggest that the complexes have stronger interaction with DNA. The intrinsic binding constants Kb of the two complexes with DNA were obtained by monitoring the changes in absorbance at 467 nm for complex 1 and 465 nm for complex 2 with increasing concentration of DNA. The intrinsic binding constants Kb of complexes 1 and 2 were 1.81 × 10 5 M − 1 and 0.89 × 10 5 M − 1, respectively. The results indicate that the binding strength of complex 1 is stronger than that of 2. Such a small change in λmax is more in keeping with groove binding, leading to small perturbations. The Kb value obtained here is lower than that reported for classical intercalator (for ethidium bromide and [Ru(phen)DPPZ] whose binding constants have been found to be in the order of 10 6–10 7 M) [28,29]. The observed binding constant is more in keeping with the groove binding with DNA, as observed in the literature [14,30].
The observed CD spectrum of calf thymus DNA consists of a positive band at 277 nm due to base stacking and a negative band at 245 nm due to helicity, which is characteristic of DNA in the righthanded B form. While groove binding and electrostatic interaction of small molecules with DNA show little or no perturbations on the base stacking and helicity bands, intercalation enhances the intensities of both the bands, stabilizing the right-handed B conformation of CTDNA. The CD spectra of DNA taken after incubation of the complexes with CT-DNA are shown in Fig. 3. In all two cases, the intensities of both the negative and positive bands decrease significantly (shifting to zero levels). This suggests that the DNA binding of the complexes induces certain conformational changes, such as the conversion from a more B-like to a more C-like structure within the DNA molecule [31]. These changes are indicative of a non-intercalative mode of binding of these complexes and offer support to their groove binding nature [32]. As shown in Fig. 4, the fluorescence intensity of complex 1 and 2 are quenched steadily with the increasing concentration of the CTDNA. The Stern–Volmer quenching plot from the fluorescence titration data is shown in Fig. 5. The fluorescence quenching constant (Ksv) evaluated using the Stern–Volmer equation is 2.46 × 10 4 M − 1 and 3.57 × 10 4 M − 1. The Stern–Volmer plot is linear, indicating that only one type of quenching process occurs. This phenomenon of the quenching of fluorescence of the complex by DNA may be attributed to the photoelectron transfer from the guanine base of DNA to the excited MLCT state of the complex [30,33–36]. Hydrodynamic methods that are sensitive to length are regarded as one of the least ambiguous and most critical tests of a binding mode in solution in the absence of crystallographic structural data. Intercalating agents are expected to elongate the double helix to accommodate the ligands in between the base leading to an increase in the viscosity of DNA. In contrast, complexes that bind exclusively in the DNA grooves by partial and/or non-classical intercalation, under the same conditions, typically cause less pronounced (positive or negative) or no change in DNA solution viscosity [37]. The values of
Fig. 1. ORTEP drawing of the compound 2 with thermal ellipsoid at 50% probability level, all hydrogen atoms are omitted for clarity.
Z. Xu et al. / Inorganic Chemistry Communications 14 (2011) 1569–1573
Fig. 2. Absorption spectra of the complex 1 (a) and 2 (b) in Tris–HCl buffer upon addition of calf-thymus DNA. [complex] = 1 × 10− 5 M, [DNA] = (0–5) × 10− 5 M. Arrow shows the absorbance changing upon increasing DNA concentrations. Inset: plots of [DNA]/(εa − εf) versus [DNA] for the titration of DNA with the complex.
(η/η0) 1/3 were plotted against [complex]/[DNA] (see Fig. S2). The results reveal that the complexes 1 and 2lead relatively to an inapparent increase in DNA viscosity, which is consistent with DNA groove binding suggested above, which is also known to enhance DNA viscosity [38]. The increased degree of viscosity, which may depend on its affinity to DNA follows the order of 1 N 2, which is consistent with our foregoing hypothesis. The cyclic voltammetric behavior of the Co and Ni complexes in the presence of CT-DNA is shown in Table 1. In the absence of CT DNA, the Epc and Epa are 1.050 and − 0.935 V for 1, and −0.117 and − 0.831 V for 2. The half-wave potentials E1/2, taken as the average of Epc and Epa
Fig. 3. CD spectra of CT-DNA (100 mM) in the absence (Solid line) and presence of compound 1 (Dash line) and 2 (Dot line) (50 mM).
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Fig. 4. Emission spectra of the complex 1 (a) and 2 (b) in Tris–HCl buffer upon addition of calf-thymus DNA. [complex] = 1 × 10− 5 M, [DNA] = (0–5) × 10− 5 M. Arrow shows the intensity changing upon increasing DNA concentrations.
are 0.058 and −0.474 V, respectively. In the absence of DNA, the separation of the anodic and cathodic peak potentials, ΔEp = 1.985 V for complex 1 and 0.714 V for complex 2. With the addition of DNA in the solution, it caused a considerable difference in the voltammetric current which changed from 1 to 0.59 for complex 1 and 1.16 for complex 2, respectively. In addition, the peak potentials, Epc and Epa, as well as E1/2 had a shift to more negative potential. The shift of the redox potential of the complexes in the presence of DNA to more negative values indicates a binding interaction between the complex and DNA that makes the complexes less readily reducible. The decreased extents of the peak currents observed for the complexes upon addition of CT DNA may indicate that complex 1 possesses higher DNA-binding affinity than complex 2 does. The results parallel the above spectroscopic and viscosity data of Co and Ni complexes in the presence of DNA. Complexes 1, 2 and the ligand (H2L) had been tested for their in vitro antibacterial activity against Escherichia coli, Staphylococcus aureus, Bacillus subtilis, comparative with free ligand, using the paper disc diffusion method [39,40] (for the qualitative determination) and the serial dilutions in liquid broth method [41] for determination of MIC. The antibacterial activity results of all of the tested compounds and their MIC values are presented in Table 2. The results evidently show that the ligand and its complexes possess antimicrobial activities against the tested bacteria at MIC values between 93 and 567 μg/ml, and that complex 1 is more active than complex 2 which is in agreement with their DNA-binding behaviors. The ligand exhibits better activity than the complexes. The presence of electron densities on aroylhydrazone group contributes positively to the increase of the activity of compounds against bacteria [42]. But the decrease of the electron densities by the coordination through the donor atoms
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Z. Xu et al. / Inorganic Chemistry Communications 14 (2011) 1569–1573 Table 2 The MICs values of ligand and its complexes (μg/ml). Compounds
Escherichia coli
Staphylococcus aureus
Bacillus subtilis
H2L 1 2 DMSO
93 170 283 –
123 170 212 –
74 140 213 –
Appendix A. Supplementary material Crystallographic data for the structure have been deposited with the Cambridge Crystallographic Data Center as supplementary material (CCDC-655710). These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by e-mailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Center, 12, Union Road, Cambridge CB2 1EZ, UK; fax: + 44 1223 336033. Supplementary data to this article can be found online at doi:10.1016/j.inoche.2011.06.005. References
Fig. 5. A Stern–Volmer quenching plot of the complex 1 (a) and 2 (b) with increasing concentrations of CT-DNA. Other conditions as in Fig. 4.
causes a decrease of activities of the complexes [43]. The antibacterial mechanism is presumably that the compounds affect the functions associated with cell division of fungi such as cell wall, protein, and/or DNA biosyntheses or kill the exponentially growing cells. In summary, two novel tetranuclear Co(II) and Ni(II) complexes have been synthesized and characterized. The DNA-binding results indicate that the complexes can bond to CT-DNA take the mode of groove binding, and complex 1has stronger binding affinity than 2. The in vitro antibacterial activity against E. coli, S. aureus, B. subtilis was screened and the antibacterial activity of complex1 is more active than complex 2 which is in consistent with their DNA-binding behaviors.
Acknowledgments The authors thank the Program for Science and Technology Innovation Talents in Universities of Henan Province (2011HASTIT029), Henan Province Science and Technology Key Project (112102310539), Natural Science Foundation of Henan Province (2010B150029) and Xuchang University for financial support.
Table 1 Cyclic voltammetric behavior of the Co and Ni complexes in the presence of CT-DNA. Complex
R
Epa/V
Epc/V
ΔEp/V
E1/2/V
ipa/ipa (R = 0)
1
0 0.04 0 0.04
− 0.935 − 0.876 − 0.831 − 0.797
1.050 1.038 − 0.117 − 0.035
1.985 1.941 0.714 0.762
0.058 0.081 − 0.474 − 0.416
1.00 0.59 1.00 1.16
2
R = [DNA]/[M].
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Inorganic Chemistry Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i n o c h e
Synthesis, characterization, DNA interaction and antibacterial activities of two tetranuclear cobalt(II) and nickel(II) complexes with salicylaldehyde 2-phenylquinoline-4-carboylhydrazone Zhi-hong Xu a,⁎, Xiao-wei Zhang a, Wan-qiang Zhang a, Yuan-hao Gao a, Zheng-zhi Zeng b, 1 a b
College of Chemistry and Chemical Engineering, Xuchang University, Xuchang, 461000, PR China College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, 730000, PR China
a r t i c l e
i n f o
Article history: Received 28 March 2011 Accepted 7 June 2011 Available online 14 June 2011 Keywords: Salicylaldehyde 2-phenylquinoline-4carboylhydrazone Tetranuclear complex Groove binding Antibacterial activity
a b s t r a c t Salicylaldehyde 2-phenylquinoline-4-carboylhydrazone (H2L), and its novel tetranuclear cobalt(II), nickel(II) complexes have been synthesized and characterized. The crystal structure of (NiL)2·[NiL(H2O)(DMF)]2· H2O·DMF obtained from DMF solutions of compound 2 was determined by X-ray diffraction analysis. The DNA-binding investigation suggests that the two complexes bind to DNA via groove binding mode. The in vitro antibacterial activity of complexes against Escherichia coli, Staphylococcus aureus, Bacillus subtilis was screened and compared to the activity of the free ligand, the antibacterial activity of complex 1 is active than complex 2 which is in consistent with their DNA-binding behaviors. © 2011 Elsevier B.V. All rights reserved.
Interaction of DNA with transition metal complexes has attracted considerable interests due to its various applications in cancer therapy and molecular biology [1–7]. Among them, Schiff base metal complex is a kind of attractive reagent due to their special activities in pharmacology and physiology [8–12]. Previous works had demonstrated that 4-quinolinecarboxylic acid amides and hydrazides, substituted at position 2, exhibit pronounced antiinflammatory and analgesic activity at a quite low toxicity [13]. However, to the best of our knowledge, less attention was paid on the interaction of DNA with Schiff base metal complexes derived from 2-phenylquinoline-4carboxylic acid. In our previous works [14–18], mononuclear complexes were obtained and exhibited different DNA-binding mode while the H2L interacted with different salts (nitrate, chloride), where intercalation mode was obtained when the salts were chloride and groove mode was obtained when the salts were nitrate. However, it was more interesting in the present work, tetranuclear cobalt(II) and nickel(II) complexes of the salicylaldehyde 2-phenylquioline-4-carboylhydrazone (H2L, Scheme 1) were obtained and exhibited groove binding mode when the salts were substituted by acetate. In addition, the antibacterial properties have been studied with a view to evaluate their pharmaceutical activities.
⁎ Corresponding author. Tel./fax: + 86 3744369257. E-mail addresses: [email protected] (Z. Xu), [email protected] (Z. Zeng). 1 Tel./fax: + 86 9318912582. 1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2011.06.005
The ligand H2L was prepared according to a method in the literature [14,19], and complexes were synthesized by reacting the ligand H2L and Co(AcO)2·4H2O and Ni(AcO)2·4H2O in ethanol [20]. The complex 2 crystallized in the triclinic lattice with a space group Pī (X-ray diffraction data shown in Table S1 and Table S2). The structure of the compound 2 is shown in Fig. 1. The structure of the compound is similar to the literature [21]. The compound consists of a tetranuclear planar neutral molecular unit. The molecule lies in the conversion center with the Ni…Ni separation of 3.098 and 7.626 Å for the phenoxide oxygen bridged pair, Ni1…Ni1(#1), and the N―N bridged pair, Ni1…Ni2 (symmetry code (#1) − x + 2,−y, − z + 2), respectively. The Ni1―N3―N2―Ni2 torsion angle is ca. 178.3(3) Å. The Ni1 is coordinated in a distorted octahedral environment to the deprotonated phenolic oxygen (O2), azomethine nitrogen (N3), and deprotonated enolimide oxygen (O1) of a ligand forming respectively five- and six-membered chelate rings, and a water O (O6) donor and an O (O5) of DMF in the opposite axial sites, the sixth coordination site is occupied by the symmetry related phenoxy oxygen of another ligand. The Ni2 bonds in the square plane to an ONO tridentate donors of a ligand and a N (N2) from another ligand. The five- and sixmembered chelate ring Ni1O1C1N3C18O2 is planar with the mean deviation of 0.018 Å. The bond distances of Ni2―O3, Ni2―N6 and Ni2―O4 agree with the bond lengths of the Ni complex of the analogous dinegative tridentate aroylhydrazone [22]. The other Ni―O and Ni―N bond distancesconsist with the Ni complex of the aroylhydrazone [23–27]. Even though the DMF is coordinated to nickel(II) in crystalline form of complex 2, the DMF will be replaced by water in aqueous solution
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Z. Xu et al. / Inorganic Chemistry Communications 14 (2011) 1569–1573
Scheme 1. 2-phenylquinoline-4-carboylhydrazone, H2L.
(See Fig. S1). The complexes will therefore exist as (NiL)2·[NiL (H2O)2]2·4H2O in our experimental section. The absorption spectra of complex 1 and 2 in the absence and presence of CT-DNA (at a constant concentration of complexes) are given in Fig. 2. In the presence of DNA, the absorption bands of 1 and 2 at about 267 nm exhibited hypochromism of about 21 and 43.2% and bathochromism of about 5 and 8 nm, respectively. The spectroscopic changes suggest that the complexes have stronger interaction with DNA. The intrinsic binding constants Kb of the two complexes with DNA were obtained by monitoring the changes in absorbance at 467 nm for complex 1 and 465 nm for complex 2 with increasing concentration of DNA. The intrinsic binding constants Kb of complexes 1 and 2 were 1.81 × 10 5 M − 1 and 0.89 × 10 5 M − 1, respectively. The results indicate that the binding strength of complex 1 is stronger than that of 2. Such a small change in λmax is more in keeping with groove binding, leading to small perturbations. The Kb value obtained here is lower than that reported for classical intercalator (for ethidium bromide and [Ru(phen)DPPZ] whose binding constants have been found to be in the order of 10 6–10 7 M) [28,29]. The observed binding constant is more in keeping with the groove binding with DNA, as observed in the literature [14,30].
The observed CD spectrum of calf thymus DNA consists of a positive band at 277 nm due to base stacking and a negative band at 245 nm due to helicity, which is characteristic of DNA in the righthanded B form. While groove binding and electrostatic interaction of small molecules with DNA show little or no perturbations on the base stacking and helicity bands, intercalation enhances the intensities of both the bands, stabilizing the right-handed B conformation of CTDNA. The CD spectra of DNA taken after incubation of the complexes with CT-DNA are shown in Fig. 3. In all two cases, the intensities of both the negative and positive bands decrease significantly (shifting to zero levels). This suggests that the DNA binding of the complexes induces certain conformational changes, such as the conversion from a more B-like to a more C-like structure within the DNA molecule [31]. These changes are indicative of a non-intercalative mode of binding of these complexes and offer support to their groove binding nature [32]. As shown in Fig. 4, the fluorescence intensity of complex 1 and 2 are quenched steadily with the increasing concentration of the CTDNA. The Stern–Volmer quenching plot from the fluorescence titration data is shown in Fig. 5. The fluorescence quenching constant (Ksv) evaluated using the Stern–Volmer equation is 2.46 × 10 4 M − 1 and 3.57 × 10 4 M − 1. The Stern–Volmer plot is linear, indicating that only one type of quenching process occurs. This phenomenon of the quenching of fluorescence of the complex by DNA may be attributed to the photoelectron transfer from the guanine base of DNA to the excited MLCT state of the complex [30,33–36]. Hydrodynamic methods that are sensitive to length are regarded as one of the least ambiguous and most critical tests of a binding mode in solution in the absence of crystallographic structural data. Intercalating agents are expected to elongate the double helix to accommodate the ligands in between the base leading to an increase in the viscosity of DNA. In contrast, complexes that bind exclusively in the DNA grooves by partial and/or non-classical intercalation, under the same conditions, typically cause less pronounced (positive or negative) or no change in DNA solution viscosity [37]. The values of
Fig. 1. ORTEP drawing of the compound 2 with thermal ellipsoid at 50% probability level, all hydrogen atoms are omitted for clarity.
Z. Xu et al. / Inorganic Chemistry Communications 14 (2011) 1569–1573
Fig. 2. Absorption spectra of the complex 1 (a) and 2 (b) in Tris–HCl buffer upon addition of calf-thymus DNA. [complex] = 1 × 10− 5 M, [DNA] = (0–5) × 10− 5 M. Arrow shows the absorbance changing upon increasing DNA concentrations. Inset: plots of [DNA]/(εa − εf) versus [DNA] for the titration of DNA with the complex.
(η/η0) 1/3 were plotted against [complex]/[DNA] (see Fig. S2). The results reveal that the complexes 1 and 2lead relatively to an inapparent increase in DNA viscosity, which is consistent with DNA groove binding suggested above, which is also known to enhance DNA viscosity [38]. The increased degree of viscosity, which may depend on its affinity to DNA follows the order of 1 N 2, which is consistent with our foregoing hypothesis. The cyclic voltammetric behavior of the Co and Ni complexes in the presence of CT-DNA is shown in Table 1. In the absence of CT DNA, the Epc and Epa are 1.050 and − 0.935 V for 1, and −0.117 and − 0.831 V for 2. The half-wave potentials E1/2, taken as the average of Epc and Epa
Fig. 3. CD spectra of CT-DNA (100 mM) in the absence (Solid line) and presence of compound 1 (Dash line) and 2 (Dot line) (50 mM).
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Fig. 4. Emission spectra of the complex 1 (a) and 2 (b) in Tris–HCl buffer upon addition of calf-thymus DNA. [complex] = 1 × 10− 5 M, [DNA] = (0–5) × 10− 5 M. Arrow shows the intensity changing upon increasing DNA concentrations.
are 0.058 and −0.474 V, respectively. In the absence of DNA, the separation of the anodic and cathodic peak potentials, ΔEp = 1.985 V for complex 1 and 0.714 V for complex 2. With the addition of DNA in the solution, it caused a considerable difference in the voltammetric current which changed from 1 to 0.59 for complex 1 and 1.16 for complex 2, respectively. In addition, the peak potentials, Epc and Epa, as well as E1/2 had a shift to more negative potential. The shift of the redox potential of the complexes in the presence of DNA to more negative values indicates a binding interaction between the complex and DNA that makes the complexes less readily reducible. The decreased extents of the peak currents observed for the complexes upon addition of CT DNA may indicate that complex 1 possesses higher DNA-binding affinity than complex 2 does. The results parallel the above spectroscopic and viscosity data of Co and Ni complexes in the presence of DNA. Complexes 1, 2 and the ligand (H2L) had been tested for their in vitro antibacterial activity against Escherichia coli, Staphylococcus aureus, Bacillus subtilis, comparative with free ligand, using the paper disc diffusion method [39,40] (for the qualitative determination) and the serial dilutions in liquid broth method [41] for determination of MIC. The antibacterial activity results of all of the tested compounds and their MIC values are presented in Table 2. The results evidently show that the ligand and its complexes possess antimicrobial activities against the tested bacteria at MIC values between 93 and 567 μg/ml, and that complex 1 is more active than complex 2 which is in agreement with their DNA-binding behaviors. The ligand exhibits better activity than the complexes. The presence of electron densities on aroylhydrazone group contributes positively to the increase of the activity of compounds against bacteria [42]. But the decrease of the electron densities by the coordination through the donor atoms
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Z. Xu et al. / Inorganic Chemistry Communications 14 (2011) 1569–1573 Table 2 The MICs values of ligand and its complexes (μg/ml). Compounds
Escherichia coli
Staphylococcus aureus
Bacillus subtilis
H2L 1 2 DMSO
93 170 283 –
123 170 212 –
74 140 213 –
Appendix A. Supplementary material Crystallographic data for the structure have been deposited with the Cambridge Crystallographic Data Center as supplementary material (CCDC-655710). These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by e-mailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Center, 12, Union Road, Cambridge CB2 1EZ, UK; fax: + 44 1223 336033. Supplementary data to this article can be found online at doi:10.1016/j.inoche.2011.06.005. References
Fig. 5. A Stern–Volmer quenching plot of the complex 1 (a) and 2 (b) with increasing concentrations of CT-DNA. Other conditions as in Fig. 4.
causes a decrease of activities of the complexes [43]. The antibacterial mechanism is presumably that the compounds affect the functions associated with cell division of fungi such as cell wall, protein, and/or DNA biosyntheses or kill the exponentially growing cells. In summary, two novel tetranuclear Co(II) and Ni(II) complexes have been synthesized and characterized. The DNA-binding results indicate that the complexes can bond to CT-DNA take the mode of groove binding, and complex 1has stronger binding affinity than 2. The in vitro antibacterial activity against E. coli, S. aureus, B. subtilis was screened and the antibacterial activity of complex1 is more active than complex 2 which is in consistent with their DNA-binding behaviors.
Acknowledgments The authors thank the Program for Science and Technology Innovation Talents in Universities of Henan Province (2011HASTIT029), Henan Province Science and Technology Key Project (112102310539), Natural Science Foundation of Henan Province (2010B150029) and Xuchang University for financial support.
Table 1 Cyclic voltammetric behavior of the Co and Ni complexes in the presence of CT-DNA. Complex
R
Epa/V
Epc/V
ΔEp/V
E1/2/V
ipa/ipa (R = 0)
1
0 0.04 0 0.04
− 0.935 − 0.876 − 0.831 − 0.797
1.050 1.038 − 0.117 − 0.035
1.985 1.941 0.714 0.762
0.058 0.081 − 0.474 − 0.416
1.00 0.59 1.00 1.16
2
R = [DNA]/[M].
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