Redox Behavior and DNA Cleavage Studies of Copper (II) Schiff Base Complex Derived From 2-Aminobenzaldehyde

 

P Jayaseelan, S Prasad and R Rajavel*

Department of Chemistry, Periyar University, Salem-11, TamilNadu.

*Corresponding Author E-mail: drrajavel@rediffmail.com

ABSTRACT:

The tetradentate Schiff base ligand was prepared by condensation of 2-aminobenzaldehyde with o-phenylenediamine. The synthesized complex has been studied by using cyclic voltammetry and DNA cleavage studies. The resulting voltammogram consists of a single quasi-reversible one electron transfer attribute to couple [Cu(II)L]/[Cu(I)L]. Trends in cathodic peak potential (Epc) values are observed which can be correlated with electron effects of Schiff base ligand, changes with basis of liquid groups are determinant for electrochemical trends. The interaction of Cu(II) complex with Calf Thymus DNA has been studied by using absorption, viscosity and cyclic voltammetry. Cyclic voltammetry studies reveal that the complex prefer to bind to DNA in Cu(II) rather than(I) oxidation state.

 

 

 

1.   INTRODUCTION:

Tetradentate Schiff base transition metal complex have been extensively investigated on electrochemical reduction process^1-3and nucleolytic activity. The tetradentate Schiff base ligand was prepared by condensation of 2-aminobenzaldehyde with o-phenylenediamine. Cyclic voltammetry has been a useful tool to investigate the mechanism of electron transfer reaction in ambopd. The complex of copper with ambopd-type ligand appear to promising ones, especially in view of the role played by copper in some aqueous biochemical process. Ambopd complexes have proved to be efficient in the cleavage of DNA and antimicrobial activity. Tetradentate schiffs base ligands with metal complex have been used as catalyst for organic reaction⁴. The electrochemical reduction of Cu(II) complex with Schiff base ligand has been studied by Zolezze et al⁵ CV using DMSO as solvent in vitreous carbon electrode. The electrochemical study of Cu(II) with Schiff base was studied by Perico Augusto⁶ by using DPV, CV, SWV. The electrochemical study of Schiff bases was studied by Ding et al ⁷ by formation of self assembled monolayer.

 

Sigman and coworkers have reported copper(II) complex on the first copper based “chemical nuclease” that cleaves DNA in presence of H₂O₂ and thiol ^8-9.

 

Similarly the anticancer, antibiotic bleomycins containing iron cleave DNA in oxidation manner¹⁰. Raman et al¹¹ have reported tetradentate Schiff base Cu(II) metal complex in nucleolytic activity.

 

The present study deals with synthesis, characteristics and DNA studies of metal complexes and redox reaction of the Cu(II) ambopd complex  using DPV, CV and SWV at the Glassy Carbon Electrode. The result shows that Cu(II)ambopd and the product of the reduction adsorb at the electrode surface with a one-electron reduction reaction through an EC mechanism. The interaction of CT-DNA was studied by UV, CV and viscosity measurements. The DNA cleavage was studied by electrophoresis method. The results shows that Cu(II) ambopd complex shows an efficient  cleaving agent.

 

2. MATERIAL AND METHODS

Synthesis of Ligand

 

Apparatus

Differential pulse stripping voltammograms were obtained with an CHI 760 electrochemical analyzer. The working electrode was glassy carbon for the differential pulse stripping and linear voltammetric techniques. Stair-case cyclic and square-wave stripping voltammetric measurements were performed with a CHI 760 voltammetric analyzer with glassy carbon electrode in the static mode.  For all experiments, the sample cells were fitted with a Ag/AgCl (3M KCl) reference electrode and a platinum wire auxiliary electrode. A magnetic stirrer and stirring bar provided the convective transport during the pre concentration step for the stripping techniques.

 

Gel Electrophoresis

The gel electrophories experiments were performed by incubation, at 35^oC for 2 hrs as follows. CT-DNA 30µM, 50μM each samples, 50 µM of H₂O₂ in50 mm bis HCl buffer (PH 7.2). The sample were electrophoresied for 2 hr at 50 V on 1% agarose gel using a Tris-acetic acid -EDTA buffer pH 7.2. After Electrophoresis the gel was strained using 1 μg /cm³ EBR and photographed under UV light¹².

 

Viscosity

Viscosity measurements were carried out using Ostwald’s Viscometer at 30.00 + 0.01^oC. Flow time was measured with three time. The average flow time was calculated. Date were presented as (η/η_o) value binding ratio (Cu/DNA). When η is a viscosity of DNA in presence of complex and η_o is the viscosity of DNA alone. Viscosity value were calculated from observed flow time of DNA containing of buffer alone to η = t - t₀

 

Chemicals and Solutions

All chemicals were of analytical reagent grade. The complex was prepared by mixing equimolar solutions of ambopd and copper(II) percholorate in ethyl alcohol. The precipitated complex was removed by filtration, washed with alcohol and placed in a dessicator containing P₂O₅. Elemental analysis gave 36.36% C, 3.40% H, 10.60% N. The copper was determined by atomic absorption spectroscopy and gave an equivalent quantity of 19.39% Cu. The stock solution of Cu(II) ambopd was prepared by dissolving the complex in dimethylsulfoxide (Merck) to a concentration of 0.01 mol.L^-1. The stock solution of a 0.2 mol.L^-1 phosphate buffer was prepared by dissolving a suitable quantity of reagent KH₂PO₄ (Sigma) in water, followed by the addition of 0.2 mol.L^-1 NaOH solution to achieve pH 7.0.

 

Procedure

A known volume (15 mL) of the supporting electrolyte solution (0.2 mol.L^-1 phosphate buffer (pH 7.0)) containing Cu(II) ambopd was added to the voltammetric cell.  The initial potential ( -0.2 V to differential pulse and +0.1V to square- wave stripping) was applied to the electrode during a selected time ( 30 seconds), while the solution was slowly stirred. The stirring was then stopped, and after 30 seconds under rest the voltammogram was recorded by applying a negative-going potential scan. The scan was terminated at -1.3 V (square-wave). The entire procedure was automated, as controlled by Stripping Analyzer. Throughout this operation, nitrogen was passed over the solution surface. The cyclic voltammograms were scanned (50 mVs^-1) in the potential range of 2.0 V to -2.0 V.

 

3. RESULTS AND DISCUSSION:

The present work reports an electrochemical behavior of copper(II) complex with the Schiff base ambopd, Cu(II) ambopd, in aqueous phosphate medium. The free Schiff base ambopd hydrolyzes in aqueous media but stabilizes after complexation with copper(II). The characterization and electrochemical studies of Cu(II)-ambopd in the present work were performed in a 0.02 mol.L^-1 phosphate buffer at pH 7.0. The electrochemical studies were realized using different voltammetric techniques (DPV, CV, and SWV). The resulting voltammograms obtained consist of a single quasi-reversible one-electron transfer attributable to the coupling of Cu(II) ambopd /Cu(I) ambopd via an EC mechanism.

 

Differential Pulse Voltammetry ( DPV)

A linear dependence of the differential pulse adsorptive stripping peak potential of the Cu (II) ambopd complex with pH was observed over a range of 5.0 to 9.0. 62mV was observed as slope. This value is very close to 59 mV which is the theoretical value for a two-electron redox reaction with ά = 0.5. This slope indicates the reversible redox reaction. The following mechanism  may be taking place.

 

(Cu(II) ambopd)_ads  + 2e^-  + 2H⁺  ↔ Cu(carbon) + ( H₂ ambopd)_ads  → (1)

 

The height of the reduction peak current due to time during which the solution was stirred, while a deposition potential is applied to Glass carbon Electrode, and this is also shown in figure1. The peak current increases linearly with 160 seconds, then the stripping  peak was decreased. This may be saturation of electrode surface and diffusion of reduced species to the bulk solution. The Adsorptive stripping potential is used to quantitation of trace level of Cu(II) ambopd complex, by using short accumulation time.

 

Figure 1. Differential pulse adsorptive stripping voltammograms obtained for increasing Cu(II)ambopd complex concentrations in 6.0 x 10^-8 mol L^-1 steps (2-6). Supporting electrolyte, 0.02 mol.L^-1 phosphate buffer (pH 7.0). Accumulation time, 30 s at -0.20V with stirring solution. The curve “a” represents the blank.

 

 

Cyclic Voltammetry

The characterization of electrochemically active system is done by CV, and also used to study the electron transfer system are coupled to chemical reaction ¹³. In cyclic voltammogram (fig 2) four signals were observed (signal 2 at -0.71V, signal 3 at -1.32V, 4 at -1.63V) The signal 2 due to quassi-irreversible Cu(II) ambopd /Cu(I) ambopd it is also confirmed by DPV. Signal 3 associated with Cu(I) to Cu(0). The signal 4 due to reduction of free primary amine nitrogen or the disproportionate of Cu(I)  ambopd (5). The signal 1 associated with Cu(II) or Cu(I) or presence of (Cu(II)(ambopd) )²⁺. Two peaks observed in reverse scan. Signal 5 related to 2, and to signal 6 related to 1. In fig 2 the cyclic votammogram of Cu(II) in buffer is shown. One little peak is in forward scan and one little peak at reverse scan. In fig 2 a small peak at (0.1) is forward scan, is due to reduction of copper-opd complex, one high peak in reverse scan at -0.05V.

(a)

 

 

(b)

Figure 2. Cyclic voltammogram for Ligand (fig a), and of the Cu(II)ambopd complex (4.0 x 10^-6 mol L^-1) (fig b) in unstirred 0.02 mol L^-1 phosphate buffer (pH 7.0). Scan rate, 50mVs^-1. Initial potential,2.0V.

 

The Cyclic voltammetry is widely used for the initial characterization of electrochemically active systems. In addition, CV can also be used for mechanistic studies of systems in which the electron transfer reactions are coupled to chemical reactions. The cyclic voltammograms for the reduction of Cu(II)ambopd in phosphate buffer (pH 7.0) is shown in Figure 2. As the potential is swept from 0 to -1.4, the reduction of Cu(II) ambopd proceeds through three distinct processes (signal 2 at -0.71 V, 3 at -1.32 V and 4 at -1.63 V). An electrochemically quasi-reversible Cu(II) ambopd /Cu(I)ambopd is responsible for signal 2, results confirmed by DC, DPP and DPV. The reduction of Cu(I)ambopd to Cu(0) is associated with signal 3 and the reduction of free primaryamine nitrogen or the disproportionate of Cu(I)ambopd (an irreversible chemical reaction) with signal 4. Signal 1 (at -0.16V) refers to the reduction of copper(I) or (II) (from different equilibrium). In the reverse scan, two peaks appear at -0.67 V (signal 5) and at -0.05V (signal 6). Signal 5 seems to be correlated with signal 2 and signal 6 with 1.   Figure.2  shows a little peak at – 0.61 V in the forward scan due to the reduction of the copper-o-phenylenediamine complex, and one higher peak on the reverse scan at -0.21 V.

The heights of the cathodic and anodic peaks are not the same, this shows that the complex in non-reversible reduction process. From Table.1, if we increase the scan rate, signal 2 shifts to more negative potentials.  The peak separation Epc - Epa becomes larger, this shows the system becomes more irreversible as the scan rate increases.  The ratio of  Ipa/Ipc remains constant in the range of 50-200mv Sec^-1. The adsorption of the Cu(II) ambopd at the electrode surface is again confirmed is the scan rate increases, the peak potential shifted to negative potential. From the above said results we predicts a mixed diffusion-adsorption reduction process.

 

The results obtained in CV shows that the mixed-diffusion-adsorption reduction process at glassy carbon electrode and this will also indicates that the Cu(II) ambopd complex and the products its reduction  adsorb at the electrode surface with one electron reduction  through a EC mechanism. The following redox reaction may be taking place.

 

 

Table 1. Effects of scan rate on the staircase cyclic voltammograms adsorptive current and potential peak for 2.0 x 10^-4 mol.L^-1 Cu(II) ambopd complex. Supporting electrolyte, 0.02 mol L^-1 phosphate buffer (pH 7.0). Initial potential: 0.0 V. Switching potential, -1.2 V. Equilibrium time, 30s.

Scan rate mV.s-1	Peak potential, mV	Epc-Epa	Ipa/ipc
50 100 150 200 300 500 800 1000	0.713 0.725 0.740 0.773 0.790 0.816 0.835 0.846	-230 -250 -265 -283 -310 -330 -360 -390	0.290 0.297 0.285 0.250 0.115 0.090 0.074 0.091

 

(Cu(II) ambopd )_ads + e^-   → (Cu(I)ambopd)_ads    →2

(Cu(II) ambopd )_ads →  Final products   → 3

 

The experiment was performed by repetitive scan. The cathodic (signal 1) and anodic (signal 6) current gradually increase with repetitive scans. The peak current is greater than that of the solution species alone (from the first scan). This results show that the formation of films at the electrode surface. This may be due to formation and reduction of Poly(Cu(I) ambopd )n film on electrode of the reduction  of Cu(II) ;or Cu(I). The metal complex from polymers on electrodes and also oxidative polymerization to form  Poly (metal(III) ambopd) . Further repetitive scan, signal 3 (fig 5 b) reduced to smaller due to quick desorption of the ambopd.

 

Square-Wave Voltammetry (SWV):

The SWV is used to study the electrode mechanism with adsorption phenomena. The redox reaction mechanism will be identified by the relationship between the SWV response and the parameters of charge transfer. The characteristics of redox mechanism will be studies by the relationship between potential and current peak, with the square wave frequency and pulse amplitude. SW voltammogram of Cu(II)ambopd complex in aqueous phosphate at pH 7.0. The signal 1 due to reduction of copper ion, signal 2 due to reduction of Cu(II) to Cu(I). The signal 3 due to reduction of Cu(I) to Cu(0). Signal 4 is due to reduction of primary amine nitrogen or the disproportion of Cu(I) ambopd. The peak potential observed in CV (fig at 0.71V peak 2) and SWV (Fig at -0.65) could be explained by stabilization of intermediate (Cu (I) ambopd) of electrode reactions 4 & 5.  In CV, DPV, and SWV on static carbon, the adsorption of Cu(II)ambopd induces this stabilization and the first one-electron transfer occurs at higher potential, while the second one-electron transfer occurs at lower potential than the two-electron transfer in the absence of the stabilization.

 

(Cu(II)ambopd)_ads + e^- ↔ (Cu(I)ambopd)_ads         → 4

(Cu(I)ambopd)_ads + e^- + 2H⁺ → Cu(carbon) + (H₂ ambopd)_ads                                                    →5

 

Cyclic voltammogram on DNA Binding studies:

The cyclic voltammetry studies is very much useful to study the interaction between complex and DNA. The voltammogram of complex with DNA and without DNA are shown below. It has been studied in DMF  using  Tertiarybutylammanium Perchlorate as supporting electrolyte. The electrochemical data obtained at glassy carbon electrode in DMSO solution. The separation of anodic and cathodic peak potential, ∆Ep is 0.15V, In absence of DNA the formal potential E_1/2 taken as negative of Epc and Epa is -0.075V. When addition of DNA the complex shift the E_1/2 to negative side and decrease ∆Ep. The ratio of anodic peak, cathodic peak current (I_pa/I_pc) decreases from 4.5 to 3.00 respectively absence and presence of DNA. The decrease in peak current can be explained in terms of an equilibrium mixture of free and DNA bound copper(II) complex to the electrode surface. From the data we can predict that the complex binds to DNA.

 

Figure 3. Square wave voltammogram for 7.5 x 10^-8 mol L^-1 of Cu(II)ambopd complex. Supporting electrolyte, 0.02 mol L^-1 phosphate buffer (pH 7.0). Initial potential, 0.4 V. Final potential, -2.0V.

 

E_1/2 value to more negative potential suggest that both Cu(II) and Cu(I) form of complex bind to DNA but with Cu(II) displaying higher DNA binding affinity that Cu(I) form. This is also illustrated by the ratio of the equilibrium constants (K⁺/K⁺⁺) for the binding of Cu(I) and Cu(II) Species to DNA. The shift in E_1/2 as the addition of DNA assuming reversible electron transfer using the following equation, E_b-E_f = 0.0591 log(K⁺/K⁺⁺).Where E_b and E_f are the formal potentials of the Cu(II)/Cu(I) couple in the bound and free forms, and K⁺ and K²⁺ is the binding constants for -/+ and 2+ species to DNA(17).

 

Figure 4. Cyclic voltammogram for Cu(II)ambopd in the presence of CT-DNA.

 

Viscosity studies:

The DNA viscosity is increased significantly due to complete or partial intercalation of molecules in DNA base stacking but it is slightly disturbed by electrostatic or covalent binding of molecules¹⁴. The change I the specific relative viscosity of DNA as addition of increasing concentration of  the complex are shown in fig.5. The decrease in relative viscosity of DNA for the complex predict that the covalent binding of complex with CT-DNA, which produced bends or kinks in the DNA and thus reduced its effective length and concomitantly its viscosity¹⁵. The result shows that the complex may be bind to DNA covalently. Effective increasing amount of complex and relative viscosity of CT-DNA at 30.00+0.01^◦C.

 

Fig. 5 Effects of increasing amount of complex  Cu(II) ambopd on the relative viscosity of CT-DNA at 30 ± 0.1^oC  [DNA] = 5 x10^–3M, pH 7.5.

Absorption Studies:

The electronic spectroscopy is verymuch useful to study the binding of comolex with DNA. The interaction  between DNA and complex is expected  to disturb the ligand centred spectral  transition of complex¹⁶. The absorption spectrum of complex around 210,250 and 530 on the incremental addition of CT-DNA to complex as small increase in the molar absorptivity accompanied by a red shif at 4 nm. The hyperchromic and hypochromic effect are the spectral features of DNA binding concerning its double helix structure. The abserved hyporchromic changes in the UV spectrum of the complex suggest strong binding of CT-DNA  mostly due to covalently.

 

Fig. 6 Absorption spectra of (a)Cu(II)ambopd  (b)Cu(II)ambopd on addition of CT-DNA. Indicates the absorbance changes upon increasing DNA concentration.

 

DNA cleavage studies:

The cleavage efficiency of the complexes compared to that of the control is due to their efficient DNA binding ability. The metal complexes were able to convert super coiled DNA into open circular DNA. The proposed general oxidative mechanisms and account of DNA cleavage by hydroxyl radicals via abstraction of a hydrogen atom from sugar units that predict the release of specific residues arising from transformed sugars, depending on the position from which the hydrogen atom is removed¹⁷. The cleavage is inhibited by free radical scavengers implying that hydroxyl radical or peroxy derivatives mediate the cleavage reaction^18-22. The reaction is modulated by a metallocomplexes bound hydroxyl radical or a peroxo species generated from the co-reactant H₂O₂. In the present study, the CT–DNA gel electrophoresis experiment was conducted at 35°C using our synthesized complexes in the presence of H₂O₂ as an oxidant. As can be seen from the results (figure 7), at very low concentration, few complexes exhibit nuclease activity in the presence of H₂O₂. Control experiment using DNA alone (lane 1) does not show any significant cleavage of CT-DNA even on longer exposure time. From the observed results, we conclude that the Cu(II)ambopd  complex cleaves DNA as compared to control DNA.

 

Fig. 7 Changes in the agarose gel electrophoretic pattern of calf-thymus DNA induced by H₂O₂ and metal complexes, Lane 1, DNA alone; Lane 2, DNA + Ligand + H₂O₂; Lane 3,DNA+Cu(II)ambopd complex + H₂O₂;

 

4. CONCLUSIONS:

The   Differential pulse voltammetry, cyclic voltammetry,  and Square wave voltametry experimental results shows that the Cu(II) ambopd and the product of its reduction, in aqueous phosphate electrolyte, adsorb at the carbon electrode surface with a one-electron reduction reaction through an EC mechanism. This study also describes the determination of trace levels of copper(II) in presence of ambopd.  Repetitive cyclic voltammograms also shows that the possible formation of a polymer film at the Glassy Carbon  electrode surface. The cyclic voltammetric study and Electronic spectral studies clearly suggest that copper(II) ambopd binds to CT-DNA through intercalating way. The elctrophoresis experiment suggest that the Cu(II) ambopd complex cleaves DNA in the presence of hydrogen peroxide.

 

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Received on 05.01.2010        Modified on 27.02.2010

Accepted on 29.03.2010        © AJRC All right reserved