Voltammetric behaviors of Copper (II) nitrogen bearing nucleobases, such as Guanine (C5H4N5O2) was studied in electro analyzer using cyclic voltammetry (CV) on a Glassy Carbon Electrode. Assessment of the chemical and physical conditions that may favor optimum current enhancement was done by studying the effect of variation of concentration of metal and ligand ions, variation of scan rate, variation of step height, variation of pH values, and variation of supporting electrolyte as (NH4)2SO4, KCl, and NaCl. It was observed that Copper and Guanine forms a 1 : 2 ratio complex. The work reflects that increasing the concentration of either metal ion or ligand ion increases the corresponding current. Increasing the scan rate increases the corresponding current linearly with the square root of the scan rate. As the step height decreases the peaks become sharp. Anodic and cathodic current increases linearly with decreasing step height. For the complex mixture the complexation occurs maximum at a pH of 2.3–7.0 and is badly restricted in the slightly alkaline medium and the complexing order of the supporting electrolyte showed a trend as
The complexation of organic compounds with selected metal ion has a wide variety of application in medicinal chemistry, surface chemistry, and analytical chemistry. Complexation of medicinal substances with ions influence the bioavailability of drugs in the body and the biological action that affects the stability of medicinal compounds since a large number of metals are taken into the body system either with drugs or in the form of diet. The complex formation has been suggested as one of the important mechanisms for certain drug action [
Toxic metals are generally more important than abundant metals, in terms of environmental pollution, because of their effects on living organisms. The study of trace metals toxicity on biological system [
Copper (II) ions (Cu2+) are soluble in water, where they function at low concentration as bacteriostatic substances, fungicides, and wood preservatives. In sufficient amounts, copper salts can be poisonous to higher organisms as well. However, despite universal toxicity at high concentrations, the Cu2+ ion at lower concentrations is an essential trace nutrient to all higher plant and animal life. In animals, including humans, it is found widely in tissues, with concentration in liver, muscle, and bone. It functions as a co-factor in various enzymes and in copper-based pigments.
Nucleobases are nitrogen-containing biological compounds found within DNA, RNA, nucleotides, and nucleosides. They are also termed as nitrogenous bases or simply bases due to their ability to form base-pairs and to stack upon one another leading directly to the helical structure of DNA and RNA. The primary Nucleobases bases are cytosine, guanine (Scheme
Chemical structure of guanine.
In DNA, guanine is paired with cytosine. With the formula C5H5N5O, guanine is a derivative of purine, consisting of a fused pyrimidine-imidazole ring system with conjugated double bonds. Being unsaturated, the bicyclic molecule is planar. Guanine has the C-6 carbonyl group that acts as the hydrogen bond acceptor, while a group at N-1 and the amino group at C-2 act as the hydrogen bond donors.
Metals in the ionic forms have essential catalytic physiological functions to perform in living systems, including human [
Guanine is a critical component of DNA; it has extra duties to perform in the cell, including defying the characteristics of the living body. In other words, they function as character determiner. Interestingly, guanine functions by binding to phosphate group and form guanosine and takes part in cell mechanism with thymine. This later involves numerous enzymatic reactions necessary for sustaining life and protecting the cell from toxins, either that produced by our cells or carcinogenic in our environment.
Thus researches on synthesis and properties of various metal-amino acid complexes are important from the view point of chemistry, bio-chemistry, and medical science. Attempts have been made in the present work to synthesize copper complex of guanine in order to gain greater insight in the metal interaction with guanine, which could help to understand the role of metal ions and Nucleobases in the biological process. Not too much research work was carried out with metal-ligand complex in searching their electrochemical behavior.
Electrochemical techniques, based on electrochemical sensors, are the most versatile analytical tool to study the redox behavior and to diagnose electro-active species in the field of chemistry, biochemistry, pharmacy, food science [
In this present work we are interested in investigating the electrochemical behavior of those complexes and their influence in our body system. Hence, the study, the redox behavior of guanine at GCE electrode and its complexation with Cu, has been investigated in aqueous medium by using electrochemical methods. Cyclic voltammetric studies of copper (II) and its complex with guanine to reveal the redox behavior in aqueous environment. Thus a systematic approach with the mode of formation and structural relationship with properties of this new compound may be derived from such study.
Analytical grade CuSO4·5H2O was purchased from E. Merck, Germany. Guanine (C5H5N5O) was purchased from China. Extra pure acetic acid (BDH, England) and phosphoric acid (BDH, England) were procured for buffer.
CuSO4·5H2O was dissolved in deionized water to prepare 0.5–2.0 mM solutions. Guanine solutions were prepared in slightly acidic medium (dil. HCl) upon warming. The sodium acetate buffer solution was prepared by taking 0.1 M CH3COONa, and 0.1 M CH3COOH of solutions at different ratios. The phosphate buffer solution was prepared by mixing 0.1 M di-sodium hydrogen phosphate, 0.1 M HCl, and 0.1 M NaOH solutions in different ratios. Milli-Q deionized water used throughout the experiment.
GCE was polished with fine alumina powder of 0.3 and then 0.03 micron sized on a wet polishing lather surface. For doing so a part of the lather surface was made wet with deionized water and alumina powder was sprinkled over it. The GCE was then polished on this surface for about 10–20 minutes by pressing softly the electrode against the polishing surface. A shiny black mirror like electrode surface was then thoroughly washed with deionized water. Under computer controlled magnetic stirring, experimental solution was deaerated by purging for at least 10 minutes with 99.9977% pure nitrogen gas before performing every experiment. Traces of dissolved oxygen gas were thus removed from the solution. Thus interference due to molecular oxygen dissolved in the solutions was eliminated from the Cyclic Voltammograms.
Three electrodes system consists of a GCE as the working electrode, Ag/AgCl (satd. KCl) as the reference electrode, and platinum wire as the counter electrode was used. Cyclic Voltammetric measurement was performed using Computerized Electrochemical System, Model HQ-2040 developed by Advanced Analytics, USA.
The redox behavior of Cu (II) in copper (II) sulphate was examined in 0.2 M KCl by Cyclic Voltammetric technique on glassy carbon electrode at room temperature.
From Figure
Cyclic Voltammogram of 0.5 mM Cu2+ in presence of 0.2 M KCl at −800 mV to 650 mV potential window, 5 mV step height and 100 mV s−1 scan rate.
Cyclic Voltammogram of 0.5 mM guanine, in presence of 0.2 M KCl at −1000 mV to 800 mV potential window, 5 mV step height and 100 mVs−1 scan rate was taken.
From Figure
Cyclic Voltammogram of 0.5 mM guanine in presence of 0.2 M KCl at −1000 mV to 800 mV potential window, 5 mV step height and 100 mV s−1 scan rate.
In Figure
Cyclic Voltammograms of copper ion, copper-guanine complex and guanine in presence of 0.2 M KCl.
Figure
In acidic medium, ring nitrogen N(3) of guanine is protonated (Scheme
Structure of guanine: (a) non-protonated, (b) protonated.
The guanine complexes of several metal ions, such as Al(III), Co(II), Cu(II), Zn(II), Pt(II), and Pd(II) have been reported earlier [
Attempts have been made to obtain the complex in solid form for further structural analysis but due to the instable nature of the complex as the individual components precipitate out with increasing pH, it was not possible to accumulate more structural analysis data to confirm the structure of the complex.
However from our observations and literature review Scheme
Proposed structure of copper (II) and guanine complex.
Cyclic Voltammogram of 0.25 mM guanine at −800 mV to 650 mV potential window and 20 mV step height and 100 mV s−1 scan rate in presence of 0.2 M KCl as supporting electrolyte and different Cu2+ ion concentrations (0.25 mM, 0.50 mM, 0.75 mM, 1.00 mM, and 1.25 mM) reflects that as the concentration of the Cu2+ increases, more reactions take place [
Cyclic Voltammogram of 0.25 mM Cu2+ at −800 mV to 650 mV potential window and 20 mV step height and 100 mV s−1 scan rate in presence of 0.2 M KCl as supporting electrolyte and different guanine concentrations (0.25 mM, 0.50 mM, 0.75 mM, 1.00 mM, and 1.25 mM) reflects that as the concentration of guanine increases the more reactions take place [
Cyclic Voltammogram of 0.6 mM Cu2+ and 1.2 mM guanine at −1000 mV to +600 mV potential window and 20 mV step height in presence of 0.2 M KCl as supporting electrolyte and different scan rates (25, 50, 75, 100, and 125 mV s−1) was taken (Figure
Cyclic Voltammogram of 0.6 mM Cu2+ and 1.2 mM guanine at −1000 mV to +600 mV potential window and 20 mV step height in presence of 0.2 M KCl as supporting electrolyte and different scan rates (25, 50, 75, 100, and 125 mV s−1).
The intensity for both the anodic and cathodic peaks is controlled by the following equation:
Two plots of anodic peak current versus square root of the scan rate and cathodic peak current versus square root of the scan rate are shown in the Figure
Peak current versus square root of the scan rate for copper and guanine complex.
The electrochemical processes are diffusion controlled which can be explained from the graph of
Cyclic Voltammogram of 0.25 mM Cu2+ and 0.50 mM guanine at −800 mV to +600 mV potential window at 100 mV/S scan rate at different step height (20, 25, 30, 35, and 40 mV) was taken. It is observed as the anodic and cathodic current decreases linearly with increasing step height.
Moreover in the lower step height the peak becomes sharper. This might happen as the interaction between the metal ions and ligand ions decreases with increasing step height and complex formation decrease and consequently anodic and cathodic current decreases with increasing step height [
Plotting peak current versus step height Figure
Peak current versus step height for Cu2+ and guanine Complex.
Cyclic Voltammogram of 0.25 mM Cu2+ and 0.50 mM guanine at −800 mV to +600 mV potential window at 100 mV/S scan rate and 20 mV step height at different pH Values (2.30, 7.00, and 10.0) was taken (Figure
Cyclic Voltammogram of 0.25 mM Cu2+ and 0.50 mM guanine at −800 mV to +600 mV potential window at 100 mV/S scan rate and 20 mV step height at different pH Values ( 2.30, 7.00, and 10.0).
Figure
Cyclic Voltammogram of 0.25 mM Cu2+ and 0.50 mM guanine at −800 mV to +600 mV potential window at 100 mV/S scan rate and 20 mV step height in presence of different Supporting electrolytes (0.2 M KCl, 0.2 M NaCl, and 0.2 M (NH4)2SO4).
Any species that compete with metal ion for the surface of the electrode would inevitably interfere in the adsorption of metal thyminate complex species hence leading to a decrease in the increase in current effect [
The results indicated that guanine is electroactive at −1000 mV to 800 mV potential window and 100 mV s−1 scan rate as it has an anodic peak at −200 mV potential having peak current of 3.7511
Complex formation of metal ion and ligands were observed according to Jobs Method and Mole Ratio Method. Cu (II) forms 1 : 2 complexes with guanine. Probable structures of the complexes of copper with guanine have been proposed, which is being supported by the previous works.
As the concentration of any component increases, interaction between the metal and ligand increases, more complexes are formed, more electrons are transferred, and consequently current increases. Scan rate was varied for all the metal and ligand pairs which indicates the increase in scan rate increases the number of time of current scanning in the system per second increases. Hence the corresponding current also increases. Plot of the peak current versus square root of scan rate gives straight line with positive slope passing through origins which indicate that the peak current is directly proportional to the square root of the scan rate.
Step height was varied from 20 mV to 40 mV for copper and guanine complex which implies that the corresponding anodic and cathodic current decreases with increasing step height, it is due to less interaction between the metal and ligand ions and fewer complexes are formed.
The pH value of the complex mixture was varied from 2.3 to 10.0 for all metal and ligand complex mixture by using acetate and phosphate buffers. At a lower pH the H+ ion compete with metal ions and less complexes are formed, less current is obtained, increasing pH value from 2.3 to 10.0, and current increases and becomes maximum and then decreases rapidly with increasing pH value. That implies the complex is favored to slightly acidic medium and slightly alkaline medium. The optimum pH range can be considered as 2.3 to 7.0 for the complexes of copper with guanine and thymine.
The supporting electrolytes were used as (NH4)2SO4, NaCl, and KCl for all metal and ligand pair complex ratio. More interaction occurs when SO4 2− containing supporting electrolyte (NH4)2SO4 is used compared to chloride containing supporting electrolyte KCl and NaCl. As Na is below K in the electrochemical series has higher tendency to be reduced compared to K, so less current is found for KCl than NaCl. So the complexation tendency of the supporting electrolytes follows the decreasing order (NH4)2SO4 > NaCl > KCl.
The authors declare that there is no conflict of interests regarding the publication of this paper.