Current Research in Chemistry

Year: 2011 | Volume: 3 | Issue: 2 | Page No.: 87-97
DOI: 10.3923/crc.2011.87.97

Electrochemical Study of Interaction of 3, 5-Ditertiarybutyl Catechol with Fe(III) Complexes of N-Salicylidene-L-Amino Acids

Pradyut Sarma and Okhil Kumar Medhi

Abstract: Interaction of Iron (III) complexes of N-Salicylidene-L-amino Acids (i.e., [Fe(sal-L-his)]Cl.H₂O and [Fe (sal-L-ala)]Cl.H₂O) with catecholate substrate encapsulated in aqueous surfactant micelle has been projected as functional model of dioxygenase enzyme. Biomimetic environment for the study has been provided by the aqueous surfactant medium. Reaction of 3, 5-ditertiarybutyl catechol with the ferric complexes are studied elaborately by electrochemical techniques. These ferric complexes exhibit irreversible redox behaviour in aqueous as well as micellar medium, however, they are found to be quasi-reversible redox process in acetonitrile medium. Interaction of these complexes with 3, 5-ditertiary butyl catechol have shown by the presence of significant redox potential of semiquinone/catechol couple. The redox potential of semiquinone/catechol couple is more positive than that of the redox potential of free 3, 5-ditertiarybutyl catechol, it reflects stabilization of 3, 5-ditertiarybutyl catechol by chelation to ferric centre. Diffusion coefficient values of the catechol coordinated ferric complexes have been calculated and it found to be of the same order as those observed for other one electron reduction processes. The cyclic voltammogram of this interaction have recorded at various scan rates and it showed that this is a diffusion controlled redox process. All these observations provide support to the novel redox active pathway for modeling dioxygenase enzyme to convert phenolic substrates into less harmful aliphatic byproducts.

Keywords: Cyclic voltammetry, 3, 5-ditertiarybutyl semiquinone, 3, 5-ditertiarybutyl catechol (DBCH2), diffusion coefficient and osteryoung square wave voltammetry

INTRODUCTION

Iron is an essential element found in living system and its importance resides on its role as a protein active site constituent. Oxygen activating enzymes with mononuclear non-heme iron active sites participate in many metabolically important reactions that have environmental, pharmaceutical and medical significance (Ferraroni et al., 2006; Biegunki et al., 2010). Among the mononuclear iron enzymes catechol dioxygenase are the most important dioxygenase enzyme. Que and Reynolds (2000) as well as Costas et al. (2004) had categorically stated that catechol dioxygenase serves as part of nature’s strategy for degrading catechol and poly catechols in the environment. They are mainly found in soil bacteria and act in the last step of transforming aromatic precursors into aliphatic products. Dioxygenase enzymes are known that contain heme iron, nonheme iron, copper or manganese. Catecholate 1,2- Dioxygenase (CTD) and Protocatechuate 3, 4-Dioxygenase (PCD) are two important such bacterial non-heme iron enzymes.

The substrates whose oxygenations are catalyzed by these enzymes are very diverse, as are the metal-binding sites; so probably several, possible unrelated, mechanisms operate in these different systems (Ohlendrof et al., 1994; Horsman et al., 2005; Fortin et al., 2005; Mayilmurugan et al., 2007).

Even before the X-ray crystal structure of PCD was obtained, a picture of the active site (Fig. 1) had been constructed by detailed spectroscopic work using a variety of methods. The two enzymes have different molecular weights and subunit composition but apparently contain very similar active site structures and functions by very similar mechanisms. In both, the resting state of the enzyme contains one Fe^III ion bound at the active site. EPR spectra show a resonance at g = 4.3, characteristic of high spin Fe^III in a so-called rhombic (low symmetry) environment and the Mossbauer parameters are also characteristic of high spin ferric. The crystal structure of the protocatechuate 3, 4-dioxygenase from Pseudomonas aeruginose reveals a trigonal bipyramidal iron site with four endogenous protein ligands (Tyr 447 and His 462 as the axial ligands and Tyr 408 and His 460 as the equatorial ligands). The third equatorial ligand is assignees to a solvent derived ligand. The iron active site sits at the end of a cleft with a hydrophobic channel that seems likely to be the substrate binding pocket (Hatta et al., 2003; Miyazawa et al., 2004). Very recently Paria et al. (2010), Dinelli et al. (2010) and Zeng et al. (2010) also highlighted a similar functional model of extradiol-cleaving dioxygenases.

This work is shaped to study the interaction of [Fe (sal-L-aa)] Cl complexs (aa = his and ala) with potential phenolate substrate like 3, 5-ditertiary butyl catechol (DBCH₂) in aqueous surfactant micelles, as models of catechol dioxygenases. The afford is directed towards investigating the reactivity and kinetics of the interaction of the ferric complexes with 3, 5-ditertiary butyl catechol by electrochemical methods. In this study, it has been tried to verify the validity of the proposed oxidative cleavage mechanism for the present Fe (III)-phenolate adduct. To correlate this work with biological systems all investigation were done in aqueous surfactant medium which could provide the hydrophobic as well as hydrophilic environment in this whole study (Ekpenyong and Antai, 2007; Abdurahman and Yunus, 2009).

MATERIALS AND METHODS

Synthesis and characterization of [Fe(sal-L-aa)]Cl complexes (aa = his and ala): The complexes [Fe (sal-L-his)]Cl and [Fe(sal-L-ala)]Cl (Fig. 2) were prepared according to the already reported procedures (Casella et al., 1987). All chemicals were purchased from Loba Chemie and used without further purification. The electronic spectra of the Fe (sal-L-aa) Cl complexes displayed several bands in the near-UV and visible regions; the absorption maxima in methanol or aqueous methanol solutions occurred near 228 nm (ε = 13000-18000 M^-1 cm^-1), 256 nm (10000-15000 M^-1 cm^-1), 288 nm (5000 M^-1 cm^-1) and in the range 450-550 nm (1000-1400 M^-1 cm^-1). The visible band was responsible for the purple colour of the complexes. Weak shoulder near 423 nm was always clearly detectable on the low energy tail of the 320 nm band. The spectra showed solvent dependence; this was partly due to solvent coordination to iron (III) but was also due to a different degree of association of the complexes in the various solvents. The spectra of Fig. 2a and b in protic solvents often show significant concentration dependence, the visible band being mostly affected in these conditions. IR spectra of the complexes hinted v (OH) band at 3000-3300 cm^-1 and imine structure of the ligands account for intense and well resolved v (C = N) bands near 1590-1610 cm^-1. Band at 1300-1330 cm^-1 indicated v (O-Ph) stretching.

Electrochemical analysis: Cyclic voltammery is the most widely used technique for acquiring information about electrochemical reactions. The power of cyclic voltammetry results from its ability to rapidly provide considerable information on thermodynamics of redox processes and the kinetics of heterogeneous electron-transfer reactions, and on coupled chemical reactions or adsorption processes. Cyclic Voltammetry (CV) is often the first experiment performed in an electro analytical study. In particular, it offers a rapid location of redox potentials of the electropositive species and convenient evaluation of the effect of medium upon the redox process. The values of E_1/2 are determined by cyclic voltammetry can confirmed by further measurement by the relatively new technique in electrochemical analysis, namely Osteryoung Square Voltammetry (OSWV). Electrochemical experiments are performed on a BAS model 100A electrochemical analyzer. Bio Analytical system; USA. A three electrode cell assembly with Nitrogen gas purging lines was used in combination with the analyzer. The working electrode is glassy carbon, reference electrode is Ag-AgCl electrode and platinum electrode is used as auxiliary electrode. The supporting electrolyte is NaNO₃ about 100 times the concentration of the electroactive species. The reference electrode potential was calibrated by measuring the E_1/2 of the ferrocene/ferrocinium couple in acetonitrile.

Diffusion coefficient: Diffusion coefficient (D₀) is a factor of proportionality representing the amount of substance diffusing across a unit area through a unit concentration gradient in unit time. It is a significant parameter to categories the reaction dynamics of redox processes in electrochemical analysis.

The values of diffusion coefficient (D₀) of DBCH₂ coordinated [Fe(sal-L-aa)]⁺ adducts in aqueous surfactant micelles were calculated by substituting the slope obtained from the linear i_pc vs. v^1/2 (v<0.05 Vs^-1) plot in the Randles- Sevcik equation (Das and Medhi, 1998; Bard and Faulkner, 2006).

Where:

UV-visible spectra were recorded in a Hitachi U-3210 model as well as in Perkin Elmer Lambda 40 uv/vis spectrometer. The oxygenation kinetics experiments were performed in a 10 mm path length Thunberg quartz cuvette. pH dependent measurements were made by Adair Dupp. and Co. India) model number DPH- 77 pH meter. The pH electrode was SCE glass combined electrode which was standardized with known buffer solution. IR spectra was recorded a Perkin Elmer RXIFT - IR spectrophotometer. We used cationic micelle Cetyltrimethyl Amonium Bomide (CTAB), anionic micelle Sodiumdodecy Sulphate (SDS) and neutral micelles polyethylene glycol p-(1, 1, 3, 3-tetramethylbutyl)-phenyl ether (TritonX-100) in our experiments. In this study we have used different concentrations of the surfactant micellar solutions and they are 2%, 4% and 3% for SDS, CTAB and TritonX- 100, respectively. These micellar concentrations are adjusted above the level of the critical micellar concentration (Bari et al., 2009; Gokturk and Var, 2011). It favors the complete micellization of the complex and also reduce the chance of any change in mid point redox potential value due to any minor fluctuation in the micelle concentration.

RESULTS AND DISCUSSION

Complexes in Fig. 2 (a, b) were highly soluble in water and also in aqueous surfactant medium. However the redox potential of Fe³⁺/Fe²⁺ couple for the complexes were different in aqueous and aqueous surfactant solutions. When these complexes got dissolved in aqueous solution the fifth position around the Fe centre of the active site got occupied by H₂O molecule and actual formula of the complexes become [Fe (sal-L-his) H₂O]⁺ and [Fe (sal-L-ala) H₂O]⁺ (Casella et al., 1987). The midpoint potential (E_1/2) of the complex [Fe (sal-L-his)]⁺ in aqueous medium was -0.120 V. But in aqueous micellar medium these E_1/2 values had shifted towards more negative potential, for example [Fe(sal-L-his)]⁺ had E_1/2 values at -0.236 V in SDS micelle, -0.400 V in CTAB micelle and -0.182 V in TritonX-100 micelle. Similar sort of variation of E_1/2 values were also observed for complex [Fe (sal-L-ala)]⁺, e.g. in aqueous medium the E_1/2 of the complex was -0.213 V whereas in SDS micelle the value was -0.601 V, in CTAB micelle it was -0.360 V and in TritonX-100 micelle E_1/2 was -0.411 V. More negative values of E_1/2 in micellar medium compared to that of the aqueous solution hinted the grater stabilization of Fe^III centre in micellar medium towards reduction.

Complexes in Fig. 2 (a, b) are also soluble in acetonitrile. 3x10^-3 molar solution of complex (Fig. 2b) in acetonitrile had prepared and its cyclic voltammetric response had investigated. A cyclic voltammogram with similar height of cathodic and anodic peak had produced (Fig. 3).

The mid point potential (E_1/2) of complex (Fig. 2b) (i.e., [Fe (sal-L-ala)]⁺) was found to be 0.159 V. This Cyclicvoltammetric (CV) response was recorded at scan rate 0.1 Vs^-1 with peak separation ΔE_p = 70 mV. Similarly Cyclic Voltammetry (CV) of complex Fig. 2a ([Fe(sal-L-his)]⁺) also recorded in acetonitrile medium and the E_1/2 value was measured as 0.150V with peak separation ΔE_p = 111 mV. Cyclic voltammetric response is significantly based on its scan rate (represented by v). If we recorded different cyclic voltammogram at different scan rates for the same complex: [Fe(sal-L-his)]Cl.H₂O or [Fe(sal-L-ala)]Cl.H₂O then it showed the peak separation ΔE_p for each graph was nearly constant for lower scan rate and it was gradually increasing with increasing scan rate (Fig. 4). Current functionincrease with increasing scan rate and i_pc/i_pa ratios was 1.23 and 0.97, respectively. All these results indicated that these redox systems were quasi-reversible. These results are consistent with the redox process of other ferric complexes involving one electron reduction reaction (Das et al., 1997; Bard and Faulkner, 2006).

Interaction of DBCH₂ with [Fe(sal-L-his)]Cl and [Fe(sal-L-ala)]Cl in surfactant medium: Interaction of 3, 5-ditertiary butyl catechol (DBCH₂) with [Fe (sal-L-aa)]⁺ complexes (aa = his and ala) could easily be interpret by the electrochemical analysis. [Fe (sal-L-aa)]⁺ complexes exhibited irreversible cyclic voltammogram but when it got coordinated with DBCH₂ at appropriated pH level, a new oxidation wave appeared and it corresponding to the DBSQ/DBCH₂ couple. Interaction of DBCH₂ with metal centre required special pH condition, at low pH value (≈ 7-8.5) its first proton got departure from OH group and DBCH₂ became DBCH^-. At this stage DBCH¯ is available for monodentate coordination with ferric complexes. On the other hand, as pH value increased, (≈ 10-11.5), proton of next OH group of DBCH^- got deprotonated and it became DBC²¯ i.e., ready for bidentate coordination with ferric complexes. These coordination possibilities of 3, 5-ditertiary butyl catechol has shown in Fig. 5. These sort ligand stereoelectronic properties are also observed during interaction of DBCH₂ with Fe (III) complexes of tetradentate monophenolate ligands (Velusamy et al., 2004).

These facts were well furnished by the following potential values: 3x10^-3 mol amount of [Fe (sal-L-his)]⁺ and equivalent amount of DBCH₂ had dissolved in SDS surfactant solution. To this mixture 2 equivalent of base (NaOH) was added and the pH had increases to 7-8.5 and it favoured the monodenated coordination of 3, 5-ditertiary butyl catechol with [Fe (sal-L-his)]⁺ complexes which became [Fe(sal-L-his)DBCH] adduct. E_1/2 of this [Fe(sal-L-his)DBCH] adduct was -0.175 V. To the same solution when we had added again two equivalents of base and the pH level increased to 10-11.5 and consequently DBCH^- became DHC²¯ which was ready to coordinate with ferric centre by bidentate mode i.e., [Fe (sal-L-his)DHC]¯ adduct. E_1/2 of this bidentate complex adduct was -0.125 V.

Their peak currents were i_pc (cathodic current) 4.15x10^-6 ampere and i_pa -4.05x10^-6 ampere. By comparison it was evident that [Fe(sal-L-his) DBC]^- (bidentate coordination) had E_1/2 value more positive than that of [Fe(sal-L-his)DBC]¯ momodentate adduct (Table 1). It revealed that DBC^2- coordinated in bidentate mode in the former case was more stabilized towards oxidation that of the latter case (Fig. 6).

The potential required for oxidation of free 3, 5-ditertiary butyl catechlol (DBCH₂) was -0.890V and when it coordinated with ferric centre, either in monodenate or bidentate fashion, it got shifted toward positive potential (i.e., -0.175 and -0.125 V). It reflected the considerable stabilization of DBCH₂ from oxidation by chelation to ferric centre. Velusamy and Palaniandavar (2003), Wang et al. (1997) and Chiou and Que (1995) had also worked in this field and they too highlighted similar type of positive shifting of E_1/2 values after coordination with chelating ligand. E_1/2 of catecholate bound adducts were more positive than that of the redox potential of the parent complex and it reflects the Lewis acidity of the iron centre in [Fe (sal-L-aa) DBC]ⁿ¯ which decreases with the increase in number of co-ordinated phenolate groups. These results were also supported by Lauffer et al. (1983) and Nanni et al. (1980).

We had recorded the cyclic voltammograms of bidentate adducts [Fe (sal-L-his) DBC]¯ and [Fe (sal-L-ala) DBC]¯ at different scar rate. These scan rate dependent cyclic voltammetric response for [Fe (sal-L-ala) DBC]¯ adduct in the CTAB micellar medium was shown in Fig. 7. For these voltammograms, at slow scan rate the peak current ratio was linear and independent of scan rate; peak current i_pc and i_pa were linearly proportional to the square root of scan rate as shown in Fig. 8. These electrochemical results indicated that Fe³⁺/Fe²⁺ redox process of the catecholate adduct of Fe (III) complexes in surfactant micelles was a diffusion controlled quasi-reversible electron transfer process (Das et al., 1997; Das and Medhi, 1998).

The values of current function (D₀= 0.56x10^-11 - 138.1x10^-11 cm² sec^-1) of the ferric complexes were of the same order as those observed for other iron (III) complexes undergoing a one electron reduction process. These values were also listed in Table 1 and it was evident that diffusion constants of the complexes were lowered in micellar medium.

This was might be because of the high viscosity provided by the micellar medium. Nematollahi et al. (2010), Dhanalakshmi et al. (2006), Mayilmurugan et al. (2007) and Viswanathan et al. (1996) had also reported many works in this field and their work supported these range of diffusion coefficient values for Fe³⁺ Fe²⁺ reduction processes. The i_pc vs v^1/2 plot is linear but the values of peak current ratio i_pa/i_pc (1.0-0.6) and the peak potential separation ΔE_p (69-140 mV) suggest fairly reversible to irreversible redox processes.

CONCLUSION

The redox potential of metal bound DBSQ/H₂DBC couple is significantly more positive than that of free H₂DBC, reflecting considerable stabilization of H₂DBC by chelation to ferric centre. Also it is more positive than that of the Fe (III)/Fe (II) couple of the parent complex and reflects the lewis acidity of the iron centre in [Fe(L)DBC]ⁿ¯ which decreases with the increases in number of coordinated phenolate groups.

It is well known that phenolic waste products from paper industries and oil refineries cause a major threat to surrounding areas. Afford to work out proper redox environment for the conversion of toxic phenolic products into less toxic aliphatic byproducts would be of great significanc. Knowledge of proper redox behavior for such [Fe (sal-L-aa)]-DBCH₂ interaction would be a right step to construct devices which can accomplished the conversion of toxic phenolate substrate into less toxic aliphatic byproducts.

ACKNOWLEDGMENT

We thank the University Grant Commission, New Delhi, for supporting this research work during the period 2007-2009 (No.F.5-294/2007-08 (MRP/NERO)/1464). The researchers wish to acknowledge the Head of the department, department of chemistry, Gauhati University, for providing the facilities to accomplish the project work.

REFERENCES