Mediated Dehalogenation of Organohalides at Both Rotating (Glassy Carbon and Pyrolytic Graphite) disk Electrodes and metmyoglobin films in Microemulsions

Abstract

Myoglobin (Horse heart, metMb) in bicontinuous microemulsion, prepared from sodium dodecylsulfate (SDS) or cetyltrimethylammonium bromide (CT AB), tetradecane. pentanol and water exhibited adsorption peaks on carbon electrodes (Glassy carbon-GC and pyrolytic graphite-PG), whereas on Platinum (Pt) potrayed diffusion-like controlled peaks. Electrode reduction of metMb had EO. of -0.244±0.0012V at PG/SDS, 0.200S±0.0063 V at PG/CT AB, -0.263±0.0008V at GClSDS, -0.228±0.0023 V at GCICTAB, -0.S20±0.0049V at PtlSDS and -0.S7S±0.0098V at PtiCT AB media. The surface concentrations of the metMb for the cathodic peaks were in the range of 1O-lomoIcm-2 at PG and 1O-12moIcm-2 at GC electrodes. Trichloroacetic acid (TCA) and dibromocyclohexane (DBCH) were reduced in the presence of metmyoglobin at the following potentials at rotating carbon electrodes (RDE): -0.262±0.0 13V vs. SCE at PG/acetate buffer solution/TCA, -0.300±0.0 II V vs. SCE at PG/SDS/DBCH, -0.330±0.0 13V vs. SCE at GC/SDS/TCA, -0.279±0.0 II V vs. SCE at GClCTAB/DBCH. The turn over rates (kcat) for the electrocatalytic reduction of TCA, catalyzed by metMb was 8.183±0.72 x 103S-1 in acetate buffer solution which was higher than in both SDS (6.83±1.177 S-I at PG and "8.93±2.S64 S-I at GC electrodes) and CTAB (9.929±0.818 S-I at PG and 9.178±0.161 S-I at GC electrodes) microemulsions. In case of DBCH, the kcat values were higher in SDS (2.0114±0.818 S-I) microemulsion than in acetate buffer solution (OA06±0.0032) S-I). Overall, the kcat values were higher for the TCA reduction than DBCH reduction. The direct reduction ofTCA and DBCH in both microemulsions on RDE occurred at -1.73V vs. SCE and -1.84V vs. SCE respectively. But under the influence of a catalyst (metMb), there was a lowering of overpotential of at least I.SV for both substrates. The catalytic efficiency, kcat/Km, as determined from linearized Michaelis-Menten equation on a linear regression program had an average of 106M-Is-I for the reduction of TCA in acetate buffer solution, 104 M-Is-I for TCA in both SDS and CTAB XIV microemulsions, 10' M-'s-' for the OBCH in acetate buffer solution and 105 M-'s-' for OBCH in SOS microemulsion at rotating carbon electrodes. The uv-visible spectra of 3.448x 1O-6MmetMb showed bands at Amax 409nm in both de-ionized water and acetate buffer solution and Amax 401nm and Amax 402nm in SDS and CTAB microemulsions respectively. The hypochromiclblue shift of the soret band of metMb in both SDS and CTAB microemulsions, compared to water and acetate buffer solutions by about 8nm indicates that the catalyst is in a microenvironment different from that of water a lone, pointing a finger to residency in interstitial sites occupied by the surfactant. Catalytic films were constructed on PG electrode by covalently binding poly-L-Iysine (PLL) onto oxidized carbon electrodes and then forming electrostatic linkages from PLL to the metMb through poly(styrenesulfonate)(PSS). Cyclic voltarnmetry of these films, "[PSS/MbIn'' in SDS microemulsion showed a pair of well defined, chemically reversible peaks at a bout -O.lS4V vs. SCE at PG electrode, characteristic of the Mb heme Fe"l/Fe" redox couple of surface bound electrochemistry. Electroactivity of these films was extended to 5 [PSS/Mbl bilayers. These films [PSS/MbJs in SDS microemulsions were used to facilitate redox reactions of polar trichloroacetic acid with significant decreases in the electrode potential required, thereby lowering the energy require to decompose the organohalide. Moreover, there was a decrease in catalytic efficiency, ic/id,with increase in scan rate, as expected in homogeneous solutions.