Role of a distal pocket in the catalytic O ₂ reduction by cytochrome c oxidase models immobilized on interdigitated array electrodes

Abstract

Five iron porphyrins with different superstructures were immobilized on self-assembled-monolayer (SAM)-coated interdigitatedarray (IDAs) gold-platinum electrodes. The selectivity of the catalysts i.e., limited formation of partially reduced oxygen species (PROS) in the electrocatalytic reduction of dioxygen, is a function of 2 rates: (i) the rate of electron transfer from the electrode to the catalyst, which is controlled by the length, and conjugation of the linker from the catalyst to the electrode and (ii) the rate of bound oxygen (superoxide) hydrolysis, which correlates with the presence of a water cluster in the gas-binding pocket influencing the rate of oxygen binding; these factors are controlled by the nature of the porphyrin superstructure. The structurally biomimetic Trisimidazole model is the most selective.electron transfer rate ͉ oxygen binding rates ͉ water cluster ͉ oxygen complex stabilization ͉ partially reduced oxygen species A recent report showed that water may inhibit oxygen binding in hemoprotein models (1). This echoes the ''water displacement model'' proposed for oxygen binding in myoglobin (2-4). This finding is relevant to cytochrome c oxidase (CcO) for 2 reasons: (i) CcO binds and reduces oxygen to form water (5) and (ii) the hydrolysis of the oxygen complex in CcO and in other enzymes is an issue and occurs when water, protons, and electrons are present (6, 7). This reaction releases partially reduced oxygen species (PROS) that are toxic to organisms (Fig. 1). The study herein examines the formation of PROS during the electrocatalytic reduction of oxygen by several site-isolated CcO models ( Fig. 2) as a function of (i) the oxygen association rate [k on (O 2 )] and the rate of hydrolysis, both associated with the presence of water in the gas binding pocket, and (ii) the availability of electrons at the oxygen reduction site associated with the rate of electron arrival from the electrode to the catalyst [k(eT)] and/or the presence of redox active species in the vicinity of the heme. The latter aspects were specifically addressed in previous studies by immobilizing the active-site models onto self-assembled-monolayer (SAM)-coated rotating ring disk gold electrodes (RRDE) (8-10): k(eT) was controlled by the length and conjugated character of the linker between the catalyst and the gold electrode, and immobilization allowed site isolation. To get better collection efficiencies for the detection of PROS and to prevent damage to the SAM associated with high rotation rates, the present study used Au-Pt interdigitated array electrodes (IDAs). The collection efficiency of these IDA electrodes is in the 65-90% range as compared with Ͻ20% in RRDE (8).IDAs have 2 sets of alternating arrays of electrodes. The electrode widths and spacing are on similar dimensions, and this distance is within diffusion layers for typical electroactive species in solution. This results in diffusion gradients of adjacent microband electrodes that overlap with each other. Redox products created at the Au generator ...

O 2 reduction by a functional heme/nonheme bis-iron NOR model complex

“…This remarkable reduction of PROS by this catalyst has been reported before by using a rotating ring disc (RRD) assembly and is due to the presence of the distal Cu and the redox-active phenol in the catalyst. Recently, we have shown that on an IDA electrode, the FeCuPhOH catalyst gives the same result as the RRD electrode (45).…”

“…The Au electrode was also electrochemically cleaned before experiments by several CV cycles between 1,650 and Ϫ200 mV at 250 mV in 0.5 H 2SO4. The azide-terminated thiols were synthesized and characterized as reported previously (33,45). The mixed thiol monolayer was deposited by immersing the cleaned IDAs in a 0.4 mM solution of a mixture of azide-terminated thiol and thiols in ethanol for 1 h.…”

Direct observation of intermediates formed during steady-state electrocatalytic O 2 reduction by iron porphyrins

“…Electrocatalytic O 2 reduction, O 2 evolution, H 2 evolution, and H 2 oxidation thus are areas of great interest and have attracted focused effort across several disciplines of science (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11). Both natural systems [metalloenzymes such as cytochrome c oxidase (CcO), hydrogenase, and glucose oxidase] and biomimetic systems (i.e., synthetic/biochemical mimics of the natural enzyme) are known to function as electrocatalysts (12)(13)(14)(15). In particular, it has been known for five decades that heme/porphyrin-based electrocatalysts (both natural and synthetic) catalyze electrochemical O 2 , H + , and CO 2 reduction (14,(16)(17)(18)(19).…”

Síntese e propriedades espectroscópicas e eletroquímicas de uma triazeno-porfirina

“…26,27 Este interesse é comprovado pelos inúmeros estudos sobre suas propriedades catalíticas e eletrocatalíticas, principalmente no que se refere aos derivados de manganês (III) e ferro (III …”

“…26,27 Este interesse é comprovado pelos inúmeros estudos sobre suas propriedades catalíticas e eletrocatalíticas, principalmente no que se refere aos derivados de manganês (III) e ferro (III O mecanismo mais provável de redução de nitro-derivados utilizando reações de redução mediadas por compostos de estanho ocorre via formação de hidroxilaminas. 70 No Esquema 12 é mostrado o mecanismo para estas reações de redução.…”