Subsequent Reoxidation Research Articles

Electrochemical examination of the ascorbic acid radical anion in non-aqueous electrolytes

A quasi-reversible redox reaction involving ascorbic acid was observed in non-aqueous electrolytes at conductive diamond electrode. The chemical reversibility of these reactions is consistent with ascorbic acid being reduced to the ascorbic acid radical anion in a one-electron process, with subsequent reoxidation to ascorbic acid. This is the first report on the electrochemical production of the ascorbic acid radical anion in non-aqueous electrolytes. Ascorbyl 6-stearate and 4-hydroxy 2(5H)-furanone, which have somewhat similar structures as ascorbic acid, also showed one-electron transfer reduction reaction producing radicals with a single negative charge, suggesting that these compounds follow the same electrochemical behavior as ascorbic acid. The double bond and hydroxyl substituent on the five-membered ring are shown to be necessary for the stabilization of the radical anions. It was confirmed by the calculation of the total energy using molecular orbital methods that resonance structures involving the double-bond and hydroxyl group provide significant stabilization of the radical anions. Electrochemical preparation may be a useful method for the detailed study of radicals, their molecular structure and reactivity.

Single‐crystal LiNbO3 surfaces processed in low‐temperature hydrogen plasma: XPS, REELS and AFM study

AbstractSingle crystals of lithium niobate, which is a widely used material in opto‐electronics, holography, surface acoustic wave devices, etc., have been processed in a low‐temperature plasma of radiofrequency discharge in hydrogen. The aim was to examine a new kind of surface modification of LiNbO3. The surface modification has been studied by XPS, reflection electron energy‐loss spectroscopy (REELS) and atomic force microscopy (AFM). The XPS study via the Nb 3d spectral lineshape revealed the plasma effect: a decomposition of the niobate structure and the appearance of niobium oxides. The subsequent REELS study performed at 4 keV electron energy showed a transfer of characteristic electron‐loss energies of relevant transitions. Oxygen and lithium escape from the surface and hydrogen is incorporated from the plasma, producing niobium hydride. The deterioration of the niobate unit cells is accompanied by an enhancement of conduction electrons. The subsequent thermal reoxidation in medical oxygen did not restore the original niobate structure. The surface morphology after the reoxidation is characterized by elongated 100 nm grains situated normally to the surface. The main constituents of this new top layers are niobium oxides Nb2O5 and NbO. The present results form part of a complex study of the plasma modification of lithium niobate. The plasma processing induces lattice defects that are similar to those of ion implantation or metal ion doping: the electrical and optical properties of the surface layer drastically differ from those of the bulk. Copyright © 2002 John Wiley & Sons, Ltd.

Poly(dibenzylidenetetrathiapentalene): A Redox-Active, Linearly Extended TTF Polymer

The π-conjugated polymers poly-1 and poly-2 composed of vinylogous tetrathiafulvalene (TTF) units were obtained by electrochemical polymerization of 2,5-di(benzylidene)-1,3,4,6-tetrathiapentalene (1) or of its corresponding dimer 1,2-bis(5-benzylidene[1,3]-dithiolo[4,5-d]1,3-dithiole-2-ylidene)-1,2-diphenylethane (2). The polymers were characterized by cyclic voltammetry (CV), in situ ESR spectroscopy and in situ conductivity measurements to gain insight into the charge−discharge behavior and the electronic properties as a function of applied potential. Our results suggest that both compounds behave as conducting polymers exhibiting multistep redox activity. In situ ESR studies show that charge−discharge phenomena differ in the nature of the charge storage configuration depending on whether they are investigated during a cathodic or anodic scan. Reduction of a polymer film generated at an applied potential of +1.6 V vs SCE allows for the detection of three clearly discernible maxima of the spin density at 0.60, 0.95, and about 1.25 V vs Ag, while subsequent reoxidation gives rise to only one spin density maximum at around 0.65 V vs Ag. Coulometric studies suggest that the spin density maximum at around 0.60−0.65 V vs Ag is associated with one positive charge per tetrathiapentalene unit of poly-1 while each monomer bears up to two charges at a potential of 1.6 V vs Ag. In situ and conventional four-probe measurements of the electrical conductivity gave values of around 5 × 10-5 S cm-1.

Chemiluminescence of 3-methylindole based on electrogeneration of superoxide ion in acetonitrile solutions

Chemiluminescence of 3-methylindole (3-MIH) based on electrogeneration of superoxide ion (O 2 − ) in air-saturated acetonitrile solutions containing no base was investigated by the use of spectroelectrochemical techniques for the first time. The emission spectrum was observed around −0.2 V versus Ag ∣ AgCl ∣ NaCl (sat) after the electrogeneration and the subsequent reoxidation of O 2 − (formal potential, E 0′ O 2/O 2 − =−0.86 V) on the electrode surface. This spectrum revealed maximum intensity around 475 nm. On the cyclic voltammogram obtained together with the chemiluminescence at the 475-nm curve, an oxidation peak apart from the reoxidation peak of O 2 − was observed at about −0.2 V. This oxidation process around −0.2 V had a close connection with the chemiluminescence, and was considered to be due to the electrooxidation of 3-MI − (i.e. a conjugate base of 3-MIH) to 3-MI (i.e. a one-electron oxidation product, a radical species of 3-MI −). The electrogenerated O 2 − could function as a proton acceptor to 3-MIH. This reaction generates 3-MI − and hydroperoxyl radical (HO 2 ). In the chemiluminescence observed around −0.2 V, the radical–radical coupling reaction between 3-MI and HO 2 was considered to result in the formation of a 1,2-dioxetane-like intermediate to lead to the emission of light. The chemiluminescent reaction mechanism of 3-MIH was discussed in detail in comparison with that of the MCLA (a Cypridina luciferin analogue)–O 2 − system.

Redox behavior and transport properties of La 0.5− xSr 0.5− xFe 0.4Ti 0.6O 3− δ (0< x

The range of perovskite-type solid solution formation in the system La 0.5− x Sr 0.5− x Ti 0.6Fe 0.4O 3− δ in oxidizing conditions is determined from X-ray diffraction and Mössbauer spectroscopy data to correspond to approximately 0–10% of the A-site concentration, similar to other numerous perovskite systems. Reduction and subsequent reoxidation of the oxides leads to a narrowing of this range and the segregation of the hematite phase at x=0.05. Increasing A-site deficiency results in the formation of oxygen vacancies and decreasing stability in reducing environments. The total conductivity of La 0.5− x Sr 0.5− x Ti 0.6Fe 0.4O 3− δ ( x=0.02–0.10) ceramics is essentially independent of composition in air and increases with increasing x in reducing atmospheres, due to increasing concentrations of n-type charge carriers and the formation of metallic iron. Partial decomposition of the perovskite phase in hydrogen, resulting in metal Fe formation, was found to be reversible when the A-site deficiency is small ( x<0.05). Mössbauer spectroscopic data showed that, contrary to other perovskite-type titanates–ferrites, the concentration of Fe 4+ cations in the perovskite lattice of oxidized La 0.5− x Sr 0.5− x Ti 0.6Fe 0.4O 3− δ is negligible.

Electrochemically initiated 1,4 additions: a versatile route to the determination of thiols

The electrochemical generation of quinoid intermediates and their subsequent reaction with sulphydryl thiols (RSH) have been investigated as a method by which the latter can be quantified. Nucleophilic addition of the thiol species to the electrogenerated quinoid structure leads to the production of a chemically reduced adduct whose subsequent re-oxidation at the electrode leads to an amplification of the oxidative current. This can be correlated to the concentration of thiol present within the medium. The influence of indicator composition on the nature of the analytical response has been assessed through examining the electrochemical properties of 15 derivatives and the validity of the proposed reaction pathway corroborated by comparison of the voltammetric response with computer simulations. Interference by other electroactive species (ascorbate, urate, tyrosine) and other physiological constituents (cystine, lysine) has also been assessed.

Deoxygenation of benzoic acid on metal oxides: 1. The selective pathway to benzaldehyde

The mechanism of the selective deoxygenation of benzoic acid to benzaldehyde was studied on ZnO and ZrO 2. The results show conclusively that the reaction proceeds as a reverse type of Mars and van Krevelen mechanism consisting of two steps: hydrogen activates the oxide by reduction resulting in the formation of oxygen vacancies. Subsequent re-oxidation of these vacancy sites by benzoic acid yields benzaldehyde. Inhibition of the deoxygenation reaction can be achieved by addition of suitable polar compounds with a high affinity for the oxygen vacancy sites such as carbon dioxide or water. Differences in the catalytic activity and selectivity of ZnO and ZrO 2 can be attributed to differences in hydrogen activation, redox properties and extent of benzoic acid coverage.

Significance of LMW-GS and HMW-GS for Dough Extensibility: «Addition» versus «Incorporation» Protocols

Low molecular weight (LMW-GS) and high molecular weight glutenin subunits (HMW-GS) were added to a base flour using both «addition» and «incorporation» protocols. «Incorporation» of glutenin subunits into the glutenin network of the base flour was performed by partial (reversible) reduction and subsequent reoxidation of the glutenin network in the presence of the added glutenin subunits whereas, in the «addition» protocol, glutenin subunits were added without reduction/oxidation. The effects of both «addition» and «incorporation» of alkylated and unalkylated LMW-GS and HMW-GS on dough extension parameters maximum resistance (MR) and extensibility (EX) were compared and thoroughly discussed. HMW-GS and LMW-GS had totally different effects on dough extensibility. «Addition» of LMW-GS significantly decreased both MR and EX whereas HMW-GS caused a significant increase in MR. «Incorporation» of LMW-GS caused a decrease in MR whereas HMW-GS clearly increased MR. The similarity in effects obtained with «addition» and «incorporation» of glutenin subunits indicated that, even with «addition», glutenin subunits can be partially incorporated into the glutenin network in the presence of oxygen. Alkylated and unalkylated glutenin subunits had different effects. This was probably caused by the effect of free sulphydryl groups in unalkylated subunits (possibility of SS/SH exchanges and/or incorporation) and/or the effect induced by introduction of alkylated derived substituents. A protocol for «incorporation with excess KIO3» was developed to exclude the possible effect of a lowering of the available oxidant concentration by oxidation of free sulphydryl groups in glutenin subunits. However, the use of high levels of oxidant in the «incorporation with excess KIO3» protocol seems to overrule the effects of added glutenin subunits or may force glutenin subunits to incorporate differently from what can be observed under gentle oxidation conditions. Therefore, «incorporation with excess KIO3» is not suitable for studying the effects of incorporation of glutenin subunits on dough extensibility.

Reactions of dimethylsulfoxide reductase from Rhodobacter capsulatus with dimethyl sulfide and with dimethyl sulfoxide: complexities revealed by conventional and stopped-flow spectrophotometry.

Improved assays for the molybdenum enzyme dimethylsulfoxide reductase (DMSOR) with dimethyl sulfoxide (DMSO) and with dimethyl sulfide (DMS) as substrates are described. Maximum activity was observed at pH 6.5 and below and at 8.3, respectively. Rapid-scan stopped-flow spectrophotometry has been used to investigate the reduction of the enzyme by DMS to a species previously characterized by its UV-visible spectrum [McAlpine, A. S., McEwan, A. G., and Bailey, S. (1998) J. Mol. Biol. 275, 613-623], and its subsequent reoxidation by DMSO. Both these two-electron reactions were faster than enzyme turnover under steady-state conditions, indicating that one-electron reactions with artificial dyes were rate-limiting. Second-order rate constants for the two-electron reduction and reoxidation reactions at pH 5.5 were (1.9 +/- 0.1) x 10(5) and (4.3 +/- 0.3) x 10(2) M-1 s-1, respectively, while at pH 8.0, the catalytic step was rate-limiting (62 s-1). Kinetically, for the two-electron reactions, the enzyme is more effective in DMS oxidation than in DMSO reduction. Reduction of DMSOR by DMS was incomplete below approximately 1 mM DMS but complete at higher concentrations, implying that the enzyme's redox potential is slightly higher than that of the DMS-DMSO couple. In contrast, reoxidation of the DMS-reduced state by DMSO was always incomplete, regardless of the DMSO concentration. Evidence for the existence of a spectroscopically indistinguishable reduced state, which could not be reoxidized by DMSO, was obtained. Brief reaction (less than approximately 15 min) of DMS with DMSOR was fully reversible on removal of the DMS. However, in the presence of excess DMS, a further slow reaction occurred aerobically, but not anaerobically, to yield a stable enzyme form having a lambdamax at 660 mn. This state (DMSORmod) retained full activity in steady-state assays with DMSO, but was inactive toward DMS. It could however be reconverted to the original resting state by reduction with methyl viologen radical and reoxidation with DMSO. We suggest that in this enzyme form two of the dithiolene ligands of the molybdenum have dissociated and formed a disulfide. The implications of this new species are discussed in relation both to conflicting published information for DMSOR from X-ray crystallography and to previous spectroscopic data for its reduced forms.

Preparation of PTCR ceramics in the BaO–Nb 2O 5–TiO 2 system

The PTCR effect was investigated in the ferroelectric BaNb 2O 6 phase doped with TiO 2. Composite ceramics formed after sintering in a reducing atmosphere and subsequent reoxidation show the PTCR effect at around 70 and 300°C, respectively. Both PTCR anomalies are associated with the formation of high resistivity grain boundaries after controlled oxidation of reduced constituent phases.

Relationship between Interfacial Roughness and Dielectric Reliability for Silicon Oxynitride Gate Dielectrics Processed with Nitric Oxide

ABSTRACTThe greatest benefits of nitrogen incorporation into gate dielectrics may be obtained by placing nitrogen preferentially at the interfacial regions of the dielectric film. One method of distributing nitrogen in this manner is by using a three-step thermal process consisting of 1.) oxynitridation in NO, 2.) subsequent reoxidation in O2, and 3.) a final NO anneal. This study investigates the effect of NO processing on substrate/dielectric interface roughness and correlates that roughness with dielectric reliability. The initial NO-containing step can roughen the interface, as can subsequent reoxidation. Increased NO exposure yields a greater nitrogen content and a concomitant increase in interface roughness. These films show a degradation in charge to breakdown (Qbd) of at least an order of magnitude when compared with similarly prepared O2-oxide films. An O2/NO process produces films with interface roughness and Qbd comparable to that of pure SiO2, independent of nitrogen content. The oxynitride reliability depends on the exact scheme for incorporating nitrogen into SiO2.

The Performance of a Pt/C Oxygen Electrode in the Presence of Dissolved Tungsten in Sulfuric Acid

Recent studies by Savadogo et al. and Kulesza et al. indicated that is an active support for the reduction of oxygen on a electrode in phosphoric and sulfuric acid. However, the mechanism involved is not clear. is slightly soluble in acidic media. The addition of of dissolved tungsten in sulfuric acid at results in a 60% increase in oxygen reduction at a current density at vs. standard hydrogen electrode on a Teflon bonded electrode . A homogeneous decomposition mechanism involving the electrochemical reduction of dissolved peroxytungstate and its subsequent reoxidation by is proposed. ©1999 The Electrochemical Society

A UV-vis spectroelectrochemical study of redox reactions of solution species at a polyaniline electrode in the conducting and the reduced state

Abstract The cathodic reduction of 1,2-naphthoquinone-4-sulfonate (NQS) at a polyaniline-coated ITO glass electrode was studied with in situ UV-vis spectroelectrochemistry. An electrocatalytic mechanism of this process is proposed, which includes the cathodic reduction of the PANI film and its subsequent reoxidation by NQS. Accordingly, PANI acts as a redox mediator cycled between its emeraldine and its leucoemeraldine form. In the equilibrium state, both redox forms of PANI are present in the film. The relative content of both forms was shown to depend on the concentration of NQS in the solution, on the PANI film thickness and on the applied electrode potential. In addition the cathodic reduction of sulfopropylviologen (SPY) and benzylviologen (BV) was studied in the potential range where the non-conducting (reduced) leucoemeraldine form of PANI exists. The slowness of the cathodic reduction of both BV and SPV, proceeding at pH 0.5, may be caused by electrostatic repulsion between the positively charged PANI polymer chain and the viologen molecule. At pH 4.0, the reduction becomes fast, probably due to deprotonation of the leucoemeraldine form of PANI, resulting in the disappearance of the electrostatic repulsion. In the middle pH region (pH 1.5), a large difference in the rate of the cathodic reduction between BV and SPV was observed. This difference may be attributed to deprotonation of the sulfogroups of SPV. Deprotonated SPV has no net electric charge, and no electrostatic repulsion should be effective. In contrast, BV has a positive electric charge independent of the pH-value of the solution.

Ceruloplasmin Ferroxidase Activity Stimulates Cellular Iron Uptake by a Trivalent Cation-specific Transport Mechanism

The balance required to maintain appropriate cellular and tissue iron levels has led to the evolution of multiple mechanisms to precisely regulate iron uptake from transferrin and low molecular weight iron chelates. A role for ceruloplasmin (Cp) in vertebrate iron metabolism is suggested by its potent ferroxidase activity catalyzing conversion of Fe2+ to Fe3+, by identification of yeast copper oxidases homologous to Cp that facilitate high affinity iron uptake, and by studies of "aceruloplasminemic" patients who have extensive iron deposits in multiple tissues. We have recently shown that Cp increases iron uptake by cultured HepG2 cells. In this report, we investigated the mechanism by which Cp stimulates cellular iron uptake. Cp stimulated the rate of non-transferrin 55Fe uptake by iron-deficient K562 cells by 2-3-fold, using a transferrin receptor-independent pathway. Induction of Cp-stimulated iron uptake by iron deficiency was blocked by actinomycin D and cycloheximide, consistent with a transcriptionally induced or regulated transporter. Cp-stimulated iron uptake was completely blocked by unlabeled Fe3+ and by other trivalent cations including Al3+, Ga3+, and Cr3+, but not by divalent cations. These results indicate that Cp utilizes a trivalent cation-specific transporter. Cp ferroxidase activity was required for iron uptake as shown by the ineffectiveness of two ferroxidase-deficient Cp preparations, copper-deficient Cp and thiomolybdate-treated Cp. We propose a model in which iron reduction and subsequent re-oxidation by Cp are essential for an iron uptake pathway with high ion specificity.

Combustion of methane over palladium/zirconia: effect of Pd-particle size and role of lattice oxygen

The catalytic combustion of methane over palladium/zirconia catalyst prepared by oxidation and subsequent reduction of Pd/Zr glassy metals has been investigated with special emphasis on the influence of the catalyst structure (particle size and specific surface area) on the catalytic performance and the impact of a redox mechanism on the product formation. The reaction of 1% methane and 4% oxygen (balance of He) in a fixed-bed microreactor was found to be strongly dependent on the particle size of palladium, which was controlled by appropriate reduction before catalytic testing. Pre-reduction of PdO resulted in an enhanced activity compared to the unreduced catalysts. The structural changes induced upon reduction were accompanied by altered physico-chemical properties, evidenced by different behaviors in the decomposition of PdO and the reduction of PdO by methane. The correlation established between catalytic performance and the rate of reduction of PdO by methane led to the postulation of a redox mechanism, involving the reaction of methane with the active palladium oxide phase and subsequent reoxidation of metallic Pd by oxygen, which was independently confirmed by the use of 18 O labeled catalysts and pulse methods.

Integrating cytosolic calcium signals into mitochondrial metabolic responses.

Stimulation of hepatocytes with vasopressin evokes increases in cytosolic free Ca2+ ([Ca2+]c) that are relayed into the mitochondria, where the resulting mitochondrial Ca2+ ([Ca2+]m) increase regulates intramitochondrial Ca2+-sensitive targets. To understand how mitochondria integrate the [Ca2+]c signals into a final metabolic response, we stimulated hepatocytes with high vasopressin doses that generate a sustained increase in [Ca2+]c. This elicited a synchronous, single spike of [Ca2+]m and consequent NAD(P)H formation, which could be related to changes in the activity state of pyruvate dehydrogenase (PDH) measured in parallel. The vasopressin-induced [Ca2+]m spike evoked a transient increase in NAD(P)H that persisted longer than the [Ca2+]m increase. In contrast, PDH activity increased biphasically, with an initial rapid phase accompanying the rise in [Ca2+]m, followed by a sustained secondary activation phase associated with a decline in cellular ATP. The decline of NAD(P)H in the face of elevated PDH activity occurred as a result of respiratory chain activation, which was also manifest in a calcium-dependent increase in the membrane potential and pH gradient components of the proton motive force (PMF). This is the first direct demonstration that Ca2+-mobilizing hormones increase the PMF in intact cells. Thus, Ca2+ plays an important role in signal transduction from cytosol to mitochondria, with a single [Ca2+]m spike evoking a complex series of changes to activate mitochondrial oxidative metabolism.

Reduction of aromatic nitrocompounds with hydrazine hydrate in the presence of an iron oxide hydroxide catalyst: II. Activity, X-ray diffraction and Mössbauer study of the iron oxide hydroxide catalyst

The catalytic activities of several iron oxides and oxide hydroxides have been tested in the reduction of substituted nitroarenes using hydrazine hydrate as reducing agent. An iron oxide catalyst, prepared by rapid precipitation of iron hydroxides from solutions of iron(III)chloride, showed the highest activity mainly due to its large BET surface area compared to other iron oxides. Mössbauer spectroscopy and X-ray powder diffraction showed this catalyst to be ferrihydrite, a poorly ordered iron oxide hydroxide. During in situ experiments, in the absence of a nitrocompound, the complete transformation of ferrihydrite to Fe 3O 4 (magnetite) was observed. After addition of 4-nitrotoluene the catalyst was reoxidized to γ-Fe 2O 3 (maghemite). The same experiment carried out in the presence of 4-nitrotoluene showed that after filtration and drying the catalyst still remained ferrihydrite. Thus, reduction of the catalyst by hydrazine hydrate and subsequent reoxidation by 4-nitrotoluene seems to occur only at the surface of the catalyst. No deactivation was observed after eight times recycling of the catalyst. Carrying out the reduction at 328 K, one could observe an increase in particle size. Increasing the reaction temperature from 328 to 373 K caused a successive increase in particle size of the ferrihydrite catalyst as well as the formation of α-Fe 2O 3 (hematite) which led to a significant decrease in activity. During the filtration procedure in air, the reduced catalyst showed pyrophoric behaviour that led to the formation of α-Fe 2O 3 (hematite).

Magneto-optical properties of nanocomposite films obtained by partial reduction of (Ni,Mg)O and (Co,Mg)O solid solutions

Metal-ceramic nanocomposite films consisting of either Ni or Co particles embedded in a MgO matrix were produced by two techniques involving the partial reduction of a mixed oxide solid solution. Transmission electron microscopy, Rutherford backscattering spectrometry, x-ray diffraction, and magneto-optical Faraday and Kerr rotation measurements were used to characterize the films. The metallic particles were well dispersed in the films, resistant to subsequent re-oxidation, with their size controlled by choice of processing conditions. The coercive field of the samples is influenced by the residual strain arising from the coefficient of thermal expansion difference between the film and the substrate upon which it is deposited. Large magneto-optical Faraday and Kerr rotations are achieved for relatively thick nanocomposite samples as compared to pure metallic films.

Influence of nanostructure size on the luminescence behavior of silicon nanoparticle thin films

We demonstrate the effect of particle size and quantum confinement on the luminescence properties of nanoscale silicon thin films. Thin films of agglomerated silicon nanoparticles are synthesized using pulsed laser ablation supersonic expansion. Following deposition, standard semiconductor processing techniques are employed to reduce the nanoparticle size. Films are oxidized both in air and chemically to reduce the silicon core dimensions, resulting in a shift of the luminescence emission peak to shorter wavelengths. Removal of the oxide using hydrofluoric acid (HF) results in further blueshifting of the luminescence, as does subsequent reoxidation in air and using nitric acid. The luminescence properties of samples are also studied as a function of excitation intensity. For room temperature excitation with a pulsed 355 nm source, a saturation of the photoluminescence intensity at high excitation intensity is observed, along with a blueshift of the peak PL wavelength. This behavior is found to persist at reduced temperature. A saturation of PL intensity, but no blueshift, is observed for high excitation intensity using a cw 488 nm source at room temperature. At reduced temperatures, no saturation of emission intensity occurs for high intensity 488 nm cw excitation. Both the irreversible shifting of the peak PL wavelength with size reducing treatments and the PL behavior at high excitation intensities indicate that quantum confinement determines the luminescence wavelength.

Characterization of Human and Pig Kidney Long‐Chain‐Acyl‐CoA Dehydrogenases and their Role in β‐Oxidation

Long-chain-acyl-CoA dehydrogenase (LCADH) has been produced by recombinant techniques from the human cDNA and purified after expression in Escherichia coli. Pig kidney LCADH was purified using an optimized method which also produces apparently pure short-chain-acyl-CoA dehydrogenase (SCADH) and medium-chain-acyl-CoA dehydrogenase (MCADH) in good yields. LCADH from both sources has a maximal turnover rate (Vmax of 650-700 min(-1) at pH 7.6) with the best substrates, which is approximately fivefold higher than reported previously. The human enzyme has an approximately fivefold higher Km compared with the pig kidney enzyme with substrates of chain length from C10 to C18 and a significantly different dependence of Vmax on the chain length. Pig kidney LCADH has a similar Vmax/Km with C10 to C14 substrates as MCADH does with C6 to C10 substrates. Recombinant human LCADH, however, is significantly less efficient (approximately fourfold with C12) than purified pig kidney enzyme. We conclude that human LCADH is either quantitatively less important in beta-oxidation than in the pig, or that post-translational modifications, not present in the recombinant human enzyme, are required to optimize human LCADH activity. Our results demonstrate that LCADH is as important as the other acyl-CoA dehydrogenases in fatty acid oxidation at physiological, mitochondrial pH with optimal substrates of chain length C10-C14. The extent of the LCADH-flavin cofactor reduction observed with most substrates and the rate of the subsequent reoxidation with oxygen are markedly different from those found with human medium chain acyl-CoA dehydrogenase. Both LCADH are inactivated by the substrate analogue 2-octynoyl-CoA, possibly via covalent modification of Glu261, the active-site residue involved in deprotonation of the substrate (alpha)C-H.