EELS spectroscopy of iron fluorides and FeFx/C nanocomposite electrodes used in Li-ion batteries.

A new type of positive electrode for Li-ion batteries has been developed recently based on FeF3/C and FeF2/C nanocomposites. The microstructural and redox evolution during discharge and recharge processes was followed by electron energy loss spectroscopy (EELS) to determine the valence state of Fe by measuring the Fe L3 line energy shift and from Fe L3/L2 line intensity ratios. In addition, transition metal fluorides were found to be electron beam sensitive, and the effect of beam exposure on EELS spectra was also investigated. The EELS results indicate that for both FeF3/C and FeF2/C nanocomposite systems, a complete reduction of iron to FeO is observed upon discharge to 1.5 V with the formation of a finer FeO/LiF subnanocomposite ( approximately 7 nm). Upon complete recharging to 4.5 V, EELS data reveal a reoxidation process to a Fe2+ state with the formation of a carbon metal fluoride nanocomposite related to the FeF2 structure.     

Microstructural and metal-support interactions of the Pt-CeO2/C catalysts for direct methanol fuel cell application

To understand the ceria promotion effect of Pt-CeO(2)/C catalysts on methanol oxidation, microstructural and metal-oxide interactions of Pt-CeO(2)/C catalysts with an atomic ratio of Pt/Ce between 0.14 and 1.4 were systematically examined using high-resolution transmission electron microscopy and electron energy loss spectroscopy (EELS). With an increasing Pt content in the catalysts, Pt particles gradually invaded into the ceria supports and decoration on Pt particles was observed. Simultaneously, the morphology of the supports was dramatically modified with nanocrystalline and amorphous ceria formed between and/or around the Pt particles. It reveals that the Pt-ceria interaction could take place in the catalysts and the influence of the interaction was enhanced with an increasing Pt/Ce ratio. The EELS study demonstrated that the strong Pt-ceria interaction was related to the redox reaction between Pt and ceria. Experimental results also suggested that the strong interaction between Pt and ceria could contribute to the promotion effect of ceria on the oxidation of methanol.     

Pulse radiolysis studies of the reactions of CO3*- and NO2* with nitrosyl(II)myoglobin and nitrosyl(II)hemoglobin

The reactions of carbonate radical anion [CO3*-, systematic name: trioxidocarbonate*1-] with nitrosyl(II)hemoglobin (HbFe(II)NO) and nitrosyl(II)myoglobin (MbFe(II)NO) were studied by pulse radiolysis in N2O-saturated 0.25 M sodium bicarbonate solutions at pH 10.0 and room temperature. The reactions proceed in two steps: outer-sphere oxidation of the nitrosyliron(II) proteins to their corresponding nitrosyliron(III) forms and subsequent dissociation of NO*. The second-order rate constants measured for the first reaction steps were (4.3 +/- 0.2) x 10(8) and (1.5 +/- 0.3) x 10(8) M(-1) s(-1), for MbFe(II)NO and HbFe(II)NO, respectively. The reactions between nitrogen dioxide and MbFe(II)NO or HbFe(II)NO were studied by pulse radiolysis in N2O-saturated 0.1 M phosphate buffer pH 7.4 containing 5 mM nitrite. Also for the reactions of this oxidant with the nitrosyliron(II) forms of Mb and Hb a two-step reaction was observed: oxidation of the iron was followed by dissociation of NO*. The second-order rate constants measured for the first reaction steps were (2.9 +/- 0.3) x 10(7) and (1.8 +/- 0.3) x 10(7) M(-1) s(-1), for MbFe(II)NO and HbFe(II)NO, respectively. Both radicals appear to be able to oxidize the iron(II) centers of the proteins directly. Only for the reactions with HbFe(II)NO it cannot be excluded that, in a parallel reaction, CO3*- and NO2* first react with amino acid(s) of the globin, which then oxidize the nitrosyliron(II) center.     

Electron energy loss spectroscopy (EELS) of iron Fischer-Tropsch catalysts.

Electron energy loss spectroscopy (EELS), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy have been used to study iron catalysts for Fischer-Tropsch synthesis. When silica-containing iron oxide precursors are activated in flowing CO, the iron phase segregates into iron carbide crystallites, leaving behind some unreduced iron oxide in an amorphous state coexisting with the silica binder. The iron carbide crystallites are found covered by characteristic amorphous carbonaceous surface layers. These amorphous species are difficult to analyze by traditional catalyst characterization techniques, which lack spatial resolution. Even a surface-sensitive technique such as XPS shows only broad carbon or iron peaks in these catalysts. As we show in this work, EELS allows us to distinguish three different carbonaceous species: reactive amorphous carbon, graphitic carbon, and carbidic carbon in the bulk of the iron carbide particles. The carbidic carbon K edge shows an intense "pi*" peak with an edge shift of about 1 eV to higher energy loss compared to that of the pi* of amorphous carbon film or graphitic carbon. EELS analysis of the oxygen K edge allows us to distinguish the amorphous unreduced iron phase from the silica binder, indicating these are two separate phases. These results shed light onto the complex phase transformations that accompany the activation of iron catalysts for Fischer-Tropsch synthesis.     

TEM investigation of nanophase aluminum powder.

Nanophase aluminum powder was characterized in a field-emission-gun transmission electron microscope (TEM). Different techniques were used to investigate the structure of the particles, including conventional bright-field and dark-field imaging, scanning transmission electron microscopy (STEM), high-resolution lattice imaging, diffraction studies, energy dispersive X-ray spectroscopy (EDS) analysis and mapping, and electron energy loss spectroscopy (EELS) analysis and mapping. It has been established that the particle cores consist of aluminum single crystals that sometimes contain crystal lattice defects. The core is covered by a passivating layer of aluminum oxide a few nanometers thick. The alumina is mostly amorphous, but evidences of partial crystallinity of the oxide were also found. The thickness of this layer was measured using different techniques, and the results are in good agreement with each other. The particles are agglomerated in two distinct ways. Some particles were apparently bonded together during processing before oxidation. These mostly form dumbbells covered by a joint oxide layer. Also, oxidized particles are loosely assembled into relatively large clusters.     

Instantaneous synthesis of stable zerovalent metal nanoparticles under standard reaction conditions

In this report is discussed a novel, easy, and general synthesis method to prepare zerovalent iron (ZVI) and copper (ZV Cu) nanoparticles (NPs), from colloid dispersions in an environmental friendly organic solvent, ethylene glycol (EG). Conventional metallic salts are used as nanoparticle precursors; sodium borohydride (NaBH4) is the reducing agent, and triethylamine (TEA) is used as the nanoparticle stabilizer. The chemical changes take place instantaneously under normal reaction conditions. Small iron (alpha-Fe0 phase) and copper (fcc phase) NPs with average diameters of 10.2 +/- 3.3 and 9.5 +/- 2.5 nm, respectively, were obtained. In both cases, the experimental evidence reveals the absence of any metal oxide shell coating the particle surfaces, and their powders remain stable, under aerobic conditions at least for 3 weeks. ZVI NPs were characterized by X-RD, M?ssbauer, and Raman spectroscopies and by EELS coupled to HR-TEM. Otherwise, copper NPs were characterized by X-RD, Z-contrast, and HR-TEM. This synthesis pathway is particularly suitable for large-scale and high-quality zerovalent metallic nanoparticle (ZV M NP) production due to its simple process and low cost.     

Identify fine microstructure of multifarious iron oxides via O K-edge EELS spectra

Relying on the electron energy loss spectrum (EELS) of metallic elements to obtain microstructure analysis is an investigation method of the reaction mechanisms of transition metal oxides (TMOs) in catalysis, energy storage and conversion. However, the low signal from K shell owing to insufficient electron beam energy, and the complicated electronic structure in L shell of the metal element restrict the analysis of the coordination environment of the TMOs. Herein, density functional theory (DFT) calculation, Fourier transform (FT) and wavelet transform (WT) were employed to probe the relationship between the four individual peaks in O K-edge spectra of iron oxides and the microstructure information (chemical bonds and atomic coordination). The findings show that the peak amplitude ration is in a linear correlation with the valence state of Fe element, and that the coordination number obtained by radial distribution function (RDF) is favorably linearly correlative with that from the standard coordination structure model. As a result, the quantitative analysis on the change of valence state and atomic coordination in microstructure can be realized by EELS O K-edge spectra. This study establishes EELS O K-edge spectrum as a promising pathway to quantitatively analyze the valence state and atomic coordination information of TMOs, and offers an effective method to conduct microstructure analysis via the EELS spectra of the non-metal element.     

Problems in quantitatively analyzing individual salt aerosol particles using electron energy loss spectroscopy

Polydisperse standard aerosols of NaCl, (NH₄)₂SO₄ and KNO₃ have been generated from their solutions by pneumatic nebulization. These aerosol particles are electron beam sensitive and therefore special precautions are necessary. A methodology is proposed for serially recording electron energy loss spectra (EELS) from sub-micrometer salt particles. The results of quantification are compared with those obtained by parallel electron energy loss spectroscopy (PEELS).Experimental conditions such as the intensity of the primary beam, time of radiation and temperature are responsible for large deviations from the theoretical values. These deviations arise because heavy mass losses occur, especially during the serial spectrum acquisition. The more time consuming serial EELS is therefore at a disadvantage relative to the parallel method.The best results are obtained for NaCl because the halogen loss can be reduced more efficiently than nitrogen and oxygen losses. The results show that nitrogen loss occurs within the first few seconds of the experiment at normal radiation doses. Even at cryogenic temperatures, losses of volatile elements cannot be avoided.     

Silica- and alkoxysilane-coated ultrasmall superparamagnetic iron oxide particles: a promising tool to label cells for magnetic resonance imaging

In this study silica- and alkoxysilane-coated ultrasmall superparamagnetic iron oxide (USPIO) particles were synthesized, and their ability to label immortalized progenitor cells for magnetic resonance imaging (MRI) was compared. USPIO particles were synthesized by coprecipitation of ferric and ferrous salts. Subsequently, the particles were coated with silica, (3-aminopropyl)trimethoxysilane (APTMS), and [N-(2-aminoethyl)-3-aminopropyl]trimethoxysilane (AEAPTMS). The size of the USPIO particles was about 10 nm without a significant increase in diameter after coating. The highest T2 relaxivity was achieved for silica-coated USPIO particles, 339.80 +/- 0.22 s-1 mM-1, as compared with APTMS- and AEAPTMS-coated ones, reaching 134.40 +/- 0.01 and 84.79 +/- 0.02 s-1 mM-1, respectively. No toxic effects on the cells could be detected by trypan blue, TUNEL, and MTS assays. Uptake of USPIO particles was evaluated by Prussian blue staining, transmission electron microscopy, T2-MR relaxometry, and mass spectrometry. It was found that cell uptake of the different USPIO particles increased for longer incubation times and higher doses. Maximum cellular iron concentrations of 42.1 +/- 4.0 pg/cell (silica-coated USPIO particles), 37.1 +/- 3.5 pg/cell (APTMS-coated USPIO particles), and 32.7 +/- 4.0 pg/cell (AEAPTMS-coated USPIO particles) were achieved after incubation of the cells with USPIO particles at a dose of 3 micromol/mL for 6 h. The decrease of the T2 relaxation time of the cell pellets was most pronounced for cells incubated with silica-coated USPIO particles followed by APTMS- and AEAPTMS-coated particles, respectively. In gelatin gels even small clusters of labeled cells were detected by 1.5 T MRI, and significant changes in the T2 relaxation times of the gels were determined for 10000 labeled cells/mL for all particles. In summary, as compared with APTMS- and AEAPTMS-coated particles, silica-coated USPIO particles provide the highest T2 relaxivity and most effectively reduce the T2 relaxation time of immortalized progenitor cells after internalization. This suggests silica-coated USPIO particles are most suited for cell labeling approaches in MRI.     

Covalency‐Driven Preservation of Local Charge Densities in a Metal‐to‐Ligand Charge‐Transfer Excited Iron Photosensitizer

Covalency is found to even out charge separation after photo‐oxidation of the metal center in the metal‐to‐ligand charge‐transfer state of an iron photosensitizer. The σ‐donation ability of the ligands compensates for the loss of iron 3d electronic charge, thereby upholding the initial metal charge density and preserving the local noble‐gas configuration. These findings are enabled through element‐specific and orbital‐selective time‐resolved X‐ray absorption spectroscopy at the iron L‐edge. Thus, valence orbital populations around the central metal are directly accessible. In conjunction with density functional theory we conclude that the picture of a localized charge‐separation is inadequate. However, the unpaired spin density provides a suitable representation of the electron–hole pair associated with the electron‐transfer process.     

Synthesis of highly crystalline and monodisperse maghemite nanocrystallites without a size-selection process.

The synthesis of highly crystalline and monodisperse gamma-Fe(2)O(3) nanocrystallites is reported. High-temperature (300 degrees C) aging of iron-oleic acid metal complex, which was prepared by the thermal decomposition of iron pentacarbonyl in the presence of oleic acid at 100 degrees C, was found to generate monodisperse iron nanoparticles. The resulting iron nanoparticles were transformed to monodisperse gamma-Fe(2)O(3) nanocrystallites by controlled oxidation by using trimethylamine oxide as a mild oxidant. Particle size can be varied from 4 to 16 nm by controlling the experimental parameters. Transmission electron microscopic images of the particles showed 2-dimensional and 3-dimensional assembly of particles, demonstrating the uniformity of these nanoparticles. Electron diffraction, X-ray diffraction, and high-resolution transmission electron microscopic (TEM) images of the nanoparticles showed the highly crystalline nature of the gamma-Fe(2)O(3) structures. Monodisperse gamma-Fe(2)O(3) nanocrystallites with a particle size of 13 nm also can be generated from the direct oxidation of iron pentacarbonyl in the presence of oleic acid with trimethylamine oxide as an oxidant.     

海胆状氧化锌/羰基铁粉核壳结构复合粒子的抗氧化及吸波性能

针对羰基铁粉吸收剂在温度较高时易被氧化的问题,采用水热法制备了氧化锌包覆羰基铁粉核壳结构复合粒子,并分别将羰基铁粉和氧化锌/羰基铁粉核壳粒子与石蜡混合,制备复合材料。结果表明,氧化锌纳米棒致密的包覆在羰基铁粉颗粒表面形成海胆状核壳结构复合粒子,正是这种结构将羰基铁粉颗粒与空气隔绝,使得复合粒子的抗氧化性能得到显著改善。与羰基铁粉复合吸波材料相比,氧化锌/羰基铁粉核壳粒子的复合材料吸收峰稍向低频移动,反射损耗小于-5dB的带宽几乎保持不变,在不改变电磁吸波性能的前提下,提高了羰基铁粉粒子的使用温度。     

Oxidation of zinc-thiolate complexes of biological interest by hydrogen peroxide: a theoretical study

Zinc-thiolate complexes play a major structural and functional role in the living cell. Their stability is directly related to the thiolate reactivity toward reactive oxygen species naturally present in the cell. Oxidation of some zinc-thiolate complexes has a functional role, as is the case of zinc finger redox switches. Herein, we report a theoretical investigation on the oxidation of thiolate by hydrogen peroxide in zinc finger cores of CCCC, CCHC, and CCHH kinds containing either cysteine or histidine residues. In the case of the CCCC core, the calculated energy barrier for the oxidation to sulfenate of the complexed thiolate was found to be 16.0 kcal mol(-1), which is 2 kcal mol(-1) higher than that for the free thiolate. The energy barrier increases to 19.3 and 22.2 kcal mol(-1) for the monoprotonated and diprotonated CCCC cores, respectively. Substitution of cysteine by histidine also induces an increase in the magnitude of the reaction energy barrier: It becomes 20.0 and 20.9 kcal mol(-1) for the CCCH and CCHH cores, respectively. It is concluded that the energy barrier for the oxidation of zinc fingers is strictly dependent on the type of ligands coordinated to zinc and on the protonation state of the complex. These changes in the thiolate reactivity can be explained by the lowering of the nucleophilicity of complexed sulfur and by the internal reorganization of the complex (changes in the metal-ligand distances) upon oxidation. The next reaction steps subsequent to sulfenate formation are also considered. The oxidized thiolate (sulfenate) is predicted to dissociate very fast: For all complexes, the calculated dissociation energy barrier is lower than 3 kcal mol(-1). It is also shown that the dissociated sulfenic acid can interact with a free thiolate to form a sulfur-sulfur (SS) bridge in a reaction that is predicted to be quasi-diffusion limited. The interesting biological consequences of the modulation of thiolate reactivity by the chemical composition of the zinc finger cores are discussed.     

Large ground-state entropy changes for hydrogen atom transfer reactions of iron complexes

Reported herein are the hydrogen atom transfer (HAT) reactions of two closely related dicationic iron tris(alpha-diimine) complexes. FeII(H2bip) (iron(II) tris[2,2'-bi-1,4,5,6-tetrahydropyrimidine]diperchlorate) and FeII(H2bim) (iron(II) tris[2,2'-bi-2-imidazoline]diperchlorate) both transfer H* to TEMPO (2,2,6,6-tetramethyl-1-piperidinoxyl) to yield the hydroxylamine, TEMPO-H, and the respective deprotonated iron(III) species, FeIII(Hbip) or FeIII(Hbim). The ground-state thermodynamic parameters in MeCN were determined for both systems using both static and kinetic measurements. For FeII(H2bip) + TEMPO, DeltaG degrees = -0.3 +/- 0.2 kcal mol-1, DeltaH degrees = -9.4 +/- 0.6 kcal mol-1, and DeltaS degrees = -30 +/- 2 cal mol-1 K-1. For FeII(H2bim) + TEMPO, DeltaG degrees = 5.0 +/- 0.2 kcal mol-1, DeltaH degrees = -4.1 +/- 0.9 kcal mol-1, and DeltaS degrees = -30 +/- 3 cal mol-1 K-1. The large entropy changes for these reactions, |TDeltaS degrees | = 9 kcal mol-1 at 298 K, are exceptions to the traditional assumption that DeltaS degrees approximately 0 for simple HAT reactions. Various studies indicate that hydrogen bonding, solvent effects, ion pairing, and iron spin equilibria do not make major contributions to the observed DeltaS degrees HAT. Instead, this effect arises primarily from changes in vibrational entropy upon oxidation of the iron center. Measurement of the electron-transfer half-reaction entropy, |DeltaS degrees Fe(H2bim)/ET| = 29 +/- 3 cal mol-1 K-1, is consistent with a vibrational origin. This conclusion is supported by UHF/6-31G* calculations on the simplified reaction [FeII(H2N=CHCH=NH2)2(H2bim)]2+...ONH2 left arrow over right arrow [FeII(H2N=CHCH=NH2)2(Hbim)]2+...HONH2. The discovery that DeltaS degrees HAT can deviate significantly from zero has important implications on the study of HAT and proton-coupled electron-transfer (PCET) reactions. For instance, these results indicate that free energies, rather than enthalpies, should be used to estimate the driving force for HAT when transition-metal centers are involved.     

Size control for two-dimensional iron oxide nanodots derived from biological molecules

We demonstrated the fabrication of size-controlled two-dimensional iron oxide nanodots derived from the heat treatment of ferritin molecules self-immobilized on modified silicon surfaces. Ferritin molecules were immobilized onto 3-aminopropyltrimethoxysilane (3-APMS)-modified silicon surfaces by electrostatic interactions between negatively charged amino acids of ferritin molecules and amino terminal functional groups of 3-APMS. Heat treatments were performed at 400 degrees C for 60 min to fabricate two-dimensional nanodots based on ferritin cores. XPS and FT-IR results clearly indicate that ferritin shells were composed of amino acids and 3-APMS modifiers on silicon surfaces were eliminated by heat treatment. Nanodots on substrate surfaces corresponded to iron oxides. The size of nanodots was tunable in the range of 0-5 (+/-0.75) nm by in situ reactions of iron ion chelators with ferritin molecules immobilized on substrates before heat treatment.     

Self-assembly approach toward magnetic silica-type nanoparticles of different shapes from reverse block copolymer mesophases

In the present study poly(isoprene-block-ethylene oxide), PI-b-PEO, block copolymers are used to structure iron oxide and silica precursors into reverse mesophases, which upon dissolution of the organic matrix lead to well-defined nanoparticles of spheres, cylinders, and plates based on the original structure of the mesophase prepared. The hybrid mesophases with sphere, cylinder, and lamellar morphologies containing the inorganic components in the minority phases are characterized through a combination of small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), and electron energy loss spectroscopy (EELS). After heat treatments the respective nanoparticles on mica surfaces are characterized by scanning force microscopy (SFM). X-ray diffraction (XRD) and superconducting quantum interference device (SQUID) magnetometer measurements are performed to demonstrate that the heat treatment leads to the formation of a magnetic gamma-Fe2O3 crystalline phase within the amorphous aluminosilicate. The results pave the way to functional, i.e., magnetic nanoparticles where the size, shape, and iron oxide concentration can be controlled opening a range of possible applications.     

RNA interference silencing the transcriptional message

We developed the microbial immobilization particle with curdlan and activated carbon, which has great adsorption capacity. The characteristics of porosity and mechanical strength of these supporting particles are dependent on manufacturing method. The supporting particle showed the best performance when the ratio of curdlan and activated carbon was 30 to 6 g/L. Brumauer-Emmett-Teller (specific surface area) and swelling capacity of the carrier were 52.63 m²/g and 17 (w/w), respectively. The immobilization characteristics of iron-oxidizing bacteria on supporting particles were observed using a scanning electron microscope. The concentration of micro-organism on the surface of supporting particle was increased with reaction time. As the number of iron oxidation batch cycles increased, the iron oxidation rate increased.     

Ultrathin carbon support films for high-resolution electron microscopy of nanoparticles.

We introduce a simple preparation method for ultrathin carbon support films that is especially useful for high-resolution electron microscopy (HREM) of nanoparticles. Oxidized iron nanoparticles were used as a test sample in a demonstration of this method. The film qualities are discussed on the basis of electron-energy-loss spectroscopy (EELS) and image analysis techniques such as thickness maps and histograms. We carried out a comparison between the homemade and commercial film qualities. The relative thickness of the homemade support films was 0.6 times less than that of the commercial films, which was calculated from the EELS analysis, whereas the thicknesses of both carbon support films varied within about 3%. The percentage of the observable area was about 67 +/- 7.6% of the support film. This was about twice as large as the commercial film (32 +/- 9.3%). The HREM image of the sample prepared with our support film improved 9% in brightness and 15% in contrast compared with images obtained with the commercial support.     

Adsorption free energy of variable-charge nanoparticles to a charged surface in relation to the change of the average chemical state of the particles

Variable-charge nanoparticles such as proteins and humics can adsorb strongly to charged macroscopic surfaces such as silica and iron oxide minerals. To model the adsorption of variable-charge particles to charged surfaces, one has to be able to calculate the adsorption free energy involved. It has been shown that the change in the free energy of variable-charge particles is related to the change in their average chemical state upon adsorption, which is commonly described using surface complexation models. In this work, expressions for the free-energy change in variable-charge particles due to changes in chemical binding are derived for three ion-binding models (i.e., the Langmuir, Langmuir-Freundlich, and NICA models) and for changes due to nonspecific binding for the Donnan model. The expressions for the adsorption free energy of the variable-charge particles to a charged surface are derived on the basis of the equality of the (electro)chemical potential of the particles in the bulk solution and adsorption phase. The expressions derived are general in the sense that they account for the competition between charge-determining ions that bind chemically to the particles, and they also apply in case of the formation of chemical bonds between particle ligands and surface sites. The derived expressions can be applied in the future to model the adsorption of variable-charge nanoparticles to charged surfaces. The results obtained for the NICA-Donnan model make it possible to apply this advanced surface complexation model to describe the adsorption of humics to minerals.     

Generation of High-Oxidation States of Myoglobin in the Nanosecond Time-Scale by Laser Photoionization

Abstract— The 248 nm laser flash photolysis of myoglobin in various redox states (oxy, met and ferryl) in neutral aqueous solution yielded hydrated electrons with concurrent changes in the visible absorption spectrum of the heme. The results could be ascribed to the photoionization of both the peptide and the heme group, in approximately equal yields. The ionization of met- and ferrylmyoglobin was biphotonic, but that of oxymyoglobin was a mixture of mono- and biphotonic processes. Using appropriate electron and radical scavengers, the changes in the heme absorption could be investigated at times ≥100 ns and were shown to be associated with a +1 increase of the formal oxidation state of the heme. Using this method, the formal iron(V) state of native myoglobin could be spectroscopically characterized for the first time. Its absorption, blue-shifted and less intense relative to the ferryl state, is reminiscent of that of the compound I of peroxidases, which contains a ferryl-oxo (iron[IV]) group and a porphyrin radical cation. On this basis, the same structure is proposed for the formal iron(V) state of native myoglobin. The transition from oxy- to metmyoglo-bin took -5 μs, which may reflect the kinetics of exchange of oxygen with water as ligand. The transitions from the met to the ferryl state, and from ferryl to iron(V) states were faster (∽250 ns), consistent with processes that involve proton or electron movements but no changes in the iron coordination state.