Fe Charge Research Articles

Synthesis, Structure Determination, and Spectroscopic/Computational Characterization of a Series of Fe(II)−Thiolate Model Complexes: Implications for Fe−S Bonding in Superoxide Reductases

A combined synthetic/spectroscopic/computational approach has been employed to prepare and characterize a series of Fe(II)-thiolate complexes that model the square-pyramidal [Fe(II)(N(His))(4)(S(Cys))] structure of the reduced active site of superoxide reductases (SORs), a class of enzymes that detoxify superoxide in air-sensitive organisms. The high-spin (S = 2) Fe(II) complexes [(Me(4)cyclam)Fe(SC(6)H(4)-p-OMe)]OTf (2) and [FeL]PF(6) (3) (where Me(4)cyclam = 1,4,8,11-tetramethylcyclam and L is the pentadentate monoanion of 1-thioethyl-4,8,11-trimethylcyclam) were synthesized and subjected to structural, magnetic, and electrochemical characterization. X-ray crystallographic studies confirm that 2 and 3 possess an N(4)S donor set similar to that found for the SOR active site and reveal molecular geometries intermediate between square pyramidal and trigonal bipyramidal for both complexes. Electronic absorption, magnetic circular dichroism (MCD), and variable-temperature variable-field MCD (VTVH-MCD) spectroscopies were utilized, in conjunction with density functional theory (DFT) and semiemperical INDO/S-CI calculations, to probe the ground and excited states of complexes 2 and 3, as well as the previously reported Fe(II) SOR model [(L(8)py(2))Fe(SC(6)H(4)-p-Me)]BF(4) (1) (where L(8)py(2) is a tetradentate pyridyl-appended diazacyclooctane macrocycle). These studies allow for a detailed interpretation of the S-->Fe(II) charge transfer transitions observed in the absorption and MCD spectra of complexes 1-3 and provide significant insights into the nature of Fe(II)-S bonding in complexes with axial thiolate ligation. Of the three models investigated, complex 3 exhibits an absorption spectrum that is particularly similar to the one reported for the reduced SOR enzyme (SOR(red)), suggesting that this model accurately mimics key elements of the electronic structure of the enzyme active site; namely, highly covalent Fe-S pi- and sigma-interactions. These spectral similarities are shown to arise from the fact that 3 contains an alkyl thiolate tethered to the equatorial cyclam ring, resulting in a thiolate orientation that is very similar to the one adopted by the Cys residue in the SOR(red) active site. Possible implications of our results with respect to the electronic structure and reactivity of SOR(red) are discussed.

Identification of interplanetary coronal mass ejections at 1 AU using multiple solar wind plasma composition anomalies

We investigate the use of multiple simultaneous solar wind plasma compositional anomalies, relative to the composition of the ambient solar wind, for identifying interplanetary coronal mass ejection (ICME) plasma. We first summarize the characteristics of several solar wind plasma composition signatures (O7/O6, Mg/O, Ne/O, Fe charge states, He/p) observed by the ACE and Wind spacecraft within the ICMEs during 1996–2002 identified by Cane and Richardson [2003], hereafter CR03. We then develop a set of simple criteria that may be used to identify such compositional anomalies and hence potential ICMEs. To distinguish these anomalies from the normal variations seen in ambient solar wind composition, which depend on the wind speed, we compare observed compositional signatures with those “expected” in ambient solar wind with the same solar wind speed. This method identifies anomalies more effectively than the use of fixed thresholds. The occurrence rates of individual composition anomalies within ICMEs range from ∼70% for enhanced iron and oxygen charge states to ∼30% for enhanced He/p (>0.06) and Ne/O and are generally higher in magnetic clouds than other ICMEs. Intervals of multiple anomalies are usually associated with ICMEs and provide a basis for the identification of the majority of ICMEs. We estimate that CR03, who did not refer to composition data, probably identified ∼90% of the ICMEs present. However, around 10% of their ICMEs have weak compositional anomalies, suggesting that the presence of such signatures does not provide a necessary requirement for an ICME. We note a remarkably similar correlation between the Mg/O and O7/O6 ratios in hourly‐averaged data within both ICMEs and the ambient solar wind. This “universal” relationship suggests that similar processes produce the first‐ionization potential bias and enhanced ion freezing‐in temperatures in the source regions of both ICMEs and the ambient solar wind.

Role of histidine in ligand binding ability of hemoglobin gene

The atomic and electronic structures of heme complexes with His, Gly, and Cys residues (Heme-His, Heme-Gly, and Heme-Cys) in the fifth coordination position of the Fe atom and with oxygen and nitrogen oxide molecules in the sixth Fe position were studied by the semiempirical quantum-chemical method PM3. A comparative analysis of internuclear distances showed that the strength of chemical bonding between the ligand molecules (oxygen and nitrogen oxide) is greater for Heme-Cys than for Heme-His and Heme-Gly complexes. Consequently, the strengthening of the chemical bond of the oxygen (or nitrogen oxide) molecule with Heme-Cys substantially weakens the chemical bond in the ligand molecule. The Mulliken population analysis showed that the electronic density of ligand (oxygen or nitrogen oxide) p-orbitals is transferred to the d-orbitals of the Fe atom, whose charge, calculated according to the Mulliken analysis, formally becomes negative. In the Heme-His complex with oxygen, this charge is substantially greater than in the complex with NO, and the oxygen molecule becomes polarized. No oxygen polarization is observed in the Heme-Cys complex, and the electron density (judging from the change in the Fe charge) is transferred to the coordinated sulfur atom. This is also characteristic of Heme-Cys complexes with nitrogen oxide. An analysis of charges on the atoms indicates that the character of chemical bonding of the oxygen molecule in Heme-Cys and Heme-Gly complexes is similar and basically differs from that in the case of the Heme-His complex. The English version of the paper: Russian Journal of Bioorganic Chemistry, 2004, vol. 30, no. 2; see also http://www.maik.ru.

Multidataset Refinement Resonant Diffraction, and Magnetic Structures.

The scope of Rietveld and other powder diffraction refinements continues to expand, driven by improvements in instrumentation, methodology and software. This will be illustrated by examples from our research in recent years. Multidataset refinement is now commonplace; the datasets may be from different detectors, e.g., in a time-of-flight experiment, or from separate experiments, such as at several x-ray energies giving resonant information. The complementary use of x rays and neutrons is exemplified by a recent combined refinement of the monoclinic superstructure of magnetite, Fe3O4, below the 122 K Verwey transition, which reveals evidence for Fe2+/Fe3+ charge ordering. Powder neutron diffraction data continue to be used for the solution and Rietveld refinement of magnetic structures. Time-of-flight instruments on cold neutron sources can produce data that have a high intensity and good resolution at high d-spacings. Such profiles have been used to study incommensurate magnetic structures such as FeAsO4 and β–CrPO4. A multiphase, multidataset refinement of the phase-separated perovskite (Pr0.35Y0.07Th0.04Ca0.04Sr0.5)MnO3 has been used to fit three components with different crystal and magnetic structures at low temperatures.

Iron charge state distributions as an indicator of hot ICMEs: Possible sources and temporal and spatial variations during solar maximum

It has been shown that Fe charge states greater than and equal to 16 are often observed in the presence of interplanetary coronal mass ejections (ICMEs) at 1 AU. These elevated charge states, representative of hot source regions on the Sun, provide an identifier of ICMEs independent of expansion processes and heliocentric distance. Using this new identifier, we present a comparative study of Fe charge state distributions as a function of latitude and time. We compare Fe charge state data from the ACE spacecraft in the ecliptic plane with that from the Ulysses spacecraft along its orbit from low to high latitudes. We discuss the frequencies of high Fe charge state distributions through a range of latitudes on the approach toward solar maximum. Close to solar maximum, we find almost an order of magnitude fewer hot ICMEs at higher latitudes than in the ecliptic. We offer an explanation for the existence of high Fe charge states in hot ICMEs as a result of magnetic connectivity to flaring regions.

The use of iron charge state changes as a tracer for solar wind entry and energization within the magnetosphere

Abstract. The variation of the charge state of iron [Fe] ions is used to trace volume elements of plasma in the solar wind into the magnetosphere and to determine the time scales associated with the entry into and the action of the magnetospheric energization process working on these plasmas. On 2–3 May 1998 the Advanced Composition Explorer (ACE) spacecraft located at the L1 libration point observed a series of changes to the average charge state of the element Fe in the solar wind plasma reflecting variation in the coronal temperature of their original source. Over the period of these two days the average Fe charge state was observed to vary from + 15 to + 6 both at the Polar satellite in the high latitude dayside magnetosphere and at ACE. During a period of southward IMF the observations at Polar inside the magnetosphere of the same Fe charge state were simultaneous with those at ACE delayed by the measured convection speed of the solar wind to the subsolar magnetopause. Comparing the phase space density as a function of energy at both ACE and Polar has indicated that significant energization of the plasma occurred on very rapid time scales. Energization at constant phase space density by a factor of 5 to 10 was observed over a range of energy from a few keV to about 1 MeV. For a detector with a fixed energy threshold in the range from 10 keV to a few hundred keV this observed energization will appear as a factor of ~103 increase in its counting rate. Polar observations of very energetic O+ ions at the same time indicate that this energization process must be occurring in the high latitude cusp region inside the magnetosphere and that it is capable of energizing ionospheric ions at the same time.Key words. Magnetospheric physics (magnetopause, cusp, and boundary layers; magnetospheric configuration and dynamics; solar wind-magnetosphere interactions)

Enhancement of Nonaqueous Fe(VI) Super-iron Primary Cathodic Charge Transfer

as a cathode is 30% lighter than the equivalent Fe(VI) salt but had exhibited poor nonaqueous charge transfer compared to the latter. Stepwise effects improving cathodic charge transfer were probed and then applied to substantially increase the observed cathode capacity to beyond that of In addition the potassium salt will not have environmental regulatory issues associated with the barium salt. capacity increased from 28, 84, 113 to 241 mAh g−1 when the solvent for 1 M electrolyte was changed from acetonitrile, ethylene carbonate-diethylene carbonate, γ-butyrolactone to propylene carbonate-dimethoxyethane (PC-DME), respectively. replacement in PC-DME, yields 247 or 260 mAh g−1 with 1 M or Polarization was 0.6, 3.0 or 5.7 mV μA−1 cm2 in LiI, or Carbon black enhanced Fe(VI) charge transfer compared to alternate conductors. With a threefold particle size decrease, the specific capacity increased from 241 to 290 at 25°C, and to 298 mAh g−1 at 50°C. Through proper selection of electrolytes, use of carbon black as the conductor, reduction of cathode particle size, and moderate increase of temperature (from 25 to 40°C), the cathodic charge transfer of Fe(VI) in nonaqueous electrolyte is enhanced, resulting in an increase of specific capacity from 290 to 370 mAh g−1. Cocathodes comprised of and exhibited high storage capacity. © 2003 The Electrochemical Society. All rights reserved.

Origin of the May 1998 suprathermal particles: Solar and Heliospheric Observatory/Charge, Element, and Isotope Analysis System/(Highly) Suprathermal Time of Flight results

The Solar and Heliospheric Observatory/Charge, Element, and Isotope Analysis System/(Highly) Suprathermal Time of Flight ((H)STOF) mass spectrometer measures the elemental composition and the charge state distribution of ions with suprathermal energies in the range 10–4000 keV amu−1 from above the solar wind particle distribution up to low‐energy flare particle energies. We analyze the time period around the large coronal mass ejection event on 2–3 May 1998. Using data from (H)STOF, we study the charge state distribution of He, the CNO group, and Fe. In addition, we investigate the energy dependence of the mean ionic charge state of Fe in the energy‐per‐nucleon range 12–100 keV amu−1. Furthermore, we also report energy spectra of H+, He++, and He+ and the variations in the elemental abundance ratios He/H, He/CNO, and Fe/CNO. These observations greatly extend the energy range in which particles associated with this time period have been measured. Because of their elevated energies, suprathermal particles are the prime seed population for further acceleration in gradual events. We identify interstellar pickup ions as an important nonsolar seed population. The observed low mean ionic charge states of Fe and the small Fe/CNO ratio are typical for large gradual events. The energy dependence of the Fe charge states may be a result of the presence of different iron populations or of the thermal history of the accelerated material. The temporal variations of Fe/CNO and He/CNO indicate a stronger confinement of low‐rigidity particles at the acceleration site.

Ionic charge states of solar energetic particles from solar flare events during the current rise of solar activity as observed with ACE SEPICA

Ionic charge states of solar energetic ions provide key information on the local temperatures and ionization processes in solar flares. In conjunction with the particle mass this measurement is a tool to study acceleration, fractionation, and transport processes. SEPICA has been designed to resolve charge state distributions in detail and to provide a large collecting power, which allows a much more detailed analysis. During November 4 – 7, 1997, a series of strong flares occurred that were magnetically well-connected. Over the energy range ≈ 0.2 – 1 MeV/nucleon a substantial increase of the charge states with energy was observed. Fe shows a charge state distribution with a distinct peak at Q ≈ 10 – 15 and a tail extending to Q ≈ 20. The newly revealed variation of the charge states with energy will potentially provide insight into modification of charge states during or after acceleration due to stripping and/or rigidity dependent acceleration processes. In addition, a sample of solar particle (SEP) events in 1998, the first year of significant solar activity during the rise of the current solar cycle, is studied for their charge state variations. Typical gradual events that resemble closely coronal composition are found with Fe charge state of ≈11 and impulsive events with high Fe abundance and charge states between 17 and 20. In addition, there are also SEP events with charge states between 12 and 15 that show a moderate, but varying enhancement in Fe. It appears that charge states of SEPs show a much richer variation than the previously reported two groups of charge states, which is also correlated with heavy ion enrichment.

Isotopic Composition of Solar Energetic Particle Events Measured byAdvanced Composition Explorer/ULEIS

Since its 1997 August launch, the Ultra Low Energy Isotope Spectrometer (ULEIS) on board the Advanced Composition Explorer (ACE) has observed a large number of solar energetic particle (SEP) events in the 0.4-3 MeV nucleon-1 range. These events show remarkable variability of both the elemental and isotopic compositions, with 22Ne/20Ne varying, from event to event, by up to a factor of 10, Fe/O by up to a factor of 100, and 3He/4He by up to a factor of 104. In this paper we present new measurements of the 22Ne/20Ne, 26Mg/24Mg, and 3He/4He ratios at ~1 MeV nucleon-1 and compare these ratios to the iron abundances measured during the same time periods. We find that the elemental abundances, Fe/O and Fe/Mg, and the isotopic abundances, 22Ne/20Ne and 26Mg/24Mg, are all well ordered by both the 3He/4He ratio and the average Fe charge state as measured by ACE/SEPICA. These variations in 22Ne/20Ne and 26Mg/24Mg are much larger than expected from the charge to mass dependence derived for the elemental abundances, and we suggest that the large fluctuations seen for the neon and magnesium isotopes are related to the amount of impulsive flare material present.

A Deep-Water Glauconitization Process on the Ivory Coast—Ghana Marginal Ridge (ODP Site 959): Determination of Fe3+-Rich Montmorillonite in Green Grains

AbstractThe mineral and chemical composition of green glauconitic grains from ODP Site 959 (2100 m water depth) located on the northern flank of the Ivory Coast—Ghana Marginal Ridge was studied. Recurrent winnowing of a 20 m thick Pleistocene succession resulted in a low accumulation rate and stratigraphic hiatuses. The green clay material typically occurs as fillings in the chambers of pelagic foraminifers. The amount of green clay present in sediments older than 1 Ma is small, and greater in younger material. Mud composed of smectite, kaolinite, traces of mica, calcite and quartz was the precursor material that filled the chambers of the foraminifers. Processes at the water-sediment interface slowly modified this composition. Kaolinite was dissolved; smectite lost Al but gained Fe, K and layer charge. In that matrix, the nanocrystals of neoformed smectite are observed. The infrared (IR) spectra showed OH-stretching and bending vibrations due to groups incorporating Fe3+. The spectra are in agreement with the crystallochemical formulae of Fe3+-rich montmorillonite as determined by point-by-point analyses on the neoformed crystallites and on the surrounding matrix. The layer charge in this Fe3+-rich montmorillonite is almost wholly octahedral as shown in crystallochemical formulae and documented independently by a new IR method. The tetrahedral charge appeared when the Fe content increased by > 1.2 Fe per formula unit. With the maturation process, the increased role of the closed layers is observed, with the color of grains becoming greener. We have documented for the first time glauconitization proceeding at a depth of 2100 m at a temperature near 3°C. The most important factors of the process are: accumulation of terrigenous clayey material in the foraminiferal chambers, Fe supply from a nearby continent, and a lengthy residence at the water-sediment interface in the zone of the winnowing and low sediment accumulation rate.

Iron charge distribution as an identifier of interplanetary coronal mass ejections

We present solar wind Fe charge state data measured on the Advanced Composition Explorer (ACE) from early 1998 to the middle of 2000. Average Fe charge states in the solar wind are typically around 9+ to 11+. However, deviations from these average charge states occur, including intervals with a large fraction of Fe≥16+ which are consistently associated with interplanetary coronal mass ejections (ICMEs). By studying the Fe charge state distribution we are able to extract coronal electron temperatures often exceeding 2 × 106 kelvins. We also discuss the temporal trends of these events, indicating the more frequent appearance of periods with high Fe charge states as solar activity increases.

Fe(VI) Catalyzed Manganese Redox Chemistry: Permanganate and Super-Iron Alkaline Batteries

In principle, alkali permanganates represent a substantial cathodic charge source for electrochemical storage, but high rate charge transfer has been inefficient. This study presents a novel Fe(VI) species (super-iron) and manganese redox chemistry synergism. Also presented is the high discharge energies from cathodes which utilize this phenomenon in a conventional cylindrical battery configuration. Batteries formed with a cathode combining BaFeO4 and KMnO4, and using a conventional zinc alkaline anode in an AAA cylindrical cell configuration, exhibit an unusually high discharge capacity of 2.2 Wh. The discharge efficiency of solid alkaline permanganate or manganate cathodes is probed, and improved charge transfer of Mn(VII) redox chemistry in the presence of Fe(VI) is demonstrated. In a reciprocal manner, enhancement of Fe(VI) charge transfer are demonstrated upon inclusion of manganese salts, and also that added cesium further improves the combined Fe(V)/Mn(VII) cathode.

Charge-equilibration of Fe ions accelerated in a hot plasma

We examine the energy-dependent rates of charge-changing processes of energetic Fe ions in the hot plasma of the solar corona. For ionization of the Fe-ion projectile in collisions with ambient protons, three different methods of estimating the corresponding ionization cross sections are presented and compared. Proton-impact ionization is found to be significant irrespective of the particular method used. Differences in the proton-impact ionization cross sections' estimates are shown to have little effect in calculating highly nonequilibrium Fe charge states during acceleration, whereas equilibrium charge states are sensitive to such differences. A parametric study of the Fe charge-equilibration comprises (i) impact of the ambient plasma density, and (ii) the energy dependence impact of the acceleration rate upon the charge-energy profiles and upon the estimated values of the density acceleration-time product. We emphasize potential importance of careful measurements of charge-energy profiles along with ion energy spectra for determining the energy dependence of ion acceleration time and the energy dependence of the leaky-box escape time.

Use of Fe charge state changes as a tracer for solar wind entry to the magnetosphere

We present a comparison of high charge state ion composition measurements from ACE located near the L1 point and POLAR in the mid‐altitude cusp/cleft, during the 13 May 1998 CME event. Using the frozen in charge state of the solar wind origin Fe ions as a tracer, we demonstrate the connectivity between the ACE observations and the high latitude measurements from POLAR. The ion charge state distributions were observed to show abrupt changes with variations in the average charge state of Fe between +6 and +15 corresponding to changes in the coronal source temperature of 4×105 to 2×106 K. A good correlation between the measurements at L1 and in the cusp/cleft was found, indicating direct entry of solar wind material during periods of southward IMF. In addition there is possible evidence for delayed entry during a period of predominantly northward IMF that may indicate a less direct entry mechanism for this configuration.

Enhanced Fe(VI) cathode conductance and charge transfer: effects on the super-iron battery

Cathodes comprising Fe(VI) salts are capable of three-electron reduction, and are useful for the formation of energetic ‘super-iron’ batteries. Material additions to the Fe(VI) cathode can be used to enhance the conductance and the efficiency of charge transfer to the cathode, and control the characteristics of the electrochemical storage. Whereas several common carbons are ineffective as conductive matrices for Fe(VI) reduction, several others such as small particle (1 μm) graphite, compressed carbon black, and fluorinated polymer graphites support efficient Fe(VI) 3e − reduction. Several inorganic salts also sustain Fe(VI) reduction, but at lower current densities. Titanates and other salts added to a K 2FeO 4 cathode improve the faradaic efficiency of Fe(VI) reduction at higher (∼3 mA cm −2) discharge current densities. Fluorinated polymer graphites provide an unusual additive to the Fe(VI) cathode mix, and at a low level (10 wt.%) addition can support efficient Fe(VI) reduction.

Charge states of solar energetic particles using the geomagnetic cutoff technique: SAMPEX measurements in the 6 November 1997 solar particle event

We report on the ionization states of ∼0.5–50 MeV/nucleon ions in the 6 November 1997 solar particle event using instrumentation on the SAMPEX satellite and the geomagnetic cutoff technique. Using the geomagnetic cutoffs of ∼10 MeV 4He and ∼20 MeV protons, we infer the ionization states of ∼0.5–50 MeV/nucleon C‐Fe by measuring their latitude distributions. The geomagnetic cutoff method extends the measurement of ionization states beyond 10 MeV/nucleon, where charge state information is inaccessible with present electrostatic deflection techniques. In contrast to an increase of the Fe charge state observed above ∼20 MeV/nucleon in events in late 1992, we find in the 6 November 1997 event that Si & Fe charge states increased dramatically across 0.5–50 MeV/nucleon. While the origin of this newly discovered energy dependence is unknown, such significant event to event variations of solar particle charge states should be considered in models of acceleration and transport processes.

Spectroscopic Study of [Fe2(O2)(OBz)2{HB(pz‘)3}2]: Nature of the μ-1,2 Peroxide−Fe(III) Bond and Its Possible Relevance to O2Activation by Non-Heme Iron Enzymes

Spectroscopic and theoretical studies of a binuclear cis μ-1,2 peroxide-bridged Fe(III) complex, [Fe2(O2)(OBz)2{HB(pz‘)3}2], where HB(pz‘)3 represents hydrotris(3,5-diisopropyl-1-pyrazolyl)borate and OBz is benzoate, are presented and discussed. It is shown that the high O−O stretching frequency of 876 cm-1 does not reflect a strong O−O bond (kO-O = 3.1 mdyn/Å) but rather results from substantial mechanical coupling between the Fe−O and O−O stretch motions. The coupling strength in μ-1,2 peroxide-bridged complexes is mainly determined by the metal−O−O bond angle, and this angle is larger in the three structurally characterized Fe−peroxide dimers than in related Cu and Co dimers due to a strong π-bonding interaction in the former. Using absorption, magnetic circular dichroism, and resonance Raman spectroscopies we can identify four peroxide-to-Fe charge transfer (CT) transitions contributing to the optical absorption spectrum below 35 000 cm-1. These transitions give rise to two similarly intense bands centered at ∼14 600 and 26 200 cm-1 (ε ∼ 3800 M-1 cm-1) and to weaker features at 12 500 cm-1 (∼400 M-1 cm-1) and 24 650 cm-1 (∼1600 M-1 cm-1). A complete band assignment based on the experimental data and assisted by density functional calculations is presented and provides significant insight into Fe−peroxide bonding. From a quantitative analysis of the optical absorption and resonance Raman data within the framework of time-dependent theory of electronic spectroscopy it follows that the π contribution to the peroxide → Fe charge donation amounts to 50−65%. The σ contribution, however, is smaller than in the well-characterized trans μ-1,2 peroxide-bridged Cu(II) dimer, and the charge on peroxide is thus similar in the two complexes, suggesting that the bridging peroxide in [Fe2(O2)(OBz)2{HB(pz‘)3}2] has a rather basic/nucleophilic character. The mechanistic implications of these results for the conversion of the putative cis μ-1,2 peroxide-bridged diiron(III) intermediate (P or Hperoxo) of methane monooxygenase to a high-valent diiron(IV)−oxo species are discussed.

Iron-sulfur cluster cysteine-to-serine mutants of Anabaena -2Fe-2S- ferredoxin exhibit unexpected redox properties and are competent in electron transfer to ferredoxin:NADP+ reductase.

The reduction potentials and the rate constants for electron transfer (et) to ferredoxin:NADP+ reductase (FNR) are reported for site-directed mutants of the [2Fe-2S] vegetative cell ferredoxin (Fd) from Anabaena PCC 7120, each of which has a cluster ligating cysteine residue mutated to serine (C41S, C46S, and C49S). The X-ray crystal structure of the C49S mutant has also been determined. The UV-visible optical and CD spectra of the mutants differ from each other and from wild-type (wt) Fd. This is a consequence of oxygen replacing one of the ligating cysteine sulfur atoms, thus altering the ligand --> Fe charge transfer transition energies and the chiro-optical properties of the chromophore. Each mutant is able to rapidly accept an electron from deazariboflavin semiquinone (dRfH.) and to transfer an electron from its reduced form to oxidized FNR although all are somewhat less reactive (30-50%) toward FNR and are appreciably less stable in solution than is wt Fd. Whereas the reduction potential of C46S (-381 mV) is not significantly altered from that of wt Fd (-384 mV), the potential of the C49S mutant (-329 mV) is shifted positively by 55 mV, demonstrating that the cluster potential is sensitive to mutations made at the ferric iron in reduced [2Fe-2S] Fds with localized valences. Despite the decrease in thermodynamic driving force for et from C49S to FNR, the et rate constant is similar to that measured for C46S. Thus, the et reactivity of the mutants does not correlate with altered reduction potentials. The et rate constants of the mutants also do not correlate with the apparent binding constants of the intermediate (Fdred:FNRox) complexes or with the ability of the prosthetic group to be reduced by dRfH.. Furthermore, the X-ray crystal structure of the C49S mutant is virtually identical to that of wt Fd. We conclude from these data that cysteine sulfur d-orbitals are not essential for et into or out of the iron atoms of the cluster and that the decreased et reactivity of these Fd mutants toward FNR may be due to small changes in the mutual orientation of the proteins within the intermediate complex and/or alterations in the electronic structure of the [2Fe-2S] cluster.

Chemical characterization of dissolved organic matter released by limed and unlimed forest soil horizons

Liming of forest soils, currently practiced in Europe, may be necessary in the future in North America to counteract calcium (Ca) depletion due to whole tree harvesting. With current concerns over increasing carbon dioxide (CO2) levels in the atmosphere, the effects of forest practices such as liming on carbon (C) cycling in forest ecosystems has been receiving increasing attention. This laboratory study investigated the effect of an increase in pH on dissolved organic matter (DOM) in water extracts of organic horizons. Organic horizons were collected from nine forested sites, some predominantly hardwood and some predominantly softwood stand types. The soils were chemically characterized. Water extracts of limed and unlimed treatments of nine organic horizons were analyzed for pH, Al, Fe, Ca, C, and DOM negative charge density. Total luminescence fluorescence spectra were also obtained for each extract. The addition of lime (CaCO3) to organic horizons caused an average of 55% more C to be released than in unamended samples. Liming caused no significant changes in negative charge density of the DOM. Liming decreased organically-complexed aluminum (Al) and increased organically-complexed Ca. There was a negative correlation between Al concentration of extracts and fluorescence intensity. Chemical changes in the DOM released by liming were suggested by decreased fluorescence emission and excitation wavelengths due to liming. Hardwood samples did not respond differently than softwood samples with respect to DOM concentration, functional group concentration, or spectroscopic characteristics. However, there was a suggestion that particular tree species, such as cedar, may have unique effects on C concentrations and DOM chemistry. Key words: Lime, forest soil, dissolved organic matter, fluorescence