Diffuse reflectance infrared Fourier transform (DRIFT) and Mössbauer spectroscopic study of Azospirillum brasilense Sp7: Evidence for intracellular iron(II) oxidation in bacterial biomass upon lyophilisation.

Microbial cells are well known to be capable of remaining viable when desiccated, and a variety of beneficial microorganisms can thus be preserved for storage. For the ubiquitous widely studied soil bacterium Azospirillum brasilense (wild-type strain Sp7), which has a significant agrobiotechnological potential owing to its plant-growth-promoting capabilities perspective for its use in biofertilisers, Fourier transform infrared (FTIR) spectroscopy (in the diffuse reflectance mode, DRIFT) was used to control the state of biomass, together with 57Fe transmission Mössbauer spectroscopy to monitor intracellular iron speciation in live rapidly frozen cell suspension and in the lyophilised biomass (both measured at T = 80 K). It has been shown for the first time that a relatively large part of ferrous iron in live cells (22% of the whole cellular iron pool, represented by two high-spin Fe(II) forms, in the 18-h culture grown on 57Fe(III) complex with nitrilotriacetic acid as the sole source of iron) gets largely oxidised upon lyophilisation. The remaining part of iron(II) in the resulting dry biomass was found to be ca. 3% only. The major part of ferric iron in the dry biomass was shown to be comprised of ferritin-like ferric species (giving a typical magnetically split sextet at T = 5 K), while the iron(III) formed from cellular iron(II) by oxidation in air in the course of drying remained in a paramagnetic state even at T = 5 K. The possibility of intracellular iron(II) oxidation to iron(III) upon desiccation may be a specific natural strategy to avoid cell damage caused by Fenton-type reactions in dormant (frozen, dried) cells. The results obtained may have important implications related to iron speciation and redox transformations in dried bacterial preparations intended for long-term storage.

Synthesis of coal fly ash zeolite for the catalytic wet peroxide oxidation of Orange II.

Fly ash, a coal combustion residue produced by Termotasajero in Colombia, has been hydrothermally treated after an alkaline fusion to produce zeolite without addition of silicon or aluminum. The starting material was thoroughly mixed with NaOH, in a 1:1.2 mass ratio, to obtain a homogeneous mixture that was heated to 100 °C during different times (6, 8, and 10 h) and three zeolite samples were produced. The samples were characterized by XRD, SEM, XRF, Mössbauer spectroscopy, and N2 physisorption. According to characterization results (high surface area and appropriate morphological properties including crystallinity) and synthesis time, zeolitic catalyst synthesized with 8 h of hydrothermal treatment was selected to perform further analysis. This sample consisted of a mixture of zeolite X and zeolite A of high surface area (301 m2 g-1) and a Fe content of 6% wt/wt. The zeolite was used as a catalyst for the Fenton oxidation of Orange II. Experiments were performed in a laboratory batch reactor at 70 °C and constant pH = 3, using different concentrations of H2O2. When the stoichiometric amount of H2O2 was used, good mineralization (XTOC = 45%), complete discoloration, and oxidant consumption were obtained after 240 min of reaction. The sample retained activity after 16 h of usage. The presence of Fe in the reaction media was always detected and a homogeneous Fenton mechanism induced by surface-leached iron is suggested.

Synthesis, characterization and applications of maghemite beads functionalized with rabbit antibodies.

Magnetic nanoparticles (NPs) have attracted great attention owing to their applications in the biomedical field. In the present work, maghemite (γFe2O3) NPs of 6.5 nm were prepared using a sonochemical method and used to prepare magnetic beads through silanization with 3-aminopropyltrimethoxysilane (APTS). Subsequently, amino groups in the resulting APTS-γFe2O3 beads were converted to carboxylic acid (CARB-γFe2O3) through the succinic anhydride reaction, as confirmed by transmission electron microscopy (TEM), Fourier transform infrared spectroscopy and dynamic light scattering (DLS) measurements. The size of these beads was measured as 12 nm and their hydrodynamic diameter as 490 nm, using TEM analysis and DLS, respectively. The CARB-γFe2O3 beads were further functionalized by immobilizing rabbit antibodies on their surfaces; the immobilization was confirmed by flow cytometry and ionic strength. The samples were further characterized by Mössbauer spectroscopy and DC magnetization measurements. Studies on magnetic relaxivities showed that magnetic beads present great potential for application in MR imaging.

Development of a biocompatible magnetic nanofluid by incorporating SPIONs in Amazonian oils.

Higher quality magnetic nanoparticles are needed for use as magnetic nanoprobe in medical imaging techniques and cancer therapy. Moreover, the phytochemistry benefits of some Amazonian essential oils have sparked great interest for medical treatments. In this work, a magnetic nanoprobe was developed, allying the biocompatibility and superparamagnetism of iron oxide nanoparticles (SPIONs) with benefits associated with Amazonian oils from Copaiba and Andiroba trees. SPIONs were obtained by two thermal decomposition procedures and different amounts of precursors (iron acetylacetonates). Their characterization was accomplished by Fourier transform infrared spectroscopy, thermogravimetric analysis, transmission electron microscopy (TEM), X-ray diffraction (XRD), Mössbauer spectroscopy and magnetization. The obtained nanoparticles composition and magnetic properties were not affected by the relative proportion of iron(II) and iron(III) in the precursor system. However, when changing the reducing and stabilizing agents the coating layer shows different compositions/relative weight - the more promising SPIONs have a coating mainly composed by oleylamine and an iron oxide:coating wt% ratio of 55:45. Nanoparticles size distributions were very narrow and centred in the average size of 6-7nm. Cellular assays confirmed the biocompatibility of SPIONs and their effective internalization in human colon cancer cells. Mössbauer/XRD results indicated maghemite as their main iron oxide phase, but traces of magnetite proved to be present. Magnetization saturations of 57emu/g at 5K and 42emu/g at 300K were achieved. With incorporation of SPIONs into Copaiba and Andiroba essential oils, these values show a 4-fold decrease, but the supermagnetic behaviour is preserved providing the effective formation of a nanofluid.

Approximate total Fe content determined by Mössbauer spectrometry: Application to determine the correlation between gamma-ray-emitter activities and total content of Fe phases in soils of the Province of Buenos Aires, Argentina.

Pearson correlation coefficients between 40K, 226Ra and 232Th activities and the total Fe phase fractions yielded by Mössbauer spectroscopy have been calculated for soils of the Province of Buenos Aires, Argentina. Total fractions of Fe phases have been obtained from the relative fractions reported in previous works weighted by the Fe soil content and the recoilless-fraction of each Fe phase. An approximate method based on the relationship between the Mössbauer spectral absorption area (obtained from the 57Fe Mössbauer data) and the total Fe concentration (determined by colorimetric methods, after microwave assisted acid digestion of soil samples) has been used for the first time to determine the Fe concentration in soils with an accuracy of 15%. Protocol to extend the method for unknown samples is also discussed. The determined new coefficients differ from those reported previously. A significant and positive correlation between the total fraction of Fe2+ and the 40K activity values has been obtained. This result validates the hypothesis put forward in a previous work, i.e., that illite captures the 40K existing in the soil. In addition, with the new approximation, the Pearson correlation coefficients for the other natural radionuclides give values that indicate that the methodology reported here is appropriate to study the correlations between the activity values with the total fractions of Fe phases.

Evidence for ferritin as dominant iron-bearing species in the rhizobacterium Azospirillum brasilense Sp7 provided by low-temperature/in-field Mössbauer spectroscopy.

For the ubiquitous diazotrophic rhizobacterium Azospirillum brasilense, which has been attracting the attention of researchers worldwide for the last 35 years owing to its significant agrobiotechnological and phytostimulating potential, the data on iron acquisition and its chemical speciation in cells are scarce. In this work, for the first time for azospirilla, low-temperature (at 80 K, 5 K, as well as at 2 K without and with an external magnetic field of 5 T) transmission Mössbauer spectroscopic studies were performed for lyophilised biomass of A. brasilense (wild-type strain Sp7 grown with (57)Fe(III) nitrilotriacetate complex as the sole source of iron) to enable quantitative chemical speciation analysis of the intracellular iron. In the Mössbauer spectrum at 80 K, a broadened quadrupole doublet of high-spin iron(III) was observed with a few percent of a high-spin iron(II) contribution. In the spectrum measured at 5 K, a dominant magnetically split component appeared with the parameters typical of ferritin species from other bacteria, together with a quadrupole doublet of a superparamagnetic iron(III) component and a similarly small contribution from the high-spin iron(II) component. The Mössbauer spectra recorded at 2 K (with or without a 5 T external field) confirmed the assignment of ferritin species. About 20% of total Fe in the dry cells of A. brasilense strain Sp7 were present in iron(III) forms superparamagnetic at both 5 and 2 K, i.e. either different from ferritin cores or as ferritin components with very small particle sizes.

Magnetic nanoparticles for in vivo use: a critical assessment of their composition.

Three different magnetic samples with particle sizes ranging from 10 to 30 nm were prepared by wet chemical methods. The powders were heated at 100, 150, 200, and 250 °C during 30 min under air. Ferrous and total iron contents were determined immediately after the synthesis and after the thermal treatments. All samples were characterized by X-ray diffraction, transmission and integral low-energy electron Mössbauer spectroscopy (ILEEMS) at 298 K. These samples are composed of a mixture of individual particles of maghemite and magnetite, which implies that once oxidation starts in this kind of material, it occurs throughout the entire particle volume. The existence of a maghemite/magnetite core-shell model was ruled out. A linear correlation between the average isomer shift and the magnetite content was found, allowing the estimation of the amounts of magnetite and maghemite in an unknown sample without the need of performing chemical analysis.

A magnetic nanogel based on O-carboxymethylchitosan for antitumor drug delivery: synthesis, characterization and in vitro drug release.

This paper studied the synthesis, characterization and use of the magnetic chitosan nanogel for carrying meleimidic compounds. The hydrogel polymer was prepared using O-carboxymethylchitosan, which was crosslinked with epichlorohydrin for subsequent incorporation of iron oxide magnetic nanoparticles. The characterization revealed that the magnetic material comprises about 10% of the hydrogel. This material is comprised of magnetite and maghemite and exhibits ferro-ferrimagnetic behavior. The average particle size is 4.2 nm. There was high incorporation efficiency of maleimides in the magnetic nanogel. The release was of sustained character and there was a greater release when an external magnetic field was applied. The mathematical model that best explained the process of drug release by the magnetic hydrogel was that of Peppas-Sahlin. The magnetic nanogel proved to be an excellent candidate for use in drug-delivery systems.

Nano-mineralogical investigation of coal and fly ashes from coal-based captive power plant (India): an introduction of occupational health hazards.

Coal derived nano-particles has been received much concern recently around the world for their adverse effects on human health and the environment during their utilization. In this investigation the mineral matter present in some industrially important Indian coals and their ash samples are addressed. Coal and fly ash samples from the coal-based captive power plant in Meghalaya (India) were collected for different characterization and nano-mineralogy studies. An integrated application of advanced characterization techniques such as X-ray diffraction (XRD), High Resolution-Transmission Electron microscopy (HR-TEM)/(Energy Dispersive Spectroscopy) EDS/(selected-area diffraction pattern) SAED, Field Emission-Scanning Electron Microscopy (FE-SEM)/EDS analysis, and Mössbauer spectroscopy were used to know their extent of risks to the human health when present in coal and fly ash. The study has revealed that the coals contain mainly clay minerals, whilst glass fragments, spinel, quartz, and other minerals in lesser quantities were found to be present in the coal fly ash. Fly ash carbons were present as chars. Indian coal fly ash also found to contain nanominerals and ultrafine particles. The coal-fired power plants are observed to be the largest anthropogenic source of Hg emitted to the atmosphere and expected to increase its production in near future years. The Multi Walled Carbon Nano-Tubes (MWCNTs) are detected in our fly ashes, which contains residual carbonaceous matter responsible for the Hg capture/encapsulation. This detailed investigation on the inter-relationship between the minerals present in the samples and their ash components will also be useful for fulfilling the clean coal technology principles.

A friendly environmental material: iron oxide dispersed over activated carbon from coffee husk for organic pollutants removal.

A friendly environmental material for organic contaminants removal was prepared in this work: small particles of iron oxide dispersed over activated carbon from coffee waste for Fenton-like application. The materials were characterized by means of XRD, N2 physisorption, Mössbauer spectroscopy and H2 pulse titration. The composites showed very good catalytic performances for methylene blue organic dye oxidation. The small goethite particles are the predominant iron oxide phase in the composites. The apparent surface area is quite high and is not very affected after iron impregnation. ESI-MS studies revealed that the dye removal occurs through a Fenton-type system by the composite whereas for the activated carbon the dye removal occurs mainly via adsorption. The iron leaching from the catalysts was negligible, less than 0.5 ppm, which evidences the occurrence of a heterogenous oxidation process. The catalyst was very active for methylene blue oxidation and could be repeatedly used for at least 5 cycles. The catalyst was also active in the oxidation of other organic compounds, such as caffeine and atrazine.

Alternative low-cost approach to the synthesis of magnetic iron oxide nanoparticles by thermal decomposition of organic precursors.

A new approach to thermal decomposition of organic iron precursors is reported, which results in a simpler and more economical method to produce well crystallized γ-Fe2O3 nanoparticles (NPs) with average sizes within the 3-17 nm range. The NPs were characterized by TEM, SAED, XRD, DLS-QELS, Mössbauer spectroscopy at different temperatures, FT-IR and magnetic measurements. The obtained γ-Fe2O3 NPs are coated with oleic acid and, in a lower quantity, with oleylamine (about 1.5 nm in thickness). It was shown that changing operative variables allows us to tune the average particle diameters and obtain a very narrow or monodisperse distribution of sizes. The γ-Fe2O3 NPs behave superparamagnetically at room temperature and their magnetization saturation is reduced by about 34% in comparison with bulk maghemite. The results indicate that the distance between two neighbour NPs, generated by the coating, of about 3 nm is insufficient to inhibit interparticle magnetic interactions when the average diameter is 8.8 nm. The good quality of the NPs, obtained through the present low-cost and easy-handling process, open a new perspective for future technological applications.

Synthesis, characterization and adsorptive properties of carbon with iron nanoparticles and iron carbide for the removal of As(V) from water.

This manuscript presents the synthesis of carbon modified with iron nanoparticles (CFe) and iron carbide (CarFe) from the pyrolyzed crown leaves of pineapple (Ananas comosus) treated with iron salts. The materials that were obtained were used for the removal of As(V) from aqueous media. The carbonaceous materials were characterized by Scanning Electron Microscopy (SEM), Transmission electron microscopy (TEM), X-ray diffraction (XRD), X-Ray Photoelectron Spectroscopy (XPS) and Mossbauer Spectroscopy. The specific area (BET), number site density and point of zero charge (pH(pzc)) were also determined. The kinetic parameters were obtained by fitting the experimental data to the pseudo-first-order and pseudo-second-order models. Different isotherm models were applied to describe the As(V) adsorption behavior. The kinetics of As(V) sorption by CFe and CarFe was well defined for the pseudo-second-order model (R(2) = 0.9994 and 0.999, respectively). The maximum As(V) uptake was 1.8 mg g(-1) for CFe and 1.4 mg g(-1) for CarFe. The results obtained indicated that both materials are equally useful for As(V) sorption. The As(V) experimental isotherm data were described by the Freundlich model for CFe and CarFe.

Synthesis, characterization and magnetic properties of polymer-Fe3O4 nanocomposite.

The chemical stability of magnetic particles is of great importance for their applications in medicine and biotechnology. The most challenging problem in physics of disordered systems of magnetic nanoparticles is the investigation of their dynamic properties. The chemical coprecipitation process was used to synthesize spherical magnetite nanoparticles of 14 nm. The as-prepared magnetite nanoparticles have been aged in the matrix. Magnetic properties and aging effect were studied by Mössbauer spectroscopy at temperatures ranging from 77 to 300 K, and X-ray diffraction. At room temperature, the Mössbauer spectrum showed superparamagnetic behavior of the particles, while well-defined sextets were observed at 77K, indicating a blocked regime. The superparamagnetic magnetite nanoparticles can be used as microbead biosensors.

Solid Eu(III) complexes studied by positron annihilation, optical and Mössbauer spectroscopies: insights on the positronium formation mechanism.

In this work, positron annihilation lifetime (PALS), Doppler broadening annihilation radiation lineshape (DBARL), Mössbauer and optical spectroscopies measurements were performed in Eu(III) dipivaloylmetanate complex, Eu(dpm)(3), at 295 and 80 K. The Eu(dpm)(3) complex is not luminescent at 298 K and does not form positronium. On the other hand, it is highly luminescent at 80K, but still does not form positronium. The absence of positronium formation at 80K cannot be explained by a ligand/metal charge transfer process. We found strong evidences that the electronic delocalization does not occur at both temperatures. Despite the Mössbauer results being inconclusive regarding the Eu(III)/Eu(II) reduction hypothesis, previous results showing positronium formation in other Eu(III) complexes suggest that this process is not occurring. Thus, more studies are needed to explain the absence of positronium in Eu(III) complexes.

Assessment of natural radioactivity levels and their relationship with soil characteristics in undisturbed soils of the northeast of Buenos Aires province, Argentina.

Surface and depth profile concentrations (down to 50 cm) of ²³²Th chain, ²²6Ra, and 4°K radionuclides were determined in undisturbed coastal and inland soils of La Plata city region, Argentina, through their gamma-ray activity using a high-purity Ge detector spectrometer. These results were compared with superficial activities determined in soils from the surroundings of the Centro Atómico Ezeiza (Ezeiza Atomic Center) located in Ezeiza, Buenos Aires Province, Argentina. The hyperfine and magnetic Fe phase's properties of soil profiles were characterized by Mössbauer spectroscopy, magnetic hysteresis loops and AC magnetic susceptibility. No dependence of the activity of the ²³²Th natural chain on depth was found, whereas variations for ²²6Ra and 4°K activities were observed. Positive correlations, determined by the Pearson correlation coefficients, were established between 4°K, ²²6Ra and ²³²Th activity concentrations for the whole set of soil samples. The annual external equivalent dose for adults was similar for La Plata and Ezeiza regions, with average values of 0.08 ± 0.01 mSv and 0.06 ± 0.02 mSv, respectively. The thermal dependence of the AC magnetic susceptibility revealed the existence of magnetite and hematite. The Mössbauer spectra of all soils were made up of signals associated with α-Fe2O3, a paramagnetic relaxation component, and Fe³âº and Fe²âº doublets. In addition, the spectra of inland soils revealed the presence of Fe3O4. A negative correlation was found between the activity concentrations and the α-Fe2O3 and Fe3O4 relative fractions, whereas a positive correlation was found between the Fe³âº relative fraction and the 4°K activity.

Bioinspired FeIIICdII and FeIIIHgII complexes: synthesis, characterization and promiscuous catalytic activity evaluation.

In this work we report on the synthesis, crystal structure, and physicochemical characterization of the novel dinuclear [Fe(III)Cd(II)(L)(µ-OAc)(2)]ClO(4)·0.5H(2)O (1) complex containing the unsymmetrical ligand H(2)L=2-bis[{(2-pyridyl-methyl)-aminomethyl}-6-{(2-hydroxy-benzyl)-(2-pyridyl-methyl)}-aminomethyl]-4-methylphenol. Also, with this ligand, the tetranuclear [Fe(2)(III)Hg(2)(II)(L)(2)(OH)(2)](ClO(4))(2)·2CH(3)OH (2) and [Fe(III)Hg(II)(L)(µ-CO(3))Fe(III)Hg(II)(L)](ClO(4))(2)·H(2)O (3) complexes were synthesized and fully characterized. It is demonstrated that the precursor [Fe(III)(2)Hg(II)(2)(L)(2)(OH)(2)](ClO(4))(2)·2CH(3)OH (2) can be converted to (3) by the fixation of atmospheric CO(2) since the crystal structure of the tetranuclear organometallic complex [Fe(III)Hg(II)(L)(µ-CO(3))Fe(III)Hg(II)(L)](ClO(4))(2)·H(2)O (3) with an unprecedented {Fe(III)(µ-O(phenoxo))(2)(µ-CO(3))Fe(III)} core was obtained through X-ray crystallography. In the reaction 2→3 a nucleophilic attack of a Fe(III)-bound hydroxo group on the CO(2) molecule is proposed. In addition, it is also demonstrated that complex (3) can regenerate complex (2) in aqueous/MeOH/NaOH solution. Magnetochemical studies reveal that the Fe(III) centers in 3 are antiferromagnetically coupled (J=-7.2cm(-1)) and that the Fe(III)-OR-Fe(III) angle has no noticeable influence in the exchange coupling. Phosphatase-like activity studies in the hydrolysis of the model substrate bis(2,4-dinitrophenyl) phosphate (2,4-bdnpp) by 1 and 2 show Michaelis-Menten behavior with 1 being ~2.5 times more active than 2. In combination with k(H)/k(D) isotope effects, the kinetic studies suggest a mechanism in which a terminal Fe(III)-bound hydroxide is the hydrolysis-initiating nucleophilic catalyst for 1 and 2. Based on the crystal structures of 1 and 3, it is assumed that the relatively long Fe(III···)Hg(II) distance could be responsible for the lower catalytic effectiveness of 2.

Adsorption of cysteine on hematite, magnetite and ferrihydrite: FT-IR, Mössbauer, EPR spectroscopy and X-ray diffractometry studies.

In the present paper, the adsorption of cysteine on hematite, magnetite and ferrihydrite was studied using FT-IR, electron paramagnetic resonance (EPR), Mössbauer spectroscopy and X-ray diffractometry. Cysteine was dissolved in artificial seawater (two different pHs) which contains the major constituents. There were two main findings described in this paper. First, after the cysteine adsorption, the FT-IR spectroscopy and X-ray diffractometry data showed the formation of cystine. Second, the Mössbauer spectroscopy did not show any increase in the amount of Fe(2+) as expected due the oxidation of cysteine to cystine. An explanation could be that Fe(2+) was oxidized by the oxygen present in the seawater or there occurred a reduction of cystine by Fe(2+) generating cysteine and Fe(3+). The specific surface area and pH at point of zero charge of the iron oxides were influenced by adsorption of cysteine. When compared to other iron oxides, ferrihydrite adsorbed significantly (p < 0.05) more cysteine. The pH has a significant (p < 0.05) effect only on cysteine adsorption on hematite. The FT-IR spectroscopy results showed that cystine remains adsorbed on the surface of the iron oxides even after being mixed with KCl and the amine and carboxylic groups are involved in this interaction. X-ray diffractometry showed no changes on iron oxides mineralogy and the following precipitated substances were found along with the iron oxides after drying the samples: cysteine, cystine and seawater salts. The EPR spectroscopy showed that cysteine interacts with iron oxides, changing the relative amounts of iron oxides and hydroxide.

Characterization of the biocompatible magnetic colloid on the basis of Fe3O4 nanoparticles coated with dextran, used as contrast agent in magnetic resonance imaging.

The magnetic resonance imaging contrast agent, the so-called Endorem colloidal suspension on the basis of superparamagnetic iron oxide nanoparticles (mean diameter of 5.5 nm) coated with dextran, were characterized on the basis of several measurement techniques to determine the parameters of their most important physical and chemical properties. It is assumed that each nanoparticle is consisted of Fe3O4 monodomain and it was observed that its oxidation to gamma-Fe2O3 occurs at 253.1 degrees C. The Mössbauer spectroscopy have shown a superparamagnetic behavior of the magnetic nanoparticles. The Magnetic Resonance results show an increase of the relaxation times T1, T2, and T2* with decreasing concentration of iron oxide nanoparticles. The relaxation effects of SPIONs contrast agents are influenced by their local concentration as well as the applied field strength and the environment in which these agents interact with surrounding protons. The proton relaxation rates presented a linear behavior with concentration. The measured values of thermo-optic coefficient dn/dT, thermal conductivity kappa, optical birefringence delta n0, nonlinear refractive index n2, nonlinear absorption beta' and third-order nonlinear susceptibility |chi(3)| are also reported.

Characterization of superparamagnetic iron oxide coated with silicone used as contrast agent for magnetic resonance image for the gastrointestinal tract.

The present work is a report of the characterization of superparamagnetic iron oxide nanoparticles coated with silicone used as a contrast agent in magnetic resonance imaging of the gastrointestinal tract. The hydrodynamic size of the contrast agent is 281.2 nm, where it was determined by transmission electron microscopy and a Fe3O4 crystalline structure was identified by X-ray diffraction, also confirmed by Mössbauer Spectroscopy. The blocking temperature of 190 K was determined from magnetic measurements based on the Zero Field Cooled and Field Cooled methods. The hysteresis loops were measured at different temperatures below and above the blocking temperature. Ferromagnetic resonance analysis indicated the superparamagnetic nature of the nanoparticles and a strong temperature dependence of the peak-to-peak linewidth deltaH(pp), giromagnetic factor g, number of spins N(s) and relaxation time T2 were observed. This behavior can be attributed to an increase in the superexchange interaction.

The modular nature of all-ferrous edge-bridged double cubanes.

Two all-ferrous, edge-bridged 8Fe-8S clusters, one capped with carbenes (2) and the other with phosphenes (3), have been characterized by (57)Fe Mossbauer spectroscopy. The clusters have diamagnetic ground states that yield spectra consisting of one quadrupole doublet with a large splitting (25% of absorption) and one (3) or two (2) doublets with much smaller splittings (75% of absorption). These patterns closely resemble those observed for all-ferrous 4Fe-4S clusters. Structurally, the 4Fe-4S fragments of 2 and 3 are remarkably similar to all-ferrous 4Fe-4S clusters, sharing with them the characteristic 3:1 pattern of the iron sites, a differentiation that has been shown previously to reflect spontaneous distortions of the cluster core. These spectroscopic and geometric similarities suggest that the diamagnetic ground state of the 8Fe-8S cluster results from antiferromagnetic exchange coupling of two identical 4Fe-4S modules, each carrying spin S(4Fe) = 4. The iron atoms with the largest quadrupole splittings are located at the opposite ends of the body diagonal containing the bridging sulfides.