James E. Penner-Hahn

Structural characterization of horseradish peroxidase using EXAFS spectroscopy. Evidence for Fe = O ligation in compounds I and II

Extended X-ray absorption fine structure spectroscopy has been utilized to determine the structural environment of the heme iron sites in horseradish peroxidase compounds I and II. For comparison, analogous studies have been undertaken on putative ferryl (Fe/sup IV/=O) porphyrin model compounds and on crystallographically characterized Cr/sup IV/=O and Cr/sup V/ identical with N porphyrins. In a preliminary communication, they suggested that a short ca. 1.6 A Fe-O bond is present in the high valent forms of both the enzyme and the synthetic porphyrins. The present work demonstrates unambiguously that a short, ca. 1.64 A, Fe-O bond length is present both in HRP compounds I and II and in their synthetic analogues. This structure is consistent only with an oxo-ferryl (Fe=O) complex as the active oxygen species in horseradish peroxidase. The structural details, their implications for heme protein mediated oxygen activation, and the difference between their results and those recently published by other workers.

Oxidation state of gold and arsenic in gold-bearing arsenian pyrite

Abstract XANES measurements on gold-bearing arsenian pyrite from the Twin Creeks Carlin-type gold deposits show that gold is present as both Au0 and AuI+ and arsenic is present at AsI-. Au0 is attributed to sub-micrometer size inclusions of free gold, whereas AuI+ is attributed to gold in the lattice of the arsenian pyrite. STEM observations suggest that AsI- is probably concentrated in angstrom-scale, randomly distributed layers with a marcasite or arsenopyrite structure. Ionic gold (AuI+) could be concentrated in these layers as well, and is present in both twofold- and fourfold-coordinated forms, with fourfold-coordinated AuI+ more abundant. Twofold-coordinated AuI+ is similar to gold in Au2S in which it is linearly coordinated to two sulfur atoms. The nature of fourfold-coordinated AuI+ is not well understood, although it might be present as an Au-As-S compound where gold is bonded in fourfold coordination to sulfur and arsenic atoms, or in vacancy positions on a cation site in the arsenian pyrite. AuI+ was probably incorporated into arsenian pyrite by adsorption onto pyrite surfaces during crystal growth. The most likely compound in the case of twofold-coordinated AuI+ was probably a tri-atomic surface complex such as Spyrite-AuI+-Sbi-sulfideH or AuI+-S-AuI+. The correlation between gold and arsenic might be related to the role of arsenic in enhancing the adsorption of gold complexes of this type on pyrite surfaces, possibly through semiconductor effects.

Sulfur speciation in heavy petroleums: Information from X-ray absorption near-edge structure

Abstract The chemical speciation of sulfur in heavy petroleums, petroleum source rock extracts, and source rock pyrolysis products was studied using X-ray absorption near-edge structure (XANES) spectroscopy. The good energy resolution (ca. 0.5 eV) at the sulfur K edge and the strong dependence of XANES on the sulfur environment combine to give excellent sensitivity to changes in the electronic and structural environment of the sulfur. This has permitted identification and approximate quantitation of different classes of sulfur-containing compounds (e.g., sulfur, sulfides (including disulfides and polysulfides as a group), thiophenes, sulfoxides, sulfones, sulfinic acids, sulfonic acids, and sulfate) in a series of petroleums and petroleum source rocks. Our results indicate that the sulfur speciation of geological samples can be correlated with differences in source depositional environment, thermal maturity, and aromaticity. We report organosulfur compositions for the asphaltene, maltene, and liquid Chromatographie fractions of two sulfur-rich oils. In addition, we find that the organosulfur species in some, but not all, oils are subject to oxidation upon storage and thus may also be susceptible to oxidation in shallow reservoirs exposed to oxic waters. This work illustrates the utility of XANES as a direct spectroscopic probe for the quantitative determination of sulfur species in geological samples.

The physical and chemical action of fire suppressants

This work measures the suppression action of inert gas phase agents in extinguishing an air/liquid organic fuel pool fire. It then gives a predictive model for determining the physical contribution involved in non-inert agent fire suppression. Chemical suppression effectiveness can then be calculated. Further, studying the CF3Y and SF5Y (Y = F, Cl, Br, I) series allows quantitation of chemical suppression action of the individual radical moieties. CF3 is shown to be a strong chemical suppressant while SF5 is a flame promoter. CF3Br (Halon 1301) suppression action is 20% physical, 25% chemical due to CF3, and 55% chemical due to Br. Such quantitation provides guidance in selecting alternative fire suppressants to replace ozone layer depleting halons.

Synthesis, characterization, and in vitro testing of superparamagnetic iron oxide nanoparticles targeted using folic Acid-conjugated dendrimers.

Organic-coated superparamagnetic iron oxide nanoparticles (OC-SPIONs) were synthesized and characterized by transmission electron microscopy and X-ray photoelectron spectroscopy. OC-SPIONs were transferred from organic media into water using poly(amidoamine) dendrimers modified with 6-TAMRA fluorescent dye and folic acid molecules. The saturation magnetization of the resulting dendrimer-coated SPIONs (DC-SPIONs) was determined, using a superconducting quantum interference device, to be 60 emu/g Fe versus 90 emu/g Fe for bulk magnetite. Selective targeting of the DC-SPIONs to KB cancer cells in vitro was demonstrated and quantified using two distinct and complementary imaging modalities: UV-visible and X-ray fluorescence; confocal microscopy confirmed internalization. The results were consistent between the uptake distribution quantified by flow cytometry using 6-TAMRA UV-visible fluorescence intensity and the cellular iron content determined using X-ray fluorescence microscopy.

X-ray absorption spectroscopy in coordination chemistry

Abstract X-ray absorption spectroscopy (XAS) is one of the premier tools for investigating the local structural environment of metal ions. XAS can be divided into X-ray absorption near edge structure (XANES), which provides information primarily about geometry and oxidation state, and extended X-ray absorption fine structure (EXAFS) which provides information about metal site ligation. One of the key attractions of XAS is that it can be used regardless of the physical form of the sample, and in particular it can be used to study non-crystalline materials. Consequently, XAS has come to be widely utilized by the biochemistry and materials science communities. Recently, however, it has become clear that there are numerous questions of interest to coordination chemists that can be addressed using XAS. In this contribution, the physical basis of XAS is reviewed, the advantages and limitations of the technique are discussed, and several examples of the applications of XAS to coordination chemistry are presented. The prospects for future applications of XAS are summarized.

Tetrathiomolybdate Inhibits Copper Trafficking Proteins Through Metal Cluster Formation

Targeting Copper Clusters Tetrathiomolybdate (TM) is a copper-depleting agent that has potential in treating copper-dependent diseases. Alvarez et al. (p. 331, published online 26 November) used spectroscopic and structural studies to show that TM inhibits the yeast copper chaperone Atx1 by forming a TM-Cu-ATx1 complex that is stabilized by a sulfur-bridged copper-molybdenum cluster. Cluster formation prevents transfer of copper from the chaperone to target enzymes. The results provide a basis for developing drugs that target metallation pathways. Complex formation between a copper chaperone and a metallo-drug prevents copper transfer to target enzymes. Tetrathiomolybdate (TM) is an orally active agent for treatment of disorders of copper metabolism. Here we describe how TM inhibits proteins that regulate copper physiology. Crystallographic results reveal that the surprising stability of the drug complex with the metallochaperone Atx1 arises from formation of a sulfur-bridged copper-molybdenum cluster reminiscent of those found in molybdenum and iron sulfur proteins. Spectroscopic studies indicate that this cluster is stable in solution and corresponds to physiological clusters isolated from TM-treated Wilson’s disease animal models. Finally, mechanistic studies show that the drug-metallochaperone inhibits metal transfer functions between copper-trafficking proteins. The results are consistent with a model wherein TM can directly and reversibly down-regulate copper delivery to secreted metalloenzymes and suggest that proteins involved in metal regulation might be fruitful drug targets.

Elucidating the protonation site of vanadium peroxide complexes and the implications for biomimetic catalysis.

Coordination complexes of vanadium(5+) played a key role in understanding the structure and mechanism of vanadium-dependent haloperoxidases, particularly the effects of protonation on peroxide coordination to dioxovanadium(5+) species, and in the activation of the peroxo-oxovanadium(5+) complex for substrate oxidation. There has been no spectroscopic evidence that could test the presence of a hydroxo intermediate in a catalytically active oxovanadium(5+) complex. Herein we report the use of the pre-edge transition in X-ray absorption spectroscopy as a spectroscopic signature for V=O bonding. Displacement of oxo donors with hydrogen peroxide or chloride donors dramatically decreases the pre-edge intensity, confirming that the source of the intense pre-edge feature is closely related to the -bonding associated with the V=O. Protonation of a catalytically active tripodal amine oxovanadium(5+) complex has no affect on the pre-edge intensity and, therefore, rules out the possibility of a hydroxo intermediate in the catalytic cycle.

A method for normalization of X-ray absorption spectra

Accurate normalization of X-ray absorption data is essential for quantitative analysis of near-edge features. A method, implemented as the program MBACK, to normalize X-ray absorption data to tabulated mass absorption coefficients is described. Comparison of conventional normalization methods with MBACK demonstrates that the new normalization method is not sensitive to the shape of the background function, thus allowing accurate comparison of data collected in transmission mode with data collected using fluorescence ion chambers or solid-state fluorescence detectors. The new method is shown to have better reliability and consistency and smaller errors than conventional normalization methods. The sensitivity of the new normalization method is illustrated by analysis of data collected during an equilibrium titration.

XAFS OF DINUCLEAR METAL SITES IN PROTEINS AND MODEL COMPOUNDS

Abstract Extensive use has been made of X-ray absorption fine structure (XAFS) spectroscopy for investigating the local structural environment of metal ions in metalloproteins. Although it is widely accepted that XAFS provides accurate structural information for the nearest neighbors to the metal (i.e., the ligands), the use of XAFS for determining metal-metal distances in multi-nuclear proteins is more problematic. We review the origin of the information in XAFS spectra and discuss some of the limitations that apply in extracting structural data from XAFS spectra. Recent advances in the theory and application of XAFS for determining metal-metal distances are reviewed, with particular emphasis on dinuclear iron and manganese proteins and models. For distances less than 3 A, it is straightforward to determine accurate metal-metal separations using XAFS. For distances > 3 A, the unique assignment of an XAFS feature to a metal-metal interaction continues to be difficult, despite recent advances in XAFS theory and analysis. However, when additional information is available to constrain the possible metal site structures, XAFS can provide very accurate metal-metal distances. Future developments that may improve this situation are discussed.