Edward I. Stiefel

Bio: Edward I. Stiefel is an academic researcher from ExxonMobil. The author has contributed to research in topics: Molybdenum & Catalysis. The author has an hindex of 45, co-authored 170 publications receiving 5406 citations. Previous affiliations of Edward I. Stiefel include Kettering University.

Resonance Raman spectroscopic characterization of the molybdopterin active site of DMSO reductase.

Abstract: Resonance Raman spectra are compared for Rhodobacter sphaeroides dimethyl sulfoxide reductase, an enzyme containing a molybdopterin cofactor, and two model compounds, I and II, which have pterin and quinoxaline, respectively, attached to a Cp2Mo[IV]-dithiolene chelate [Cp = cyclopentadienyl]. The effect of 34S incorporation was also determined. Several bands in the 200-500 cm-1 region show remarkably similar patterns of frequencies and isotope shifts between protein and models: a band at 351 cm-1 shifts 6-8 cm-1, and bands at lower and higher frequencies show smaller shifts upon 34S substitution. A normal coordinate analysis on II indicates the 351 cm-1 mode to be the symmetric Mo-S[dithiolene] stretch and the remaining low-frequency modes to contain contributions from deformations of the quinoxaline ring as well as from Mo-S stretching. The similarity in the low-frequency spectra between the model compounds and the enzyme strongly supports a dithiolene chelate as the mode of Mo-pterin interaction in the cofactor. Resonance enhancement of both high- and low-frequency quinoxaline or pterin modes is observed for both model compounds, implicating the heterocyclic rings as part of the electronic system involved in the Mo-dithiolene charge transfer transitions. RR spectra of 6-methylpterin and biopterin are reported and used to identify the pterin and quinoxaline high-frequency bands in the model compound spectra. The dithiolene C = C stretch is tentatively assigned to bands at 1506 cm-1 in I and 1515 cm-1 in II.(ABSTRACT TRUNCATED AT 250 WORDS)

Induced internal electron transfer chemistry in rhenium sulfide systems

Abstract: This paper demonstrates the proclivity with which high-valent rhenium sulfur complexes undergo internal electron transfer. Specifically, reaction of [Et{sub 4}N][ReS{sub 4}] with 1.5 molar equiv of tetraalkylthiuram disulfide in acetonitrile gives the dinuclear Re(IV) complexes, Re{sub 2}({mu}-S){sub 2}(S{sub 2}CNR{sub 2}), 1, in very high yield. This dimer reacts with an additional equivalent of tetraalkylthiuram disulfide in the presence of excess Lewis acids, or with 0.5 molar equiv of tetraalkylthiuram disulfide and 1 molar equiv of [Cp{sub 2}Fe][PF{sub 6}], to give the dinuclear Re(III) species [Re{sub 2}({mu}-S-S{sub 2}CNR{sub 2}){sub 2}(S{sub 2}CNR{sub 2}){sub 3}]{sup +}, 2, in high yield. The reaction of [ReS{sub 4}]{sup {minus}} with 3 molar equiv of tetraalkylthiuram disulfide in a mixture of dichloromethane and acetonitrile gives the mononuclear Re(V) species [Re(S{sub 2}CN(R){sub 2}){sub 4}]-[Cl], 3, in high yield. Each of these reactions involves induced internal electron transfer in which the formal oxidation state of the metal center is reduced by the addition of an oxidant (i.e., tetraalkylthiuram disulfide). The bound sulfide is the reductant both for the metal and the external oxidant. The reformation of 1 from 2, in which the metal is oxidized, can be effected using reductants such as H{sub 2}. Electrochemical properties and chemical reactivitiesmore » of the complexes are presented.« less

Secondary building units, nets and bonding in the chemistry of metal–organic frameworks

Abstract: This critical review presents a comprehensive study of transition-metal carboxylate clusters which may serve as secondary building units (SBUs) towards construction and synthesis of metal–organic frameworks (MOFs). We describe the geometries of 131 SBUs, their connectivity and composition. This contribution presents a comprehensive list of the wide variety of transition-metal carboxylate clusters which may serve as secondary building units (SBUs) in the construction and synthesis of metal–organic frameworks. The SBUs discussed here were obtained from a search of molecules and extended structures archived in the Cambridge Structure Database (CSD, version 5.28, January 2007) which included only crystals containing metal carboxylate linkages (241 references).

The Mononuclear Molybdenum Enzymes

Abstract: Molybdenum is the only second-row transition metal required by most living organisms, and is nearly universally distributed in biology. Enzymes containing molybdenum in their active sites have long been recognized,1 and at present over 50 molybdenum-containing enzymes have been purified and biochemically characterized; a great many more gene products have been annotated as putative molybdenum-containing proteins on the basis of genomic and bioinformatic analysis.2 In certain cases, our understanding of the relationship between enzyme structure and function is such that we can speak with confidence as to the detailed nature of the reaction mechanism and, with the availability of high-resolution X-ray crystal structures, the specific means by which transition states are stabilized and reaction rate is accelerated within the friendly confines of the active site. At the same time, our understanding of the biosynthesis of the organic cofactor that accompanies molybdenum (variously called molybdopterin or pyranopterin), the manner in which molybdenum is incorporated into it, and then further modified as necessary prior to insertion into apoprotein has also (in at least some cases) become increasingly well understood. It is now well-established that all molybdenum-containing enzymes other than nitrogenase (in which molybdenum is incorporated into a [MoFe7S9] cluster of the active site) fall into three large and mutually exclusive families, as exemplified by the enzymes xanthine oxidase, sulfite oxidase, and DMSO reductase; these enzymes represent the focus of the present account.3 The structures of the three canonical molybdenum centers in their oxidized Mo(VI) states are shown in Figure 1, along with that for the pyranopterin cofactor. The active sites of members of the xanthine oxidase family have an LMoVIOS-(OH) structure with a square-pyramidal coordination geometry. The apical ligand is a Mo=O ligand, and the equatorial plane has two sulfurs from the enedithiolate side chain of the pyranopterin cofactor, a catalytically labile Mo–OH group, and most frequently a Mo=S. Nonfunctional forms of these enzymes exist in which the equatorial Mo=S is replaced with a second Mo=O; in at least one member of the family the Mo=S is replaced by a Mo=Se, and in others it is replaced by a more complex –S–Cu–S–Cys to give a binuclear center. Members of the sulfite oxidase family have a related LMoVIO2(S–Cys) active site, again square-pyramidal with an apical Mo=O and a bidentate enedithiolate Ligand (L) in the equatorial plane but with a second equatorial Mo=O (rather than Mo–OH) and a cysteine ligand contributed by the protein (rather than a Mo=S) completing the molybdenum coordination sphere. The final family is the most diverse structurally, although all members possess two (rather than just one) equiv of the pyranopterin cofactor and have an L2MoVIY(X) trigonal prismatic coordination geometry. DMSO reductase itself has a catalytically labile Mo=O as Y and a serinate ligand as X completing the metal coordination sphere of oxidized enzyme. Other family members have cysteine (the bacterial Nap periplasmic nitrate reductases), selenocysteine (formate dehydrogenase H), –OH (arsenite oxidase), or aspartate (the NarGHI dissimilatory nitrate reductases) in place of the serine. Some enzymes have S or even Se in place of the Mo=O group. Members of the DMSO reductase family exhibit a general structural homology to members of the aldehyde:ferredoxin oxidoreductase family of tungsten-containing enzymes;4 indeed, the first pyranopterin-containing enzyme to be crystallographically characterized was the tungsten-containing aldehyde:ferredoxin oxidoreductase from Pyrococcus furiosus,5 a fact accounting for why many workers in the field prefer “pyranopterin” (or, perhaps waggishly, “tungstopterin”) to “molybdopterin”. The term pyranopterin will generally be used in the present account. Open in a separate window Figure 1 Active site structures for the three families of mononuclear molybdenum enzymes. The structures shown are, from left to right, for xanthine oxidase, sulfite oxidase, and DMSO reductase. The structure of the pyranopterin cofactor common to all of these enzymes (as well as the tungsten-containing enzymes) is given at the bottom.