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.

Structural results relevant to the molybdenum sites in xanthine oxidase and sulfite oxidase. Crystal structures of MoO2L, L = (SCH2CH2)2NCH2CH2X with X = SCH3, N(CH3)2

Abstract: Single-crystal x-ray diffraction have been completed for these two compounds: (1) X = SCH/sub 3/; and (2) X = N(CH/sub 3/)/sub 2/. Bond angles and bond lengths are given. The x-ray absorption fine structure (EXAFS) analysis of 1 and 2 are compared with the spectra of oxidized forms of xanthine oxidase and sulfur oxidase. Curve fitting analysis of the data, in their oxidized forms, both enzymes contain the MoO/sub 2//sup 2 +/ unit. The average M=O distances, 1.71 A, are the same for both proteins. Sulfur atoms are present at average distances of 2.42 and 2.54 A for sulfite and xanthine oxidase, respectively. A more distant sulfur is present in both enzymes at 2.85 A. Similarities in the EXAFS spectra for 1 and sulfite oxidase Mo--O and Mo--S bond lengths are apparent. Comparisons of the Mo--S distances in the enzymes with those of 1 and 2 suggests configurational features about the molybdenum sites in the enzymes. The thiolate sulfurs in 1 and 2 are trans to each other and cis to the oxo groups with the Mo--thiolate distances all between 2.40 and 2.42 A. The distances are consistent with other known structures containing thiolates cis to Mo=O. Sulfite oxidase hasmore » similar Mo--S distances, but in xanthine oxidase a longer distance (2.54 A) is observed. Since a pronounced trans effect is known to exist for M=O bonds, this longer distance may be characteristic of a thiolate sulfur trans to a Mo--O. 2 figures, 1 table.« less

Syntheses of metal dithiolene complexes from thiometalates by induced internal redox reactions

Abstract: A variety of new and known transition metal dithiolene complexes has been synthesized from thiometalates via induced internal electron-transfer reactions. Treating MS42- (M = Mo, W) with stoichiometric amounts of bis(trifluoromethyl)-1,2-dithiete ((CF3)2C2S2) results in rapid formation of the respective tris(dithiolene) complexes M(tfd)32- (tfd = [(CF3)2C2S2]2-). These complexes are isostructural and adopt twisted trigonal prismatic coordination (ca. 18° from a perfect trigonal prism). Crystal data: Mo(tfd)32-, monoclinic, space group C2/c, with a = 18.905(4) A, b = 13.732(3) A, c = 17.101(3) A, β = 110.29(3)°, and Z = 4; W(tfd)32-, monoclinic, space group C2/c, with a = 18.933(4) A, b = 13.728(3) A, c = 17.096(3) A, β = 110.26(3)°, and Z = 4. In contrast, Mo(tfd)3 (synthesized as reported previously) has nearly perfect trigonal prismatic coordination. Crystal data: hexagonal, space group P63/m, with a = 9.6795(14) A, b = 9.6795(14) A, c = 13.951(3) A, and Z = 2. Treating WOS32- and MoO2S22- with (CF3)2...

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.