Syntheses, structures, and spectroscopic properties of six-coordinate mononuclear oxo-molybdenum(V) complexes stabilized by the hydrotris(3,5-dimethyl-1-pyrazolyl)borate ligand

Cleland, W. E.; Barnhart, Kerry M.; Yamanouchi, Kuniko; Collison, David; Mabbs, Frank E.; Ortega, Richard; Enemark, John H.

doi:10.1021/ic00254a012

Cited by 158 publications

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“…EPR Spectroscopy-The room temperature isotropic solution X-band EPR spectra for [Tp*MoO(S 2 DIFPEPP)] is shown in Figure 11; the isotropic EPR spectrum for may be taken as an indication that the dithiolene sulfur atoms in these complexes are better donors in the Mo(V) oxidation state than those present in Tp*MoO(bdt) [55][56]. Unlike the other dithiolene ligands depicted in Figure 10 The low-temperature anisotropic EPR spectra for [Tp*MoO(S 2 DIFPEPP)] is presented in Figure 12 and that for [Tp*MoO(S 2 PEPP)] is available as Supplemental Material.…”

Section: Spectroscopic Studiesmentioning

confidence: 99%

Synthesis, characterization, and spectroscopy of model molybdopterin complexes

Burgmayer

Kim

Petit

et al. 2007

Journal of Inorganic Biochemistry

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The preparation and characterization of new model complexes for the molybdenum cofactor are reported. The new models are distinctive for the inclusion of pterin-substituted dithiolene chelates and have the formulation Tp*MoX(pterin-R-dithiolene) (Tp* = tris(3,5,-dimethylpyrazolyl)borate), X= O, S, R= aryl or −C(OH)(CH 3 ) 2 ). Syntheses of Mo(4+) and (5+) complexes of two pterin-dithiolene derivatives as both oxo and sulfido compounds, and improved syntheses for pterinyl alkynes and [Et 4 N][Tp*Mo IV (S)S 4 ] reagents are described. Characterization methods include electrospray ionization mass spectrometry, electrochemistry, infrared spectroscopy, electron paramagnetic resonance and magnetic circular dichroism. Cyclic voltammetry reveals that the Mo(5+/4+) reduction potential is intermediate between that for dithiolene with electron-withdrawing substituents and simple dithiolate chelates. Electron paramagnetic resonance and magnetic circular dichroism of Mo(5+) complexes where X = O, R = aryl indicates that the molybdenum environment in the new models is electronically similar to that in Tp*MoO(benzenedithiolate).

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Section: Spectroscopic Studiesmentioning

confidence: 99%

Synthesis, characterization, and spectroscopy of model molybdopterin complexes

Burgmayer

Kim

Petit

et al. 2007

Journal of Inorganic Biochemistry

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“…Though there are quite large differences in the parameters of the different signals, which undoubtedly have their origins in defined structural changes, nevertheless, drawing specific structural conclusions is not easy. As is emphasised by the work of Cleland et al (1987) on a unique series molybdenum(V) model compounds, EPR parameters are extremely sensitive to changes Table 2. Effect of structural changes on Mo(V) EPR parameters of model compounds and of some molybdoenzyme species.…”

Section: General Structural Inferences From the Epr Parametersmentioning

confidence: 99%

“…Values of g,, and of the rhombicity and anisotropy parameters are listed. Replacement of an oxygen ligand by sulphur, as when LMoO(OCH,CH,O) is compared with LMoO(OCH,CH,S), gives rise to increases in both g,, and the rhombicity parameter and, indeed, this holds also for replacement of either oxygen or nitrogen by sulphur, in each of eight pairs of derivatives of the ligand, L [hydrotris(3,5-dimethyl-1 -pyrazolyl)borate], studied by Cleland et al (1987). The changes in parameters are comparable (Table 2), when the Slow signal from desulpho xanthine oxidase is compared with the Rapid signal from this enzyme in the functional state.…”

Section: General Structural Inferences From the Epr Parametersmentioning

confidence: 99%

“…Much effort has been expended (e.g. George and Bray, 1988;Cleland et al, 1987;Greenwood et al, 1993) in attempting, with the help of model compounds and of isotopic substitution of the enzymes, to define the precise ligation state and coordination geometry of the metal that gives rise to the individual signals.The present studies concern the enzyme dimethylsulphoxide reductase (Me,SO reductase) from the photosynthetic bacterium Rhodobacter capsulatus. This enzyme (McEwan et al, 1991) and the very closely related species from Rhodobacter sphaeroides (Satoh and Kurihara, 1987) both have molybdenum as the sole redox centre, in a protein consisting of a single polypeptide chain of M, approximately 82000.…”

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Multiple States of the Molybdenum Centre of Dimethylsulphoxide Reductase from Rhodobacter Capsulatus Revealed by EPR Spectroscopy

Bennett

Benson

McEwan

et al. 1994

European Journal of Biochemistry

137

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The dimethylsulphoxide reductase of Rhodobacter capsulatus contains a pterin molybdenum cofactor molecule as its only prosthetic group. Kinetic studies were consistent with re-oxidation of the enzyme being rate limiting in the turnover of dimethylsulphoxide in the presence of the benzyl viologen radical. EPR spectra of molybdenum(V) were generated by reducing the highly purified enzyme under a variety of conditions, and with careful control it was possible to generate at least five clearly distinct EPR signals. These could be simulated, indicating that each corresponds to a single chemical species. Structures of the signal-giving species are discussed in light of the EPR parameters and of information from the literature. Three of the signals show coupling of molybdenum to an exchangeable proton and, in the corresponding species, the metal is presumed to bear a hydroxyl ligand. One signal with g,, 1.96 shows a very strong similarity to a signal for the desulpho form of xanthine oxidase, while two others with g,, values of 1.98 show a distinct similarity to signals from nitrate reductase of Escherichia coli. These data indicate an unusual flexibility in the active site of dimethylsulphoxide reductase, as well as emphasising structural similarities between molybdenum enzymes bearing different forms of the pterin cofactor. Interchange among the different species must involve either a change of coordination geometry, a ligand exchange, or both. The latter may involve replacement of an amino acid residue co-ordinating molybdenum via 0 or N, for a cysteine co-ordinating via S. Since the two signals with g,, 1.96 were obtained only under specific conditions of reduction of the enzyme by dithionite, it is postulated that their generation may be triggered by reduction of the pteridine of the molybdenum cofactor from a dihydro state to the tetrahydro state.Enzymes that depend on the pterin molybdenum cofactor (Rajagopalan, 1991) are widely distributed, have a variety of important roles in different organisms and have been extensively studied in many laboratories (for recent reviews, see Spiro, 1985;Bray, 1988;Solomonson and Barber, 1990;Wootton et al., 1991 ;Enemark and Young, 1993). Detailed structural information from X-ray crystallography is not yet available on any of these enzymes (however, see Romao et al., 1993a,b). All but one of the enzymes contain, in addition to molybdenum, redox centres such as flavin, haem or ironsulphur. These complicate spectroscopic investigations and, in particular, their strong absorption in the ultraviolethisible region effectively masks the rather weak absorption of the molybdenum chromophore. Nevertheless, other spectroscopic methods have provided important insights into the local structure around the molybdenum atom. EPR spectroscopy has been very important in this context and has been supplemented particularly by extended X-ray absorption fine structure (EXAFS) and to a lesser extent by magnetic circular Abbreviations. MCD, magnetic circular dichroism ; EXAFS, extended X-ray absorpt...

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