Molybdenum spinning

Drawing and spinning Spinning and cupping are facilitated if molybdenum is gently heated suitable temperatures can be found in Reference . 

A second unusual EPR spectrum was observed in the oxidized (as-isolated) protein (Fig. 3). This spectrum, which was assigned to an S = z system, was not reminiscent of any Fe-S cluster. Indeed, with g-values of 1.968, 1.953, and 1.903, it looked more like a molybdenum or tungsten spectrum. However, chemical analysis ruled out the possibility that this EPR spectrum arose from Mo or W, and the spectrum was assigned to an Fe-S center instead. The spin concentration, however, was sub stoichiometric and sample-dependent. Furthermore, when the as-isolated protein was oxidized with ferricyanide, it became EPR silent. This, together with the iron determination and the fingerprint of the reduced protein, led Hagen and colleagues to the... 

FPR studies at low temperature detect the presence of one iron-sulfur center and molybdenum. At low temperature a sample of nitrate reductase reduced by dithionite shows a rhombic signal (gm,x = 2.04, gmed = 1.94, and gnm = 1.90). This signal accounts for 0.84 spins/... 

Table 3.4 lists values for A Eq and for some important oxidation and spin states found in bioinorganic molecules. Data are taken from reference 24 and from Table 1 of reference 25 for hemoglobin, myoglobin, and the picket-fence porphyrin model compound, FeTpivPP(l-Melm).25 The myoglobin and hemoglobin model compounds are discussed in Section 4.8.2. Reference 26 provides the Table 3.4 data on iron sulfur clusters found in many bioinorganic species.26 The unusual iron-sulfur and iron-molybdenum-sulfur clusters found in the enzyme nitrogenase are discussed more fully below and in Chapter 6. 

Spectroscopic studies on the Fe-Mo protein by EPR and Mossbauer spectroscopy have shown six iron atoms each in a distinctive magnetic environment coupled to an overall S=3/2 spin system (6,7,8) and electron nuclear double resonance (ENDOR) studies suggest one molybdenum per spin system (8). The 5 Fe signals (five or six doublets) observed in the ENDOR spectra (8) indicate a rather asymmetric structure for the Fe/Mo/S aggregate in which the iron atoms roughly can be grouped into two sets of trios, each set having very similar hyperfme parameters. 

The complex [Fe"(bpca)2][Fe"(hfac)2]2, where Hbpca = bis(2-pyridylcarbonyl)amine, (230), consists of a pair of high-spin tris-/ -diketonate units bridged by a low-spin Ng entity." [Fe" (hfac)2] can bridge through the pyridyltriazine tptz, (231), to molybdenum(O), in Mo(CO)4( tptz), and rhenium(I) in ReCl(CO)3( tptz). ... 

One-electron reduction of chicken liver sulfite oxidase produces a species in which the molybdenum centre is Mo(V) and the b-type heme is low-spin Fe(III).90... 

A typical hyperfine spectrum is shown in Fig. 1 in which the spectrum of an aqueous solution of K3Mo(CN)8 is given. This spectrum is a plot of the first derivative of the absorption curve as a function of the magnetic field, the frequency being held constant. (Most ESR spectra are displayed as the first derivative of the absorption, although some workers are now plotting the second derivative instead.) Seventy-five per cent of the molybdenum atoms are isotopes with no nuclear spin, whereas 25 per cent are the isotopes... 

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