Elicitation of thiomolybdates from the iron‐molybdenum cofactor of nitrogenase

Abstract

Aerial oxidation of the iron-molybdenum cofactor (FeMoco) of Azotobacter vinelandii nitrogenase has been shown to yield either the tetrathiomolybdate ion ([MoS4I2-) or the oxotrithiomolybdate ion ([MoOS3l2-), depending on the reaction conditions. Thus, when N-methylformamide (NMF) solutions of FeMoco either were titrated with measured aliquots of air or were diluted with air-saturated NMF, [MoOS3I2-was found to be the predominant product while dilution of NMF solutions of FeMoco with air-saturated methanol produced [MoS4I2-almost exclusively. Similar aerial oxidation of solutions of chemically synthesized Fe-Mo-S clusters showed that significant information about the molybdenum environment in these species could be deduced from the nature of the elicited thiomolybdates. The differences in decomposition products as a function of solvent are postulated to be due to the loss through precipitation of the reducing agent sodium dithionite on addition of methanol but not NMF. These overall decomposition results are discussed in the context of recent X-ray absorption spectroscopic data which suggest the presence of an 'MoS3' core in FeMoco. A possible mechanism whereby [MoS4I2-might be rapidly formed from this core is presented.The iron-molybdenum cofactor (FeMoco) is the extruded, putative Nz-reducing site of nitrogenase [I], obtained by acid treatment of the molybdenum-iron protein of that enzyme followed by extraction with the organic solvent N-methylformamide (NMF) [2, 31. Nitrogenase contains two such cofactors, each of which accounts for one Mo and 6 -8 atoms of Fe and probably 8 -9 atoms of sulfur arranged in a cluster [2-41 so far unrecognized elsewhere in biology [3-91. Although numerous attempts have been made to simulate the structure and/or chemical properties of FeMoco via synthesis and study of Mo-Fe-S cluster compounds, a close chemical model of the site has remained elusive [lo-131. The most incisive structural information on FeMoco has come from Xray absorption spectroscopy (XAS), both near edge spectra (XANES) and fine structure analysis (EXAFS), with the recent data [14, 151 suggesting a molybdenum environment which is comprised of three 0 (or N) and three S atoms from anionic ligands. However, although EXAFS is very precise in identifying the type of donor atom and its distance from the absorber (Mo), it is very much less precise in defining the number of such atoms. Thus, in order to obtain independent, complementary insight into the nature of the molybdenum environment in FeMoco, especially with respect to the number of S donor atoms, we have carried out oxidative decomposition studies of this entity and herein report our results, which Correspondence to W. E.

New, man-made N 2 -fixing systems

“…No X -ray diffraction d ata are available because crystals of the appropriate quality have yet to be produced. No recognizable Fe-S clusters can be extruded from FeMoco (K urtz et al 1979), but its degradation by oxygen produces [M oO S3] 2~ and [M oS4] 2- (Newton et al 1985(Newton et al , 1986. The latter results are consistent with our X -ray absorption spectroscopic results, both xanes (Conradson et al 1985) and exafs (extended X -ray absorption fine structure) (Conradson et al 1987), at the M o K-edge.…”

Observation of Terahertz Vibrations in the Nitrogenase FeMo Cofactor by Femtosecond Pump–Probe Spectroscopy

“…Figure 1(a) shows the steady state absorption spectrum for this FeMoco solution, together with the spectra of the pump and probe pulses. The relatively featureless FeMoco absorption spectrum is one indicator of sample integrity, because air-oxidized FeMoco exhibits a variety of distinct features in its visible spectrum [30] . No changes in the absorption spectrum were observed during the ICVS experimental sessions.…”

“…No changes in the absorption spectrum were observed during the ICVS experimental sessions. Over a period of several weeks of storage, the sample bleached and a distinct absorption band grew in at 470 nm (Supporting Information), indicating the presence of air-oxidized FeMoco [30] . These observations indicate that the integrity of the sample was maintained during the period when the ICVS measurements were conducted.…”

Observation of Terahertz Vibrations in the Nitrogenase FeMo Cofactor by Femtosecond Pump–Probe Spectroscopy

“…The relatively featureless FeMoco absorption spectrum is one indicator of sample integrity, because air-oxidized FeMoco exhibits a variety of distinct features in its visible spectrum. [30] No changes in the absorption spectrum were observed during the ICVS experimental sessions. Over a period of several weeks of storage, the sample bleached and a distinct absorption band grew in at 470 nm (see the Supporting Information), indicating the presence of air-oxidized FeMoco.…”

“…Over a period of several weeks of storage, the sample bleached and a distinct absorption band grew in at 470 nm (see the Supporting Information), indicating the presence of air-oxidized FeMoco. [30] These observations indicate that the integrity of the sample was maintained during the period when the ICVS measurements were conducted. probe data present a strong signal at zero time delay that lasts for about 100 fs.…”