The resting state of nitrogenase shows an S = 3/2 electron paramagnetic resonance (EPR) signal resulting from the FeMo-cofactor (MoFe7S9:homocitrate) of the MoFe protein. When the enzyme undergoes turnover under a CO atmosphere, this signal disappears and two new ones appear:  one under low pressure of CO (denoted lo-CO; 0.08 atm) and the other under high pressure of CO (denoted hi-CO; 0.5 atm). Our recent Q-band (35 GHz) 13C and 57Fe electron nuclear double resonance (ENDOR) studies demonstrated that one CO is bound to the FeMo-cofactor of lo-CO and two to the cofactor of hi-CO. [Christie, P. D.; Lee, H. I.; Cameron, L. M.; Hales, B. J.; Orme-Johnson, W. H.; Hoffman, B. M. J. Am. Chem. Soc. 1996, 118, 8707−8709. Pollack, R. C.; Lee, H. I.; Cameron, L. M.; DeRose, V. J.; Hales, B. J.; Orme-Johnson, W. H.; Hoffman, B. M. J. Am. Chem. Soc. 1995, 117, 8686−8687.] In the present report, we examine the CO-bound FeMo-cofactor in both the lo- and hi-CO forms of the MoFe protein from Azotobacter vinelandii by complete orientation-selective 13C and 1H ENDOR measurements. 1H ENDOR studies reveal that well-resolved signals from a solvent-exchangeable proton seen in the resting state FeMo-cofactor are lost in both of the CO-inhibited forms, indicating a loss in hydrogen bonding as compared to the resting state. This supports the hypothesis that CO binds near the “waist” of the cofactor. Determination of 13C hyperfine tensors of bound 13CO to lo-CO and hi-CO leads to the suggestion that the single CO bound to the FeMo-cofactor of lo-CO may bridge or semibridge two iron ions, while each of the two CO bound to hi-CO is a terminal ligand. These ENDOR measurements and recent FTIR results of Thorneley and co-workers [George, S. J.; Ashby, G. A.; Wharton, C. W.; Thorneley, R. N. F. J. Am. Chem. Soc. 1997, 119, 6450−6451] provide strong mutual support.

A new nitrogenase from Azotobacter vinelandii has been isolated and characterized. It consists of two proteins, one of which is almost identical with the Fe protein (component 2) of the conventional enzyme. The second protein (Av1'), however, has now been isolated and shown to differ completely from conventional component 1, i.e., the MoFe protein. This new protein consists of two polypeptides with a total molecular weight of around 200,000. In place of Mo and Fe it contains V and Fe with a V:Fe ratio of 1:13 +/- 3. The ESR spectrum of Av1' also differs from conventional component 1 in that lacks the g = 3.6 resonance that arises from the FeMo cofactor but contains an axial signal with gav less than 2 as well as inflections in the g = 4-6 region possibly arising from an S = 3/2 state. This new enzyme can reduce dinitrogen, protons, and acetylene but is only able to utilize 10-15% of its electrons for the reduction of acetylene.

The paramagnetism of cigarette tar is found to be associated with at least four different types of species. One of the types is responsible for over 80 percent of the total paramagnetism and has a signal intensity that is independent of temperature from 60 to 250 K. This non-Curie-Weiss temperature dependence indicates that the principal paramagnetic species in tar is not an organic monoradical (doublet) species but instead is a donor-acceptor excimer with a paramagnetic excited state and a diamagnetic ground state. Modeling experiments suggest that the excimer consists of quinone (Q) and hydroquinone (QH2) molecules held in a tar matrix. Since such Q-QH2 species are catalysts for the oxidation of hydrocarbons and are very active redox systems, this paramagnetic species may be implicated in the cocarcinogenic properties of tar. Alternatively, since semiquinone radicals are known to bind to DNA, the tar paramagnetic species may be directly involved in the carcinogenic properties of tar.

The resting state of nitrogenase shows an S = 3/2 electron paramagnetic resonance (EPR) signal resulting from the FeMo-cofactor (M-center; inorganic portion, [Mo, Fe7, S9]) of the MoFe-protein. When the enzyme undergoes turnover under a CO atmosphere, this signal disappears and two new ones appear:  one under low pressure of CO (denoted lo-CO; 0.08 atm) with g = [2.09, 1.97, 1.93] and the other under high pressure of CO (denoted hi-CO; 0.5 atm) with g = [2.06, 2.06, 2.17]. Our recent Q-band (35 GHz) 57Fe and 13C electron nuclear double resonance (ENDOR) studies clearly identified [FeMo-cofactor][CO] n , as the origin of the EPR signals from both lo-CO (n = 1) and hi-CO (n = 2) [Christie, P. D.; Lee, H. I.; Cameron, L. M.; Hales, B. J.; Orme-Johnson, W. H.; Hoffman, B. M. J. Am. Chem. Soc. 1996, 118, 8707−8709 and Pollack, R. C.; Lee, H. I.; Cameron, L. M.; Derose, V. J.; Hales, B. J.; Orme-Johnson, W. H.; Hoffman, B. M. J. Am. Chem. Soc. 1995, 117, 8686−8687], and a previous paper discusses CO binding in detail [Lee, H. I.; Cameron, L. M.; Hales, B. J.; Hoffman, B. M. J. Am. Chem. Soc. 1997, 119, 10121−10126]. We now present complete orientation-selective 57Fe ENDOR measurements of the CO-bound FeMo-cofactor in both lo- and hi-CO forms of the MoFe-protein from Azotobacter vinelandii. The 57Fe ENDOR signals associated with the seven Fe ions of the FeMo-cofactor of lo-CO can be completely assigned and interpreted in terms of four magnetically distinct iron signals. Analysis of these signals following the procedures of Mouesca et al. [Mouesca, J.-M.; Noodleman, L.; Case, D. A.; Lamotte, B. Inorg. Chem. 1995, 34, 4347−4359] has led us to propose valence assignments and charges for the cofactor cluster, [Mo, Fe7, S9]+ = [Mo4+, Fe3+ 1, Fe2+ 6, S2- 9]+, organized into one Fe2.5+ pair and five Fe2+ ions, [Mo4+, (2Fe2.5+)1, Fe2+ 5, S2- 9]+. The result is a formal d-electron count of 43. ENDOR and functional studies indicate that the lo-CO, hi-CO, and resting states of the M-center are all at the same oxidation level. Hence, the proposed valency assignments apply to all three states.