title: Variable temperature magnetic circular dichroism studies of reduced nitrogenase iron proteins and [4Fe-4S]+ synthetic analog clusters.
abstract: Variable temperature magnetic circular dichroism (VTMCD) and EPR spectroscopies have been used to investigate the ground and excited-state properties of [4Fe-4S]+ clusters in Mo- and V-nitrogenase Fe-proteins from Azotobacter vinelandii and two synthetic analog clusters, [Fe4S4(SEt)4]3- and [Fe4S4(SC6H11)4]3-. The results indicate similar [4Fe-4S]+ clusters with analogous S = 1/2 and S = 3/2 ground states in both Fe-proteins. However, the Fe-proteins do differ in terms of the medium effects on the S = 1/2 and S = 3/2 spin mixtures in frozen solution. By utilizing medium effects in both Fe-proteins, the VTMCD characteristics of both the S = 1/2 and S = 3/2 forms of the [4Fe-4S]+ have been determined. Together with the VTMCD studies of [Fe4S4(SEt)4]3- and [Fe4S4(SC6H11)4]3-, which are shown to be predominantly S = 1/2 and 3/2, respectively, in frozen DMF/toluene solutions, the results demonstrate that the form of the VTMCD spectra provides a means of identifying and distinguishing S = 1/2 and S = 3/2 [4Fe-4S]+ clusters. Ground state zero-field splitting parameters for the S = 3/2 clusters are determined for both Fe-proteins. In addition to spin state heterogeneity, samples of the Mo-nitrogenase Fe-protein in the presence of 50% (v/v) ethylene glycol were found to exhibit heterogeneity in the S = 1/2 resonance. A rapidly relaxing axial resonance, g perpendicular = 1.94 and g parallel = 1.82, was observed in addition to the characteristic rhombic resonance, g = 2.05, 1.94 and 1.87. The origin of the heterogeneity exhibited by [4Fe-4S]+ clusters in frozen solution is discussed in light of these results.
authors.affiliation: Department of Chemistry, University of Georgia, Athens 30602.
authors.forename: Y A
authors.initials: YA
authors.lastname: Onate
authors.forename: M G
authors.initials: MG
authors.lastname: Finnegan
authors.forename: B J
authors.initials: BJ
authors.lastname: Hales
authors.forename: M K
authors.initials: MK
authors.lastname: Johnson
mesh_terms: D002942:Circular Dichroism; D019855:Ethylene Glycol; D005026:Ethylene Glycols; D007501:Iron / Q000737:chemistry*; D009591:Nitrogenase / Q000737:chemistry*; D010084:Oxidation-Reduction; D010088:Oxidoreductases; D013455:Sulfur / Q000737:chemistry*; D014508:Urea
chemical_list: D005026:Ethylene Glycols; D013455:Sulfur; D014508:Urea; D007501:Iron; D010088:Oxidoreductases; D009591:Nitrogenase; C029130:nitrogenase reductase; D019855:Ethylene Glycol