VARIABLE-TEMPERATURE MAGNETIC CIRCULAR-DICHROISM STUDIES OF REDUCED NITROGENASE IRON PROTEINS AND [4FE-4S]+ SYNTHETIC ANALOG CLUSTERS

Variable temperature magnetic circular dichroism (VTMCD) and EPR spect roscopies have been used to investigate the ground and excited-state p roperties of [4Fe-4S]+ clusters in Mo- and V-nitrogenase Fe-proteins f rom Azotobacter vinelandii and two synthetic analog clusters, [Fe4S4(S Et)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 e ffects on the S = 1/2 and S = 3/2 spin mixtures in frozen solution. By utilizing medium effects in both Fe-proteins, the VTMCD characteristi cs of both the S = 1/2 and S = 3/2 forms of the [4Fe-4S]+ have been de termined. Together with the VTMCD studies of [Fe4S4(SEt)4]3- and [Fe4S 4(SC6H11)4]3-, which are shown to be predominantly S = 1/2 and 3/2, re spectively, in frozen DMF/toluene solutions, the results demonstrate t hat 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 ze ro-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) ethylen e glycol were found to exhibit heterogeneity in the S = 1/2 resonance. A rapidly relaxing axial resonance, g(perpendicular-to) = 1.94 and g( parallel-to) = 1.82, was observed in addition to the characteristic rh ombic resonance, g = 2.05, 1.94 and 1.87. The origin of the heterogene ity exhibited by [4Fe-4S]+ clusters in frozen solution is discussed in light of these results.