ANALYSIS OF THE FE-57 HYPERFINE COUPLING-CONSTANTS AND SPIN STATES INNITROGENASE P-CLUSTERS

We present a new analysis of the EPR data and Mossbauer parameters of the redox state P-OX of the P-cluster of nitrogenase. In our model, ea ch half of the cluster is formally equivalent to a ''classic'' [Fe4S4] (+) unit containing one ferric and three ferrous ions. However, due to the presence of an additional serine ligand at one iron site, the spi n properties of the two halves are distinct; one has a typical spin S- 1 = 1/2 and the other, close to the serine, a spin S-2 = 7/2 (We consi dered also the case S-2 = 5/2) A parallel coupling of these two subspi ns (resulting in the state \S-1, S-2, S-t] with S-t equal to 4) is fou nd to be more likely than the antiparallel \1/2, 7/2, 3]. This conclus ion is based on two lines of evidence. First, the ferrous ions (identi fied from: their large isomer shifts) present both positive and negati ve hyperfine parameters, which points to a ferromagnetic coupling of t he two cubane subspins S-1 and S-2. Second, we propose the use of a si mple empirical quantity a(test) = Sigma A(iz) where A(iz) are the meas ured hyperfine parameters for P-OX. Comparison of the values of a(test ) (which is markedly St-dependent) for P-clusters from Clostridium pas teurianum (Cp) and from Azotobacter vinelandii (Av) with theoretical e stimates for the possible parallel-coupled states \1/2, 5/2, 3] and \1 /2, 7/2, 4] clearly favors the latter. Our spin coupling model predict s a 5:3 (5 negative and 3 positive)pattern for the hyperfine parameter s rather than 4:4 as originally measured in Av or 6:2 as in Cp (and al so in Av after experimental reanalysis). This model for the P-OX state is combined with other information to provide a consistent picture of oxidation states and spin coupling patterns in all four observed P-cl uster redox states.