Moessbauer and integer-spin EPR of the oxidized P-clusters of nitrogenase: POX is a non-Kramers system with a nearly degenerate ground doublet

The molybdenum-iron protein of nitrogenase contains 2 Mo atoms and ca. 28-30 Fe atoms. Approximately 16 Fe atoms belong to the P-clusters, a novel type of iron-sulfur cluster of unknown structure. Mossbauer studies have established that P-clusters are diamagnetic in the semireduced state, P N . Upon oxidation of the protein with redox dyes such as thionin the state P OX is attained. Previous studies have revealed that the low-temperature (≤4.2 K) Mossbauer spectra of P OX exhibit magnetic hyperfine patterns even in the absence of external magnetic fields