註釋 Abstract: Nitrogenase enzymes catalyze the reduction of atmospheric dinitrogen to ammonia utilizing a Mo-7Fe-9S-C active site, the so-called FeMoco cluster. FeMoco and an analogous small-molecule (Et4N)[(Tp)MoFe3S4Cl3] cubane have both been proposed to contain unusual spin-coupled MoIII sites with an S(Mo)=1/2 non-Hund configuration at the Mo atom. Herein, we present Fe and Mo L3-edge X-ray magnetic circular dichroism (XMCD) spectroscopy of the (Et4N)[(Tp)MoFe3S4Cl3] cubane and Fe L2,3-edge XMCD spectroscopy of the MoFe protein (containing both FeMoco and the 8Fe-7S P-cluster active sites). As the P-clusters of MoFe protein have an S=0 total spin, these are effectively XMCD-silent at low temperature and high magnetic field, allowing for FeMoco to be selectively probed by Fe L2,3-edge XMCD within the intact MoFe protein. Further, Mo L3-edge XMCD spectroscopy of the cubane model has provided experimental support for a local S(Mo)=1/2 configuration, demonstrating the power and selectivity of XMCD 