Structural correlations of nitrogenase active sites using nuclear resonance vibrational spectroscopy and QM/MM calculations

The biological conversion of N-2 to NH3 is accomplished by the nitrogenase family, which is collectively comprised of three closely related but unique metalloenzymes. In the present study, we have employed a combination of the synchrotron-based technique of Fe-57 nuclear resonance vibrational spectroscopy together with DFT-based quantum mechanics/molecular mechanics (QM/MM) calculations to probe the electronic structure and dynamics of the catalytic components of each of the three unique M N(2)ase enzymes (M = Mo, V, Fe) in both the presence (holo-) and absence (apo-) of the catalytic FeMco clusters (FeMoco, FeVco and FeFeco). The results described herein provide vibrational mode assignments for important fingerprint regions of the FeMco clusters, and demonstrate the sensitivity of the calculated partial vibrational density of states (PVDOS) to the geometric and electronic structures of these clusters. Furthermore, we discuss the challenges that are faced when employing NRVS to investigate large, multi-component metalloenzymatic systems, and outline the scope and limitations of current state-of-the-art theory in reproducing complex spectra.