Observation of terahertz transitions in iron-molybdenum cofactor from nitrogenase via multiple spectroscopies

Inelastic x-ray scattering spectroscopy (IXS) is a modern synchrotron based vibrational spectroscopy, which measures transitions in the terahertz vibrations. This paper presents the first IXS on a metallo-enzyme derived sample, namely, the iron-molybdenum cofactor (FeMoco) of nitrogenase. We have also measured the corresponding far infrared absorption spectrum (far IR spectrum) of FeMoco and compared these data to previously published nuclear resonant vibrational spectra (NRVS) of FeMoco and far infrared spectra of the model Mo-dioxo-homocitrate complex K2[MoO2(R,S-H2homocitrate)2]·2H2O. Although NRVS offers several clear advantages, IXS can probe vibrations not involving iron motion and is still sensitive to heavier elements. This provides a useful method to study molybdenum vibrations, such as the ν(Mo-O) mode at 530 cm-1 in FeMoco, which is observable by IXS but not by NRVS. The comparison of the vibrational frequencies of FeMoco with those of the Mo-complex supports an earlier proposal that the homocitrate ligand in FeMoco is protonated. This work, which introduces IXS to bioinorganic chemistry, concludes by examining both the opportunities and the challenges of applying IXS to other biologically relevant systems.