Nuclear resonance vibrational spectroscopy of a protein active-site mimic

For many years, Mössbauer spectroscopy has been applied to measure recoilless absorption of x-ray photons by nuclei. Recently, synchrotron radiation sources have enabled the observation of weaker features separated from the recoilless resonance by the energy of vibrational quanta. This enables a form of vibrational spectroscopy with a unique sensitivity to the probe nucleus. Biological applications are particularly promising, because it is possible to selectively probe vibrations of a single atom at the active site of a complex biomolecule, while avoiding interference from the vibrations of thousands of other atoms. In contrast with traditional site-selective vibrational spectroscopies, nuclear resonance vibrational spectroscopy (NRVS) is not hampered by solvent interference and faces selection rule limitations only if the probe nucleus lies on a symmetry element. Here, we formulate a mathematical language appropriate for understanding NRVS measurements on molecular systems and apply it to analyse NRVS data recorded on ferrous nitrosyl tetraphenylporphyrin, Fe(TPP)(NO). This compound mimics the haem group found at the active site of many proteins involved in the biological usage of oxygen and nitric oxide. Measurements on such model compounds provide a baseline for evaluating the extent to which vibrations are localized at the active site of a protein, with the goal of elucidating the mechanisms of biological processes, such as intersite communication in allosteric proteins.