Nuclear resonance vibrational spectroscopy of a protein active-site mimic

For many years, Mossbauer spectroscopy has been applied to measure recoille ss absorption of x-ray photons by nuclei. Recently, synchrotron radiation s ources have enabled the observation of weaker features separated from the r ecoilless resonance by the energy of vibrational quanta. This enables a for m of vibrational spectroscopy with a unique sensitivity to the probe nucleu s. Biological applications are particularly promising, because it is possib le to selectively probe vibrations of a single atom at the active site of a complex biomolecule, while avoiding interference from the vibrations of th ousands of other atoms. In contrast with traditional site-selective vibrati onal spectroscopies, nuclear resonance vibrational spectroscopy (NRVS) is n ot hampered by solvent interference and faces selection rule limitations on ly if the probe nucleus lies on a symmetry element. Here, we formulate a ma thematical language appropriate for understanding NRVS measurements on mole cular systems and apply it to analyse NRVS data recorded on ferrous nitrosy l tetraphenylporphyrin, Fe(TPP)(NO). This compound mimics the haem group fo und at the active site of many proteins involved in the biological usage of oxygen and nitric oxide. Measurements on such model compounds provide a ba seline for evaluating the extent to which vibrations are localized at the a ctive site of a protein, with the goal of elucidating the mechanisms of bio logical processes, such as intersite communication in allosteric proteins.