Iron porphyrins reinvestigated by a new method: Mössbauer spectroscopy using synchrotron radiation

Nuclear resonant forward scattering (NFS) of synchrotron radiation represents Mossbauer spectroscopy in the time domain. This new technique complements the conventional nuclear resonance absorption, e.g. Mössbauer spectroscopy in the energy domain, by supplying highly brilliant, polarized, collimated and timed radiation. In NFS the hyperfine interaction of coherently excited nuclei manifests itself as quantum beats, i.e. as modulation of the time-dependent intensity of the transmitted radiation, which is delayed with respect to the incoming synchrotron pulse. We have investigated ⁵⁷Fe-enriched iron porphyrins to test NFS for first biophysical applications. NFS spectra of the diamagnetic porphyrin FeO₂(SC₆HF₄)(TP_pivP) and of the paramagnetic porphyrin [Fe(CH₃COO)(TP_pivP)]^- were recorded at various temperatures, with and without reference scatterer, with and without applied field. Dynamic molecular properties, e.g. dynamic structural disorder or spin-lattice relaxation document as variation of the time-delayed count rate. Measured NFS spectra were analysed theoretically by programs which are basically the analogue in the time domain compared to the usual calculations in the energy domain.