Iron porphyrins reinvistigated by 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. Mossbauer 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 Fe02(SC6HF4)(TPpivP) and of the paramagnetic porphyrin [Fe(CH3COO)(TPpiVP)]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. INTRODUCTION Nuclear resonant scattering techniques (l) , i. e. nuclear forward scattering (hW) (2-5) and nuclear inelastic scattering (NIS) (64, have become available as spectroscopic methods with the 2nd and 3'd generation sources of synchrotron radiation. In the present contribution we focus on NFS, which represents Mossbauer spectroscopy in the time domain, and which complements the conventional nuclear resonance absorption, e.g.-Mossbauer spectroscopy in the energy domain. NFS makes use of the specific properties of synchrotron radiation, e.g. high brilliance (large flux of light within a narrow bandwidth of energy), high degree of polarization, timed structure of radiation (narrow and equidistant pulses of light) and high degree of beam collimation (1 mm2 at the 3'd generation machines). N F S is used to study hyperfine interactions and, via effective sample thickness, also molecular dynamics. Here we present first biophysical applications, i. e. on the "picketfence" porphyrins FeO2(SC6HF4)(TPpi,P) and [Fe(CH3COO)(TPpivP)]~. With respect to NIS, which we have employed to study molecular vibrations in an iron(Q spin-crossover complex, we refer to separate contributions which we present at this conference (9) and at the Intemational Conference on the Applications of the Mossbauer Effect (ICAME 97) (10). EXPERIMENTAL The time dependence of NFS intensity was obtained with the nuclear resonance station at beamline RW4 (1 1) of HASYLAB, DESY in Hamburg, Germany (Fig. 1). The beam of white X-rays was monochrornatized by a double-crystal Si( 11 1) premonochromator to a bandwidth of about 2.5 eV at.tbe resonance ener-