Mössbauer spectroscopy of mesohaem and protohaem myoglobins and their fluoride complexes

Abstract

The Mössbauer spectra of 57Fe-enriched ferric myoglobins, pH 6, containing both protohaem and mesohaem prosthetic groups, have been measured under a variety of conditions of temperature and field. In applied fields at 4.2°K well-resolved paramagnetic hyperfine spectra are observed. These are consistent with a high-spin model with axial distortion. In small fields internal fields of order 300 kgauss are observed, corresponding to saturation fields of 493 ± 4 and 498 ± 4 kgauss for meso and proto forms, respectively. A quadrupole splitting of 0.65 ± 0.05 mm/sec is observed in each implying that the splitting in absence of magnetic interaction should be 1.3 mm/sec. This is in good agreement with the value of quadrupole splitting observed at 195°K where relaxation effects suppress magnetic interaction. The field dependence of the low temperature spectra are consistent with an axial splitting, 2 D, of 20 ± 2 cm −1, in agreement with susceptibility results. The fluoride complexes of these proteins are also high spin and have similar low temperature spectra. The saturation fields are 525 ± 3 kgauss, 2 D is 12.6 ± 1 cm −1, and the quadrupole splitting is 0.40 ± 0.05 m/sec, implying a splitting of 0.8 mm/sec in the absence of magnetic interaction. At 195°K the spectrum is unresolved as a result of residual paramagnetic effects. Again, measurements suggest that the internal field of proto form may be a few tenths of a percent higher than that of the meso form.