Multi-frequency EPR spectroscopy of myoglobin. Spectral effects for high-spin iron(III) ion at high magnetic fields

Abstract

We report the result of a series of EPR experiments performed at microwave frequencies from 1 to 285 GHz on the S = 5/2 iron(III) ion in acid metmyoglobin. Two important phenomena occur upon increasing the frequency. Firstly, the effective g_⊥-value decreases notably as the Zeeman energy approaches the value of D, the zero-field splitting. Secondly, the linewidth of the spectra increases anomalously above 130 GHz. The dependence of observed linewidth on the microwave frequency shows three distinct phases. At low frequencies, broadening by unresolved hyperfine splittings is dominant. It is shown that from X-band frequencies onwards, a distribution in the value of g_⊥ and D causes the measured dependence of linewidth on frequency. The implications of this experiment for the study of randomly distributed spin systems at high frequencies are discussed.