A combined X-band and frequency-domain Fourier-transform THz electron
paramagnetic resonance (FD-FT THz-EPR) approach has been employed to determine
heme Fe(III) S = 5/2 zero-field splitting (ZFS) parameters of frozen metHb and
metMb solutions, both with fluoro and aquo ligands. Frequency-domain EPR
measurements have been carried out by an improved synchrotron-based FD-FT THz-
EPR spectrometer. ZFS has been determined by field dependence of spin
transitions within the mS = ±1/2 manifold, for all four protein systems, and
by zero-field spin transitions between mS = ±1/2 and mS = ±3/2 levels, for
metHb and metMb flouro-states. FD-FT THz-EPR data were simulated with a novel
numerical routine based on Easyspin, which allows now for direct comparison of
EPR spectra in field and frequency domain. We found purely axial ZFSs of D =
5.0(1) cm−1 (flouro-metMb), D = 9.2(4) cm−1 (aquo-metMb), D = 5.1(1) cm−1
(flouro-metHB) and D = 10.4(2) cm−1 (aquo-metHb)
