Multi-Frequency High-Field EPR Study of Iron Centers in Malarial Pigments

Abstract

The multi-frequency high-field electron paramagnetic resonance (HFEPR) was used to study the magnetic properties of malarial pigment hemozoin and its synthetic analogue, β-hematin. (FeIII−protoporphyrin−IX)2 dimers containing five-coordinate high-spin FeIII, S = 5/2, are the building blocks of these pigments. The fit of EPR spectra that were acquired in an unprecedented wide range of microwave frequencies of 34 and 94 GHz for hemozoin and 27−500 GHz for β-hematin yielded a complete set of intrinsic spin Hamiltonian parameters: D = +5.85(1) cm-1, E = 0, g ⊥ = 1.95(1), g ∥ = 2.00(1). These results point to the existence of largely axial symmetry of the iron environment in the bulk phase of hemozoin and β-hematin.

A perspective on applications of ligand-field analysis: inspiration from electron paramagnetic resonance spectroscopy of coordination complexes of transition metal ions

“…The final system to be mentioned briefly is HS d 5 92 Ligand-field analyses were not performed in these studies, which focused on the detailed EPR experiments. Fe III is of the greatest interest, but in porphyrinic complexes often exhibits large ZFS, due to the highly tetragonally distorted geometry.…”

“…An EPR transition near zero-field directly yielded 2|D| = 11.7 cm -1 . 92 Note also the use of ENDOR spectroscopy to determine ZFS in HS Fe III heme proteins, 75 as mentioned above.…”

“…Thus these parameters are difficult to correlate with coordination environment. 91 The situation today is conducive to more work of this nature given the number of interesting HS d 5 metalloprotein systems (e.g., SOD, and the plethora of heme proteins), the availability of techniques such as HFEPR to provide accurate and precise spin Hamiltonian parameters, even for large ZFS, 92 and software (e.g., Ligfield) for the necessary analysis.…”

“…As an sample calculation, we can employ the crystalfield parameters estimated for a porphyrin (Dq = 2500, Ds = 2500, Dt = 2000 cm -1 ) combined with Racah and spin-orbit coupling parameters similar to those given by Bencini et al 97 (B = 900, C = 3600, ζ = 350 cm ), 92 but the parameters used here are a very crude estimate. In contrast to the cases of rhombic S = 1, 3/2, 2 (equations 5, 7, 11, respectively), the intrinsic g values are not easily calculated for a rhombic S = 5/2 system, as the eigenvalues even of the zero-field spin Hamiltonian are the very unwieldy roots of cubic equations.…”

Biomimetic Approaches to Understanding the Mechanism of Haemozoin Formation

“…The determination of the relationship between the external macroscopic morphology of the crystal (crystal habit) and the unit cell structure by Buller et al (2002) (Figure 3) has been important in later work that has attempted to understand the molecular mechanism of haemozoin formation. Magnetic properties have been investigated using X-band electron paramagnetic resonance (EPR) spectroscopy and magnetic Mössbauer spectroscopy and have unequivocally demonstrated that the Fe(III) centre exists in a high spin S = 5 / 2 state (Bohle et al 1998) with very weak to negligible magnetic exchange between Fe(III) centres later confirmed by multifrequency high field EPR (Sienkiewicz et al 2006). The uv-visible spectrum of -haematin has also been determined and its luminescence properties investigated (Bellemare et al, 2009).…”