Electronic state of iron in enterobactin using Mössbauer spectroscopy

Abstract

Well-resolved paramagnetic hyperfine structure is observed in the Mössbauer spectra of 57Fe complexed by enterobactin, an iron transport compound found in many enteric bacteria. Spectra were observed under varied conditions of temperature (4.2–77 K) and applied magnetic field (0–6 kOe). The spectra for the high spin ferric ion (S=5/2) are well explained using a spin Hamiltonian with rhombic symmetry including a fourth order term in the electronic spin. This Hamiltonian has partially rhombic symmetry with the ratio E/D = 0.46. An isotropic ESR signal at g = 4.3 is predicted from the spin Hamiltonian if 4a = 3D (1-3 E/D), except when E≡D. A derivation of this relationship is given, which includes the special case of E = D/3, a = 0, previously used as the explanation for the so-called ’’rhombic’’ ESR signal at g = 4.3. The zero field splitting parameters are found to be D = 0.50 cm−1, λ = E/D = 0.46, and μ = a/D = −0.27. The magnetic hyperfine coupling constant is A = 1.51 mm/sec, the quadrupole coupling constant is e2qQ/4 = −0.15 mm/sec, η = 3λ = 1.38, and the isomer shift is +0.28 mm/sec.