Moessbauer and EPR Studies of the Binuclear and Trinuclear Antiferromagnetically Coupled Iron(III)-Binding Sites in Ferreascidin

Previous studies of the interaction of iron(III) with ferreascidin, a glycoprotein isolated from the blood cells of the stolidobranch ascidian Pyura stolonifera, have shown that the iron’s coordination sphere involves two (3,4-dihydroxyphenyl)alanine (DOPA) residues and possibly one tyrosine (Taylor, S. W.; Hawkins, C. j.; Winzor, D. j. Inorg. Chem. 1993, 32, 422). Herein we report variable-field (0–11.1 T) low-temperature Mossbauer spectra which reveal the existence of at least three distinct iron(iii)-binding sites. In conjunction with electron paramagnetic resonance spectroscopy of ferreascidin and model complexes, site 1 is found to be a strongly antiferromagnetically coupled (|J| > 100 cm) binuclear iron(III) center which may possess a μ-oxo bridging group, while site 2 is also an antiferromagnetically coupled (∼20 J producing a paramagnetic S = 5/2 ground state. The absence of sulfur and sulfide in ferreascidin indicates that the bridging atoms in these three clusters (sites) must be oxygen and/or nitrogen atoms. Site 3 represents the first example of a trinuclear iron(III) cluster with a paramagnetic S =5/2 ground state to be found in metalloproteins. Structures for the metal ion binding sites are proposed.