Ferric Ion Complexes of a DOPA-Containing Adhesive Protein from Mytilus edulis

Iron(III) binding to the DOPA-containing Mytilus edulis adhesive protein (Mefp1) has been studied by spectrophotometric titrations, electron paramagnetic resonance (EPR), and resonance Raman spectroscopies. At pH 7.0, two different forms of the iron−protein complex exist: one purple (λmax = 548 nm) and one pink (λmax = 500 nm). The pink form is favored at high DOPA:Fe ratios and the purple at low DOPA:Fe ratios. Resonance Raman spectroscopy of both forms demonstrates that the chromophores are ferric catecholate complexes. EPR spectra of both forms of the protein measured at the same iron concentration reveal a g ≈ 4.3 resonance of approximately 4 times the intensity in the spectrum of the pink complex compared with that of the purple form. On the basis of the collective evidence obtained here, a model for the purple form of the ferric Mefp1 involving bis(catecholato) coordination of ferric ions with most of the iron(III) complexed as EPR-silent μ-oxo- or μ-hydroxo-bridged binuclear clusters is suggested. In the pink form, in contrast, the ferric iron is EPR-active, mononuclear, and present in high-spin tris(catecholato) complexes. The biological implications of these complexes are discussed.