FERRIC ION COMPLEXES OF A DOPA-CONTAINING ADHESIVE PROTEIN FROM MYTILUS-EDULIS

Iron(III) binding to the DOPA-containing Mytilus edulis adhesive prote in (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 pur ple (lambda(max) = 548 nm) and one pink (lambda(max) = 500 nm). The pi nk form is favored at high DOPA:Fe ratios and the purple at low DOPA:F e ratios. Resonance Raman spectroscopy of both forms demonstrates that the chromophores are ferric catecholate complexes. EPR spectra of bot h forms of the protein measured at the same iron concentration reveal a g approximate to 4.3 resonance of approximately 4 times the intensit y 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) coo rdination of ferric ions with most of the iron(III) complexed as EPR-s ilent mu-oxo- or mu-hydroxo-bridged binuclear clusters is suggested. I n the pink form, in contrast, the ferric iron is EPR-active, mononucle ar, and present in high-spin tris(catecholato) complexes. The biologic al implications of these complexes are discussed.