title: Axial coordination of ferric Aplysia myoglobin.
abstract: Resonance Raman spectra of ferric Aplysia myoglobin in the ligand-free and the azide-bound forms have been studied over a wide pH range to determine the coordination states of the heme iron atom. In the hydroxide form at high pH (approximately 9) the iron is six-coordinate and is in a high/low spin equilibrium. As the pH is lowered below the acid/alkaline transition (pKa = 7.5), the heme becomes five-coordinate. When the pH is lowered even further no other changes in the resonance Raman spectrum are detected; thus, the heme remains five-coordinate down to pH 4, the lowest value studied. For ferric azide-bound Aplysia myoglobin, the iron is six-coordinate in a high/low spin equilibrium at all pH values (4.8-9). These data indicate (i) that the unusual reactivity toward azide previously observed at neutral pH is indeed related to the absence of a coordinated water molecule, and (ii) that causes other than the heme coordination are responsible for the spectral differences and the ligand-binding kinetics differences observed below pH 6.
authors.affiliation: AT&T Bell Laboratories, Murray Hill, New Jersey 07974.
authors.forename: D L
authors.initials: DL
authors.lastname: Rousseau
authors.forename: Y C
authors.initials: YC
authors.lastname: Ching
authors.forename: M
authors.initials: M
authors.lastname: Brunori
authors.forename: G M
authors.initials: GM
authors.lastname: Giacometti
mesh_terms: D000818:Animals; D001048:Aplysia / Q000032:analysis*; D001386:Azides / Q000378:metabolism; D006736:Horses; D006863:Hydrogen-Ion Concentration; D007501:Iron / Q000378:metabolism*; D009211:Myoglobin / Q000378:metabolism*; D013053:Spectrophotometry; D013059:Spectrum Analysis, Raman
chemical_list: D001386:Azides; D009211:Myoglobin; D007501:Iron