title: Spectroscopic and theoretical studies of Ga(III)protoporphyrin-IX and its reactions with myoglobin.
abstract: Ga(III)protoporphyrin-IX (Ga-PP) has been proposed as a model for the key interporphyrin interactions in malaria pigment. Unlike the paramagnetic parent iron heme derivatives, Ga-PP is readily soluble in methanol (MeOH). We report optical, mass spectroscopic, and theoretical results for Ga-PP as well as its reactions with myoglobin. UV-visible absorption and MCD spectroscopy show that Ga-PP exhibits a typical spectrum for a main group metal: a Q-band at 539 nm and a B band at 406 nm when dissolved in MeOH. We also report optical data for Zn(II)protoporphyrin IX (Zn-PP) dissolved in MeOH, which exhibits a Q-band at 545 nm and a B band at 415 nm. ESI mass spectral data for Ga-PP dissolved in MeOH show the presence of predominantly monomers, with smaller fractions of dimers [(Ga-PP)(2)] and trimers. UV-visible and MCD absorption spectroscopy and ESI mass spectral data demonstrate the successful insertion of monomeric Ga-PP into apo-Mb. Ga-PP-Mb exhibits a B band at 417 nm and Q bands at 545 and 584 nm, which are all red-shifted from the free Ga-PP values. The calculated electronic structures and frontier molecular orbitals of Ga-PP, (Ga-PP)(2) and Zn-PP fit the previously reported trends in band energies and oscillator strengths as a function of molecular orbital energies. These new data can be applied to explain the experimentally observed optical spectroscopy. The observed Q-band energies are accounted for by calculated (HOMO-LUMO) gap of the frontier MOs, while the split in the two top occupied MOs accounts for the magnitude of the Q-band oscillator strength as well as the experimentally observed Q to B band energy separation. Although Ga-PP shares more spectroscopic properties with Zn-PP than it does with Fe(III)PPIX, the trivalent oxidation state allows this molecule to be used as a model for ferric hemes in heme proteins.
authors.affiliation: Department of Chemistry, The University of Western Ontario, London, Ontario, Canada.
authors.forename: Tyler B J
authors.initials: TB
authors.lastname: Pinter
authors.forename: Erin L
authors.initials: EL
authors.lastname: Dodd
authors.forename: D Scott
authors.initials: DS
authors.lastname: Bohle
authors.forename: Martin J
authors.initials: MJ
authors.lastname: Stillman
mesh_terms: D005708:Gallium / Q000737:chemistry*; D009211:Myoglobin / Q000737:chemistry*; D011524:Protoporphyrins / Q000737:chemistry*; D021241:Spectrometry, Mass, Electrospray Ionization; D013056:Spectrophotometry, Ultraviolet
chemical_list: D009211:Myoglobin; D011524:Protoporphyrins; C028025:protoporphyrin IX; D005708:Gallium