title: Electronic and nuclear structural snapshots in ligand dissociation and recombination processes of iron porphyrin in solution: a combined optical/X-ray approach.
abstract: The photodissociation and recombination of CO and 1-methylimidazole (Im) from iron protoporphyrin IX (FePP-ImCO) dissolved in a 30% v/v aqueous solution of Im was studied using ultrafast optical transient absorption (TA) and X-ray transient absorption (XTA) spectroscopies. FePP-ImCO was shown to lose the CO ligand upon excitation at the Q bands, with 3.8 ps vibrational cooling and 21.6 ps intersystem crossing time constants derived from optical TA experiments, followed by ligation of a second Im on the nanosecond time scale. The penta-coordinate FePP-Im intermediate which forms following CO dissociation adopts a square pyramidal geometry with a "domed" iron center that is reminiscent of that formed upon loss of CO from carbonmonoxymyoglobin (MbCO). Unlike MbCO, which typically retains its newly generated penta-coordinated geometry until CO recombination, FePP can adopt a hexa-coordinate geometry by binding an additional Im ligand (FePP-(Im)2), allowing the porphyrin to exist in the low-spin electronic state even without the CO attached. The second Im ligand remains bound until CO recombination occurs with a time constant of 283 μs. The photodissociated states of FePP-ImCO and MbCO 100 ps after photoexcitation have similar iron site geometries, implying that the protein matrix in MbCO maintains minimum potential energy in the heme center despite the large-scale reorganization in the protein secondary and tertiary structure that arises from the dynamic active site/matrix interaction.
authors.affiliation: Department of Chemistry, Northwestern University , 2145 Sheridan Road, Evanston, Illinois 60208-3113, U.S.A.
authors.forename: Michael W
authors.initials: MW
authors.lastname: Mara
authors.forename: Megan
authors.initials: M
authors.lastname: Shelby
authors.forename: Andrew
authors.initials: A
authors.lastname: Stickrath
authors.forename: Mike
authors.initials: M
authors.lastname: Harpham
authors.forename: Jier
authors.initials: J
authors.lastname: Huang
authors.forename: Xiaoyi
authors.initials: X
authors.lastname: Zhang
authors.forename: Brian M
authors.initials: BM
authors.lastname: Hoffman
authors.forename: Lin X
authors.initials: LX
authors.lastname: Chen
mesh_terms: D002248:Carbon Monoxide / Q000737:chemistry; D004583:Electrons; D007093:Imidazoles / Q000737:chemistry; D007700:Kinetics; D008024:Ligands; D009211:Myoglobin / Q000737:chemistry / Q000378:metabolism; D010782:Photolysis; D011524:Protoporphyrins / Q000737:chemistry*; D012996:Solutions / Q000737:chemistry; D013056:Spectrophotometry, Ultraviolet; D014867:Water / Q000737:chemistry; D056928:X-Ray Absorption Spectroscopy
chemical_list: D007093:Imidazoles; D008024:Ligands; D009211:Myoglobin; D011524:Protoporphyrins; D012996:Solutions; C018013:carboxymyoglobin; C448299:iron protoporphyrin IX; D014867:Water; D002248:Carbon Monoxide; C018100:1-methylimidazole