Electronic and Nuclear Structural Snapshots in Ligand Dissociation and Recombination Processes of Iron Porphyrin in Solution: A Combined Optical/X-ray Approach

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• 作者：Michael W. Mara ; Megan Shelby ; Andrew Stickrath ; Mike Harpham ; Jier Huang ; Xiaoyi Zhang ; Brian M. Hoffman ; Lin X. Chen
• 刊名：Journal of Physical Chemistry B
• 出版年：2013
• 出版时间：November 14, 2013
• 年：2013
• 卷：117
• 期：45
• 页码：14089-14098
• 全文大小：418K
• 年卷期：v.117,no.45(November 14, 2013)
• ISSN：1520-5207

文摘

The photodissociation and recombination of CO and 1-methylimidazole (Im) from iron protoporphyrin IX (FePP鈥揑mCO) 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鈥揑mCO 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鈥揑m intermediate which forms following CO dissociation adopts a square pyramidal geometry with a 鈥渄omed鈥?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鈥?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 渭s. The photodissociated states of FePP鈥揑mCO 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.