jp407094u_si_001.pdf (260.61 kB)
Electronic and Nuclear Structural Snapshots in Ligand Dissociation and Recombination Processes of Iron Porphyrin in Solution: A Combined Optical/X-ray Approach
journal contribution
posted on 2013-11-14, 00:00 authored by Michael
W. Mara, Megan Shelby, Andrew Stickrath, Mike Harpham, Jier Huang, Xiaoyi Zhang, Brian M. Hoffman, Lin X. ChenThe
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.