Structural changes upon oxygenation of an iron(II)(porphyrinato)(imidazole) complex (Ibers 1978)

Porphyrins are very important structures found in hemes such as those seen in hemoglobin. Trying to model them can be quite difficult because the protein’s porphyrins are attached to have more function than you would think. When reading this dense but short paper which attempts to explore and model a “T” state of hemoglobin consider the difficulties of preforming this task.

This paper ties in many of the concepts we learned in class such as high spin/ low spin, coordination numbers, and molecular orbits. Be prepared to get picked on so brush up on that material.


6 Responses to “Structural changes upon oxygenation of an iron(II)(porphyrinato)(imidazole) complex (Ibers 1978)”

  1. Hunter Burgin Says:

    Carbon Monoxide is a well known competitive inhibitor for the heme active site that when in high concentrations binds to the Fe prosthetic group in place of oxygen. This paper explains the structural changes that occur within the iron heme complex upon oxidation. My question asks what structurally makes CO a competitive inhibitor to oxygen and why is it a more desirable bond for iron to make rather than oxygen?

  2. Josh Ellsworth Says:

    It was interesting to see that the porphyrin ring contracted when the Oxygen molecule coordinated to the complex. I hadn’t pictured the pi back-bonding occuring on an as needed basis, although it makes sense that as the Iron becomes more electron deficient it pulls more electron density from the ligands.

    I’m curious as to the role of the picket fence ligands. Why are they included at all if the main purpose of the study is to ascertain the buckling of the porphyrin ring? Also, when the authors mention that Oxygen binds better to the desolvated complex than in ethanol, are they attributing it to the lack of Hydrogen bonding between the ethanol and 2-methyl imidazole? What could be expected to happen in the presence of an aprotic solvent?

  3. Daniel Begay Says:

    In the last paragraph, they say that solid-gas measurements were done. Were you able to look up what this actually entailed? What kind of instrumentation was used to get these results? My question is how the solvate ethanol change the behaviors in its strength and affinities when present (or not present)? Is this something that was later investigated in depth later on?

  4. Kevin Greenwood Says:

    In the diagrams at the bottom of the page the iron is seen to be dipping below the plane of the aromatic system by about 0.4 angstroms. When oxygen binds, the iron crosses the plane and actually bows up a little bit for the less sterically hindered 1-methylimidazole ligand, while the sterics of 2-methylimidazole keep the iron just below the plane in the oxygen-bound state. I was kind of surprised that the metal center crossed the plane in the first example. How well does this model translate to biological systems? If iron is pulling harder on the ring nitrogens as a result of oxygen bonding, why does it bow out towards oxygen and not stop at being planar with the porphyrin ring?

  5. Porter Marsh Says:

    The diagrams at the bottom show mostly 2MIM and one 1MIM. In the diagrams with 2MIM, the porphyrin ring is above the iron while in the 1MIM diagram the iron is in the plane. When there isn’t a methyl group to push the porphyrin ring above the iron, the iron is in plane. Does causing a conformational change in the porphyrin ring act as an energy barrier to binding? I would imagine that the stability of the iron in plane with the porphyrin ring would act as an energy barrier to binding.

  6. Adam Settimo Says:

    I am confused about the top paragraph of the second column of the first page where there is talk about the “doming” of the porphyrinato skeleton being due to the 2-methylimidozole ligand that they both have. Is this steric interaction between the methyl group of 2-meIm and the bottoms of the benzene rings attaching the “picket fence post” to the porphyrin ring, due to eclipsed conformation?

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