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Related Experiment Videos

Carbon monoxide binding kinetics in "capped" porphyrin compounds

E J Rose, P N Venkatasubramanian, J C Swartz

    Proceedings of the National Academy of Sciences of the United States of America
    |September 1, 1982
    PubMed
    Summary

    Capped porphyrins show no steric hindrance to carbon monoxide (CO) binding, with rates comparable to high-affinity hemoglobin. This suggests their potential use in studying heme protein interactions.

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    Area of Science:

    • Coordination Chemistry
    • Biophysical Chemistry
    • Chemical Kinetics

    Background:

    • Heme proteins, crucial for biological oxygen transport and metabolism, feature a ferrous iron (Fe(II)) center within a porphyrin ring.
    • Carbon monoxide (CO) is a known ligand that binds to the heme iron, and its binding kinetics are vital for understanding protein function and dysfunction.
    • Steric hindrance around the heme pocket can significantly modulate ligand binding affinity and kinetics in hemoproteins.

    Purpose of the Study:

    • To investigate the effect of steric hindrance on carbon monoxide (CO) binding kinetics to novel capped porphyrin complexes.
    • To compare the CO binding rate constants of these synthetic porphyrins with those of natural hemoproteins.
    • To elucidate the kinetic mechanism governing CO binding to these specifically designed porphyrin systems.

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    Main Methods:

    • Synthesis and characterization of two novel capped porphyrin complexes: 5,10,15,20-[pyromellitoyl(tetrakis-o-oxyoxyphenyl)]porphyrin and 5,10,15,20-[pyromellitoyl(tetrakis-o-oxypropoxyphenyl)]porphyrin.
    • Kinetic measurements of carbon monoxide (CO) binding to the five-coordinate ferrous iron complexes using stopped-flow spectrophotometry.
    • Analysis of kinetic data to determine second-order rate constants and propose a reaction mechanism.

    Main Results:

    • The measured second-order rate constants for CO binding to both capped porphyrins were found to be independent of the 'cap' size.
    • These rate constants (k5 approximately 4 X 10(6) M-1s-1) are comparable to those observed for the high-affinity state of hemoglobin.
    • The findings indicate that the employed capped porphyrin structures do not impose significant steric hindrance on CO binding.

    Conclusions:

    • The studied capped porphyrins serve as effective models for investigating heme-ligand interactions without introducing steric impediments.
    • The lack of steric hindrance suggests these compounds can be valuable tools for studying the intrinsic kinetics of CO binding to heme centers.
    • A kinetic model involving an unusual seven-coordinate porphyrin intermediate was proposed to explain the observed binding behavior.