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Related Experiment Videos

Why is gramicidin valence selective? A theoretical study.

S S Sung, P C Jordan

    Biophysical Journal
    |April 1, 1987
    PubMed
    Summary

    Cesium and chloride ion solvation energies in gramicidin-like channels reveal distinct profiles. Chloride ions face an energy barrier at the channel entrance, explaining their lack of permeation or channel blockage.

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    Area of Science:

    • Biophysics
    • Computational Chemistry
    • Ion Channel Function

    Background:

    • Gramicidin channels are crucial for ion transport across lipid bilayers.
    • Understanding ion solvation is key to predicting channel permeation and selectivity.
    • Cesium and chloride ions serve as model cations and anions for transport studies.

    Purpose of the Study:

    • To computationally investigate and contrast the channel solvation energy for cesium (cation) and chloride (anion) ions within gramicidin-like channels.
    • To elucidate the energetic determinants of ion entry and stability within the channel.
    • To correlate calculated solvation energies with observed ion permeation and blockage phenomena.

    Main Methods:

    • Molecular dynamics simulations and free energy calculations were employed.
    • The solvation energy profiles for cesium and chloride ions were computed along the channel axis.
    • Energy barriers and wells at the channel entrance and within the single-file region were analyzed.

    Main Results:

    • Cesium ions displayed a deep well at the channel entrance and relatively constant solvation energy within the channel.
    • Chloride ions encountered a significant energy barrier at the channel entrance, with stability predicted if this barrier were overcome.
    • A solvation energy difference of approximately 15 kcal mol-1 was calculated between anions and cations at the channel entrance.

    Conclusions:

    • The calculated energy profiles are consistent with experimental observations of chloride ions neither permeating nor blocking gramicidin channels.
    • The high energy barrier for chloride entry significantly restricts its transport rate, estimated to be far slower than channel dissociation.
    • Solvation energy calculations provide critical insights into the molecular mechanisms governing ion selectivity and transport in biological channels.

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