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Related Experiment Videos

Two-dimensional protonic percolation on lightly hydrated purple membrane.

J A Rupley1, L Siemankowski, G Careri

  • 1University Department of Biochemistry, Biological Sciences West, University of Arizona, Tucson, AZ 85721.

Proceedings of the National Academy of Sciences of the United States of America
|December 1, 1988
PubMed
Summary

Protonic conduction in purple membranes shows a sharp increase at a specific hydration level. This ion transport mechanism, similar to lysozyme, aligns with a two-dimensional percolation model.

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Area of Science:

  • Biophysics
  • Membrane Biophysics
  • Ion Transport

Background:

  • Purple membrane from Halobacterium halobium is a biological system with potential for ion transport.
  • Understanding the mechanisms of proton conduction in biological membranes is crucial for various applications.
  • Previous studies on lysozyme suggested a percolation model for protonic processes.

Purpose of the Study:

  • To investigate the electrical properties of purple membrane as a function of hydration and frequency.
  • To determine the mechanism of protonic conduction in purple membrane.
  • To compare the conduction properties with those of lysozyme and evaluate the applicability of a percolation model.

Main Methods:

  • Capacitance and dielectric loss factor measurements were performed on purple membrane samples.

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  • Measurements were conducted across a range of hydration levels (0.017 to >0.2 g water/g membrane).
  • Frequency-dependent measurements were carried out from 10 kHz to 10 MHz, including deuterium isotope effect analysis.

    • Main Results:

    • Capacitance and conductivity exhibited a significant increase above a threshold hydration level (h(c) ≈ 0.0456).
    • A deuterium isotope effect (H/(2)H = 1.38) was observed, supporting a protonic conduction mechanism.
    • The critical exponent (t = 1.23) aligns with a two-dimensional percolation process, while h(c) showed no deuterium isotope effect or frequency dependence.

    Conclusions:

    • Protonic conduction in purple membrane can be explained by a percolation model, similar to lysozyme.
    • The results suggest that ion transport in membranes may not rely solely on fixed structures like proton wires.
    • Preferred regions for proton conduction exist, and the dielectric properties are more complex than those of lysozyme.