Diffuse-Double Layer At A Membrane-Aqueous Interface Measured With X-Ray Standing Waves

SCIENCE, no. 4951 (1990): 52-56

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摘要

The ion distribution in an electrolyte solution in contact with a charged polymerized phospholipid membrane was directly measured with long-period x-ray standing waves. The 27-angstrom-thick lipid monolayer was supported on a tungsten/silicon mirror. X-ray standing waves were generated above the mirror surface by total external reflection...更多

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简介
  • The ion distribution in an electrolyte solution in contact with a charged polymerized phospholipid membrane was directly measured with long-period x-ray standing waves.
  • To show the versatility and power of this new method, the authors have chosen to measure the ion distribution profile above a charged phospholipid monolayer membrane that has been deposited onto the surface of an x-ray mirror
  • This ultrathin organic film is an important model system, since the biological membrane is in essence a bimolecular phospholipid leaflet, in and on which are situated a variety of proteins and other molecules.
  • Given the multifaceted effects of membrane surface charges, the need to
重点内容
  • The ion distribution in an electrolyte solution in contact with a charged polymerized phospholipid membrane was directly measured with long-period x-ray standing waves
  • X-rays, which can penetrate through millimeters ofwater, should, in principle, be ideally suited for solving this long-standing problem in electrochemistry
  • In situ surface extended x-ray absorption fine structure measurements [4] and conventional x-ray standing wave (XSW) measurements [5] have been successfully used in determining the bond length distance between a chemically adsorbed atom layer and the surface atom layer at the liquid-solid interface. These x-ray techniques, which can be used to measure the distance between atoms that are separated by a few angstroms, cannot be used to measure the ion distribution profile of the diffuse layer that extends tens of angstroms away from the interface
  • We demonstrate how a long-period x-ray standing wave, generated by total external reflection, can be used to directly measure the ion distribution profile in a solution layer above a mirror surface
  • To show the versatility and power of this new method, we have chosen to measure the ion distribution profile above a charged phospholipid monolayer membrane that has been deposited onto the surface of an x-ray mirror
  • We have demonstrated how long-period x-ray standing waves can be used to directly measure the ion distribution profile in an electrolyte solution in contact with a charged surface
结果
  • Using known bulk rate constants [16] for the formation of the lipid-zinc complex LZn'+ and the mass-action law, the authors calculated that the fractional condensation should be 0.46, that is, 46 percent of the phospholipid molecules (L'-) are bonded to a Zn2+ ion making an LZn'+ complex and the remaining 54 percent exist free as Ll-.
结论
  • The authors have demonstrated how long-period x-ray standing waves can be used to directly measure the ion distribution profile in an electrolyte solution in contact with a charged surface.
  • The measurements show a qualitative agreement with the Gouy-.
  • Chapman-Stem model which predicts that the charged surface can be partially neutralized by a condensed layer of counterions and that the ion distribution in the solution will form a diffuse layer with an exponential decay functional form
表格
  • Table1: X-ray standing wave measured values of the Zn2+ excess surface concentration, N,, and Debye length, L', for different pH levels. The listed values and standard deviations for Nc and L' were determined by a x2 fit of
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基金
  • Using known bulk rate constants [16] for the formation of the lipid-zinc complex LZn'+ and the mass-action law, we calculated that the fractional condensation should be 0.46, that is, 46 percent of the phospholipid molecules (L'-) are bonded to a Zn2+ ion making an LZn'+ complex and the remaining 54 percent exist free as Ll-
引用论文
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  • We thank the CHESS staff, J. S. Schildkraut, R. K. Smither, M. McMillan, H. D. Abrunia, and J. H. White for technical help, and B. W. Batterman, D. H. Bilderback, and K. Finkelstein for reading the manuscript. This experiment was made possible by a 1-month dedicated run at CESR. The undulator project was carried out jointly by CHESS and Argonne National Laboratory. We thank the accelerator staff at CESR for their effbrts in reconfiguring the storage ring to nmeet undulator requirements. Supported by the U.S. National Science Foundation under grants DMR-87-19764 and CHE-87-05769, by the U.S. National Institutes of Health under grant DK-36849, by the U. S. Department of Energy under contract W-31-109-ENG-38, and by the College ofAgriculture and Life Science at Comell. 26 January 1990; accepted 2 March 1990
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