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Experimental Methods for Trapping Ions Using Microfabricated Surface Ion Traps
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Identification of ions in experimental electrostatic potential maps.

Jimin Wang1, Zheng Liu2, Joachim Frank2,3

  • 1Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, CT 06520-8114, USA.

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Summary

Cryo-electron microscopy (cryo-EM) reveals atomic charge information by imaging electrostatic potential. This technique can distinguish between cations and anions in macromolecules, aiding in charge determination.

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Mott equationcryo-EMelectron-scattering lengthribosometransition B factor

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Area of Science:

  • Biophysics
  • Structural Biology
  • Biochemistry

Background:

  • X-ray crystallography provides limited atomic charge information.
  • Cryo-electron microscopy (cryo-EM) directly images electrostatic potential (ESP) in macromolecules.
  • Differences in electron scattering between ions and neutral atoms impact cryo-EM data.

Purpose of the Study:

  • To leverage cryo-EM data for determining the sign and magnitude of ion charges within macromolecules.
  • To analyze how resolution and computational smearing affect the visibility of charged species in cryo-EM maps.
  • To investigate the ESP variations of ions in isolation versus within ionic pairs.

Main Methods:

  • Analysis of cryo-electron microscopy (cryo-EM) maps at varying resolutions.
  • Computational simulation of map smearing to assess resolution effects.
  • Comparison of electrostatic potential (ESP) values for isolated ions and ions within macromolecular structures.

Main Results:

  • Anion peaks in cryo-EM maps are less prominent than cation peaks, especially at resolutions below 2.5 Å.
  • Computational map smearing diminishes anion peak size while increasing cation peak size.
  • Observed ESP values for Mg2+ relative to C1' atoms ranged from 0.57 to 1.27.

Conclusions:

  • Cryo-EM can differentiate and quantify ion charges in macromolecules based on peak characteristics in electron potential maps.
  • Resolution-dependent peak behavior in cryo-EM provides a method for identifying and estimating ion charges.
  • Understanding ESP variations is crucial for accurate structural interpretation of charged species in cryo-EM studies.