Pair Distribution Function from Electron Diffraction in Cryogenic Electron Microscopy: Revealing Glassy Water...

Area of Science:

• Structural biology
• Materials science
• Biophysics

Background:

• Cryogenic electron microscopy (Cryo-EM) is a powerful technique for determining the structure of biological macromolecules in near-native, hydrated environments.
• The structure of amorphous ice, crucial for cryo-EM sample preparation, has been previously studied using neutron and X-ray diffraction on larger volumes.
• Understanding the local structure of glassy water under cryo-EM conditions is essential for accurate macromolecular structure determination.

Purpose of the Study:

• To investigate the pair-distribution function, g(r), of glassy water specifically under cryo-electron microscopy (Cryo-EM) conditions.
• To compare the structural properties of amorphous ice under cryo-EM conditions with known states like low-density amorphous ice and supercooled water.
• To explore potential structural changes, such as ice nucleation, induced by electron exposure in cryo-EM samples.

Main Methods:

• Utilized electron diffraction data obtained from glassy water samples prepared under cryo-EM conditions.
• Analyzed the pair-distribution function, g(r), to characterize the local atomic/molecular structure of the amorphous ice.
• Observed and analyzed the effects of electron exposure on the glassy water samples.

Main Results:

• The pair-distribution function, g(r), of glassy water under cryo-EM conditions was determined for the first time.
• The determined g(r) values were found to lie between those of low-density amorphous ice and supercooled water.
• Thicker glassy water samples exhibited nucleation of cubic ice regions upon exposure to the electron beam.

Conclusions:

• This study provides the first quantitative structural information of glassy water using the pair-distribution function, g(r), directly from cryo-EM data.
• The findings offer a more precise understanding of the amorphous ice environment relevant to cryo-EM.
• The observed ice nucleation highlights the importance of sample thickness and electron dose in cryo-EM studies.