Microscopic charge fluctuations cause minimal contrast loss in cryoEM

Area of Science:

• Physics
• Materials Science
• Biophysics

Background:

• The "bee swarm" effect, or fluctuating granularity, in transmission electron microscopy is attributed to microscopic charge fluctuations induced by the electron beam.
• Concerns exist in high-resolution single particle electron cryomicroscopy (cryoEM) that these charge fluctuations could lead to defocus-dependent Thon ring fading, degrading image quality.

Purpose of the Study:

• To analyze the impact of charge fluctuations on image quality in cryoEM.
• To quantify the defocus-dependent blurring effect caused by the bee swarm effect on lattice fringes.

Main Methods:

• Analysis of 2.35 Å fringes from the (111) reflection of gold nanoparticles in amorphous ice.
• Comparison of fringe blurring in gold nanoparticles with that of a pure gold foil.

Main Results:

• A small, detectable defocus-dependent blurring of lattice fringes was observed in gold nanoparticles compared to pure gold foil.
• The transverse electric field from charge fluctuations on the specimen causes local electron beam deflection and image blurring.
• The effect reduces the amplitude of the 2.35 Å reflection by ~7% (intensity by 14%) at 10 µm defocus.

Conclusions:

• The bee swarm effect causes minor image degradation, reducing contrast by approximately 14% at 10 µm defocus.
• For typical cryoEM resolutions and defocus values (2-4 µm) using a field emission gun (FEG), the bee swarm effect and source incoherence are negligible.
• Specimen movement due to radiation damage remains the primary cause of contrast loss in cryoEM micrographs.