Cryo-Electron Microscopy (Cryo-EM): Principles, Data Collection, and Reconstruction

Comprehensive vocabulary flashcards covering the instrumentation, physical principles, sample preparation, image formation, CTF theory, and 3D reconstruction methodologies of cryo-electron microscopy.

Cryo-Electron Microscopy (cryo-EM)

A form of electron microscopy where biological specimens are examined in their native state at cryogenic temperatures without fixation or staining, characterized by radiation sensitivity, high resolution, and low-contrast raw images.

Negative Staining

An electron microscopy method in which heavy metal salts surround a specimen to create a cast or outline of the molecule of interest.

Field Emission Gun (FEG)

An electron source utilizing a tungsten tip coated with zirconium oxide ($\text{ZrO}_2$), which lowers the work function to maximize electron emission and ensure long-term stability.

Thermal Surface Diffusion

The mechanism by which fresh zirconium and oxygen continuously creep down the tungsten tip barrel toward the apex to replenish evaporated atoms in an FEG.

Liquid Ethane

A cryogen used for rapid plunge-freezing of aqueous macromolecular samples to achieve vitrification without forming crystalline ice.

Cryo-EM Grid

A circular mesh support (typically copper) coated with a holey carbon film (such as $\sim 1.2\,\mu\text{m}$ holes with $\sim 1.3\,\mu\text{m}$ spacing) onto which aqueous macromolecular samples are applied and vitrified.

Single Particle Analysis

A cryo-EM approach applied to individual protein complexes (typically $> 50\,\text{kDa}$) trapped in random orientations within vitrified ice, requiring no crystallization and achieving resolutions routinely around $2\text{--}3\,\text{Å}$.

2D Crystals (Cryo-EM)

Ordered, planar crystalline arrays of proteins that yield diffraction amplitudes and require data collection at multiple tilt angles, achieving resolutions around $3\,\text{Å}$.

Tubular Crystals (Cryo-EM)

Helical or cylindrical crystalline sample arrangements (such as acetylcholine receptor or $\text{Ca}^{2+}\text{-ATPase}$) that do not yield diffraction but provide isotropic projection data, resolving to approximately $5\,\text{Å}$.

Radiolysis Damage

Radiation-induced structural degradation of protein specimens during electron imaging, characterized by blurring of features and the formation/merging of hydrogen gas bubbles with cumulative dose.

Low-Dose Microscopy

A photographic strategy where setup operations such as focusing and astigmatism corrections are performed on sacrificial areas adjacent to the target area of interest ($X$) to minimize radiation damage.

Search Mode (Cryo-EM)

A low-magnification operation mode ($500\text{--}100\times$) used to locate suitable areas on a grid while applying minimal electron exposure.

Complementary Metal Oxide Semiconductor (CMOS)

A direct electron detector featuring fast acquisition speeds ($30\text{--}120\,\text{fps}$) and physical pixels significantly larger than smartphone camera pixels to maximize electron capture and minimize noise.

Contrast Transfer Function (CTF)

A mathematical function in reciprocal space describing the spatial frequency-dependent alterations, phase reversals, and contrast loss caused by electron microscope optical aberrations.

Thon Rings

Concentric oscillations of alternating bright and dark rings visible in the power spectrum (Fourier transform) of an electron micrograph, produced by the Contrast Transfer Function.

Astigmatism (Cryo-EM)

An optical aberration resulting from asymmetric focal power in orthogonal planes of an objective lens, evidenced by elliptical rather than circular Thon rings in the Fourier power spectrum.

Defocus (Cryo-EM)

The controlled underfocusing of the objective lens (e.g., $-1.5\,\mu\text{m}$ to $-3.0\,\mu\text{m}$) to enhance phase contrast at low spatial frequencies at the expense of introducing more frequent CTF oscillations.

Envelope Function

A damping profile that attenuates the amplitudes of the Contrast Transfer Function at increasing spatial frequencies due to chromatic aberration, beam divergence, and specimen motion.

Projection Theorem

A fundamental theorem stating that the 2D Fourier transform of a projection of a 3D density equals a central section of the 3D Fourier transform of that density, oriented perpendicular to the projection direction.

Cross-Common Lines

An unsupervised orientation determination method based on the principle that any two central slices in 3D Fourier space intersect along a line, defining their relative angular orientations.

Supervised Classification

A sorting method where experimental single-particle images are categorized according to their correlation with a predefined template or reference model.

Unsupervised Classification

A reference-free sorting technique that groups single-particle images into classes based strictly on intrinsic statistical similarities between the images.

Model Bias

An artifact in single-particle reconstruction in which pure noise is aligned to match a reference model, erroneously reproducing features of the reference in the final 3D map.

Sharpening (B-factor Correction)

A post-processing procedure that restores high-resolution detail in a cryo-EM density map by boosting the dampened amplitudes of high-frequency Fourier coefficients while preserving phase values.

Fourier Shell Correlation (FSC)

A resolution assessment technique calculating the cross-correlation coefficient between Fourier coefficients of two independently reconstructed half-datasets over concentric shells in reciprocal space.

Coot

A macromolecular graphics application used for building, fitting, and manually inspecting atomic coordinate models within electron density and cryo-EM potential maps.

Phenix

A software package used for automated map sharpening, real-space coordinate refinement, density modification, and validation of macromolecular cryo-EM and crystallographic models.