Chemistry Labs

Biochemistry and biomedical chemistry

Cryo-EM, protein crystallography

Structural biologists infer molecular architecture from scattered electrons or X-ray diffraction, with distinct sample preparation and data-processing limits.

1 Lower secondary2 Upper secondary3 Undergraduate4 Advanced5 Research

IntuitionIntuition: the central idea

Structural biologists infer molecular architecture from scattered electrons or X-ray diffraction, with distinct sample preparation and data-processing limits.

Explore the schematic model; it illustrates a concept rather than replacing experimental evidence.

SchoolSchool level: key concepts and a first application

Definition: Core concept

Cryo-EM averages many particle images and assesses map agreement, commonly using Fourier shell correlation. Crystallography derives electron density from diffraction amplitudes and phases.

Cryo-EM images flash-frozen particles in vitreous ice; X-ray crystallography needs ordered crystals. Each method depends on different physics and validation statistics.

Key terms and meaning
TermInterpretation
FSCFourier shell correlation between half-datasets in cryo-EM.
CC½Correlation between half-datasets in crystallographic data processing.
Electron densityThe reconstructed distribution into which a structural model is built.

Example: Apply the idea

Why should a reported resolution number not be interpreted as a uniform positional accuracy for every atom?

Solution

Resolution summarizes information at a scale; local disorder, occupancy, model restraints and map quality vary across a structure, so local confidence differs.

UndergraduateUniversity level: quantitative description

FSC(s)=∑F1(s)F2∗(s)∑∣F1(s)∣2∑∣F2(s)∣2\mathrm{FSC}(s)=\frac{\sum F_1(s)F_2^*(s)}{\sqrt{\sum|F_1(s)|^2\sum|F_2(s)|^2}}

Resolution estimates are derived from correlation thresholds. They are sensitive to model bias, sharpening, masking and how the dataset is split; always interpret them with map quality.

AdvancedAdvanced: assumptions and mechanistic detail

Cryo-EM can resolve flexible regions and heterogeneous states; crystallography can yield very high resolution but needs ordered crystals. Time-resolved and in-cell approaches are extending both.

ResearchResearch frontier: open questions and current practice

References