- Resolution (electron density)
Resolution in terms of electron density is a measure of the resolvability in the electron density map of a molecule. In
X-ray crystallography, resolution is the highest resolvable peak in the diffraction pattern. While cryo-electron microscopyis a frequency space comparison of two halves of the data, which strives to correlate with the X-ray definition. Frank, 2006, p250-251]
structural biology, resolution can be broken down into 4 groups:
* sub-atomic, individual elements are distinguishable and quantum effects can be studied
* atomic, individual atoms are visible and an accurate three-dimensional model can be construction
secondary structure, such as alpha helicesand beta sheets
* domain, no secondary structure is resolvable
As the crystal's repeating unit, its
unit cell, becomes larger and more complex, the atomic-level picture provided by X-ray crystallography becomes less well-resolved (more "fuzzy") for a given number of observed reflections. Two limiting cases of X-ray crystallography are often discerned, "small-molecule" and "macromolecular" crystallography. "Small-molecule crystallography" typically involves crystals with fewer than 100 atoms in their asymmetric unit; such crystal structures are usually so well resolved that its atoms can be discerned as isolated "blobs" of electron density. By contrast, "macromolecular crystallography" often involves tens of thousands of atoms in the unit cell. Such crystal structures are generally less well-resolved (more "smeared out"); the atoms and chemical bonds appear as tubes of electron density, rather than as isolated atoms. In general, small molecules are also easier to crystallize than macromolecules; however, X-ray crystallography has proven possible even for viruses with hundreds of thousands of atoms.
cryo-electron microscopy, resolution is typically measured by the Fourier shell correlation (FSC)Harauz & van Heel, 1986] , a three-dimensional extension of the Fourier ring correlation (FRC)van Heel, 1982] Saxton & Baumeister, 1982] . The FSC is a comparison of two different Fourier transforms over different shells on frequency space. To measure the FSC, the data needs to be separated into two groups. Typically, the even particles form the first group and odd particles the second based on their order. This is commonly referred to as the even-odd test. Most publications quote the FSC 0.5 cutoff, which refers to the when the correlation coefficient of the Fourier shells is equal to 0.5Böttcher et al., 1997] .
Determining the resolution remains a controversial topic. Many other criteria using the FSC curve exist, including 3-σ criterion, 5-σ criterion, and the 0.143 cutoff. In 2007, a resolution criterion independent of the FSC was developed using the correlation between neighboring Fourier to distinguish signal from noise.Sousa & Grigoreiff, 2007]
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