Resolution: Difference between revisions

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''Resolution'', in structure determinations, is the distance corresponding to the smallest observable feature: if two objects are closer than this distance, they appear as one combined blob rather than two separate objects. Resolution is the smallest distance between crystal lattice planes that is resolved in the diffraction pattern. High numeric values of resolution, such as 4 Å, mean poor resolution, while low numeric values, such as 1.5 Å, mean good resolution.

'''2.05 Å''' is the '''median''' resolution for [[X-ray crystallography|X-ray crystallographic]] results in the Protein Data Bank (88,701 on May 15, 2014). For comparison, the van der Waals diameter of a carbon atom is 3.4-3.7 &Aring;<ref name="atomsize">See [http://en.wikipedia.org/wiki/Atomic_radii_of_the_elements_(data_page) Atomic radii of the elements (Wikipedia)], and [http://chemwiki.ucdavis.edu/Theoretical_Chemistry/Chemical_Bonding/General_Principles/Covalent_Bond_Distance,_Radius_and_van_der_Waals_Radius values from an Inorganic Chemistry textbook].</ref>, and the length of a covalent carbon-carbon bond is 1.5 &Aring;<ref name="atomsize" />.

Here is a quick guide for interpreting values of resolution.  

*1.2 Å Excellent -- backbone and most sidechains very clear. Some [[Hydrogen in macromolecular models|hydrogens]] may be resolved, see [[Hydrogen in macromolecular models]].

*2.5 Å Good -- backbone and many sidechains clear.

*3.5 Å OK -- backbone and bulky sidechains mostly clear.

*5.0 Å Poor -- backbone mostly clear; sidechains not clear.

The '''images at right''' show how the electron density map<ref name="edm">Electron density maps are the results of [[X-ray crystallography]] experiments.</ref><ref name="stills">These are "perfect" electron density maps calculated from the atomic model (R factor = 0.0%, perfect phases and amplitudes, contoured at 1 sigma). Electron density maps based on experimental data would fit the true conformation less well. Because these electron density maps were calculated from an atomic model that lacked hydrogen atoms, the electron densities for hydrogen atoms that would appear with experimental data at a resolution of 0.5 &Aring; do not appear.</ref> becomes more accurate and detailed as the resolution value decreases from 5.0 &Aring; to 0.5 &Aring;.

<center><big>

[http://proteopedia.org/wiki/images/1/1a/Resolution_holton.mpeg '''PLAY MOVIE''']

</big>(Movie by James Holton at the Advanced Light Source of the Berkeley Laboratory at the University of California<ref name='movie' />.)</center>

Each diffraction spot (also called "reflection" or "measurement") in a diffraction pattern has a nominal resolution. The higher the diffraction angle (i.e. the further from the center of the diffraction image where the incoming X-ray beam would hit), the higher the resolution. In a diffraction experiment, the goal is to collect as many reflections as possible. However, reflections of high resolution are more difficult to measure because the intensity of reflections drops off at higher diffraction angles (and with that, higher resolution). In a diffraction image, you will see high intensity spots near the center, and more and more faded spots as you move away from the center. The overall resolution of a diffraction data set refers to the resolution range of reflections measured. For example, "data were collected from 20.0 Å to 2.3 Å with an overall completeness of 96.5%" means that most reflections in this range were collected, and the data set would be described in brief as "2.3 Å resolution" data set, referring to the high resolution limit of the data collection.