Relationship
between resolution of an x-ray structureand information available in the structure
Relationship
between resolution of an x-ray structureThe following table is taken from Alan Fersht, "Enyzme Structure and Mechanism", 2nd edition, Freeman & Co., New York, 1985, p. 6. It gives you an idea of how accurate you can expect an x-ray crystal structure to be at various resolutions. Note that at moderate resolutions (such as 2.5 Å) the atoms are located only within 0.4 Å or so. This means that the lengths of hydrogen bonds calculated from the PDB file (for example, by RasMol) have at least this much uncertainty.
|
Resolution (Å) |
Structural features observable in a good map |
|
5.5 |
Overall shape of the molecule. Helices as rods of strong intensity. |
|
3.5 |
The main chain (usually with some ambiguities). |
|
3.0 |
The side chains partly resolved. |
|
2.5 |
Side chains well resolved. The plane of the peptide bond resolved. Atoms located to about ± 0.4 Å. |
|
1.5 |
Atoms located to about ± 0.1 Å (the limit of protein crystallography as of 1985--it's a little bit lower now) |
|
0.77 |
Bond lengths in small crystals measured to 0.005 Å. |
Note that some information is available at resolution that doesn't resolve individual atoms. Exciting new structures appear at >5 Å resolution which are very informative. These often are later superseded by structures at atomic resolution--see page on PDB files. The following are good examples:
"Yeast RNA polymerase II at 5 Å resolution". Fu et al., Cell 98, 799-810 (1999).
"Placement of protein and RNA structures into a 5 Å-resolution map of the 50S ribosomal subunit". Ban et al., Nature 400, 841-847 (1999).
"Structure of a bacterial 30S ribosomal subunit at 5.5 A resolution". Clemons et al., Nature 400, 833-840 (1999).
An extremely interesting new structure of an open complex between RNA
polymerase and a promoter is only at 6.5 Å, but gives major
insights into the probable structure of the open complex:
"Structural basis of transcription initiation: An RNA polymerase holoenzyme-DNA complex". Murakami et al., Science 296, 1285-1290 (2002). The link will take you to PubMed; from University computers you should have a link to the Science web site and the article itself.