5.12 Rotamers

The rotamers are generated ¹ from the backbone independent sidechain library of the Richardsons group ².

The m, t and p stand for “minus (-60)”, “trans (180)” and “plus (+60)”. There is one letter per \chi angle.

Use keyboard <.> and <,> to cycle round the rotamers.

5.12.1 Auto Fit Rotamer

“Auto Fit Rotamer” will try to fit the rotamer to the electron density. Each rotamer is generated, rigid body refined and scored according to the fit to the map. Fitting the second conformation of a dual conformation in this way will often fail - the algorithm will pick the best fit to the density - ignoring the position of the other atoms.

The algorithm doesn't know if the other atoms in the structure are in sensible positions. If they are, then it is sensible not to put this residue too close to them, if they are not then there should be no restriction from the other atoms as to the position of this residue - the default is “are sensible”, which means that the algorithm is prevented from finding solutions that are too close to the atoms of other residues. (set-rotamer-check-clashes 0) will stop this.

There is a scripting interface to auto-fitting rotamers:

(auto-fit-best-rotamer resno alt-loc ins-code chain-id imol-coords imol-map clash-flag lowest-rotamer-probability)

where:

resno is the residue number

alt-loc is the alternate/alternative location symbol (e.g. "A" or "B", but most often "")

ins-code is the insertion code (usually "")

imol-coords is the molecule number of the coordinates molecule

imol-map is the molecule number of the map to which you wish to fit the side chains

clash-flag should the positions of other residues be included in the scoring of the rotamers (i.e. clashing with other other atoms gets marked as bad/unlikely)

lowest-rotamer-probability: some rotamers of some side chains are so unlikely that they shouldn't be considered - typically 0.01 (1%).

You can change the auto-fit rotamer fitting algorithms using

(set-rotamer-search-mode mode)

where mode is one of (ROTAMERSEARCHAUTOMATIC), (ROTAMERSEARCHLOWRES) (i.e. "Backrub Rotamers" (vide infra)) or (ROTAMERSEARCHHIGHRES) (the conventional/high-resolution method using rigid-body fitting).

By default, the auto-fit rotamer method is (ROTAMERSEARCHAUTOMATIC).

5.12.1.1 Backrub Rotamers

By default, Auto Fit Rotamer will switch to “Backrub Rotamer” ³ mode when fitting against a map of worse than 2.7Å. This search mode moves the some atoms of the mainchain of the neighbouring residues. After rotation of the central residue and neighbouring atoms around the “backrub vector”, the individual peptides are back-rotated (along the peptide axis) so that the carbonyl oxygen are placed as near as possible to their original position. The Ramachandran plot is not used in this fitting algorithm.

5.12.2 De-clashing residues

Sometimes you don't have a map ⁴ but nevertheless there are clashing residues ⁵ (for example after mutation of a residue range) and you need to rotate side-chains to a non-clashing rotamer. There is a scripting interface:

(de-clash imol chain-id start-resno end-resno)

start-resno is the residue number of the first residue you wish to de-clash

end-resno is the residue number of the last residue you wish to de-clash

imol is the molecule number of the coordinates molecule

This interface will not change residues with insertion codes or alternate conformation. The lowest-rotamer-probability is set to 0.01.