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Command: altlocs

The altlocs command lists and manages alternate locations, where an atomic structure contains multiple different positions for the same set of atoms. In PDB format, for example, alternate locations are indicated by a character identifier in column 17 of ATOM and HETATM records. Most of the command capabilities are also available in the Altloc Explorer graphical interface.

ChimeraX uses only one of the alternate locations at a time for display and for any calculations. For each contiguous (through-bond) set of atoms with alternate locations, the location used initially is that with the highest average atomic occupancy, then as a tie-breaker, the lowest average atomic B-factor, and if a tie-breaker is still needed, by alphabetical order of the alternate location ID.

See also: swapaa, torsion, build, attributes

altlocs list  residue-spec 
In the Log, report any alternate locations of the specified residues, and for each residue with alternate locations, which location is being used. For example, to list all residues in chain A that have alternate locations:
altlocs list /A
altlocs change  altloc-ID  residue-spec 
Use the indicated alternate location for all specified residues that contain it. Even if only part of a residue is specified (e.g., a single atom), the same alternate location will be used for any other atoms in that residue and any contiguous (through-bond) set of atoms for which the same alternate location identifier is available, including atoms in other residues. For example, to use alternate location C of residue 171 in chain A:
altlocs change C /A:171
altlocs clean  residue-spec 
Delete all but the currently used alternate location for the specified residues and remove their alternate location IDs. Examples:
altlocs clean /A:171
altlocs clean /A
altlocs clean
The last example deletes all unused alternate locations.

UCSF Resource for Biocomputing, Visualization, and Informatics / June 2021