Atom selection

Atom selections are enclosed in double quotes and have the following general form:

"[operator] {atom} [residue] {condition} {operator {atom} [residue] {condition}}"

Items in square brackets are optional and items in curly braces can occur zero or more times. An atom selection can have the following elements:

operator

An operator to combine the selection that follows it with the previous selection

atom

An atom name, possibly containing wildcards ("?" or "*" replace exactly one or any number of characters, respectively)

~atom

An atom name, possibly containing wildcards, as in the residue library, i.e. before a possible renaming with the library rename command

&atomtype

An atom type, possibly containing wildcards, as given in the residue library

residue

A residue selection, as explained below

condition

An additional condition, as explained below

The following special atom names can be used to select groups of atoms:

AMIDE nitrogen atoms and hydrogen and pseudo atoms con¬nected to nitrogens ASSIGNED atoms with assigned chemical shift values (in the first chemical shift list) BACKBONE main chain atoms (in proteins: N, C, C’) BB atoms whose position does not depend on side-chain tor¬sion angles (in proteins: N, H, C, H, C, C’, O) CARBON carbon atoms CBOUND atoms covalently bound to carbon atoms HBOUND atoms covalently bound to hydrogen atoms HEAVY all atoms except hydrogen or pseudo atoms HYDROGEN hydrogen atoms HYDROXYL oxygen atoms and hydrogen and pseudo atoms con¬nected to oxygens METHINE methine (CH1) groups, including the car¬bon and the hydrogen atom METHYL methyl (CH3) groups, including the car¬bon atom, three hydrogen atoms and the pseudo atom. In the case of isopropyl groups, e.g. in valine and leucine, the pseudo atom that represents both methyl groups is also selected. METHYLENE methylene (CH2) groups, including for each methylene group the car¬bon atom, two hydrogen atoms and the pseudo atom NBOUND atoms covalently bound to nitrogen atoms NH3 NH3 groups, including for each NH3 group the nitrogen atom, three hydrogen atoms and the pseudo atom NITROGEN nitrogen atoms OBOUND atoms covalently bound to oxygen atoms OXYGEN oxygen atoms PBOUND atoms covalently bound to phosphorus atoms PHOSPHOR phosphorus atoms PSEUDO pseudo atoms RENAMED atoms for which an external name is defined SBOUND atoms covalently bound to sulfur atoms SELBOUND atoms bound to a selected atom SELECTED already selected atoms SULFUR sulfur atoms WITHCOORD atoms for which Cartesian coordinates are present

A residue selection consists of one or several of the following elements: @name a residue name, possibly containing wildcards @~name a residue name, possibly containing wildcards, as in the residue library, i.e. before a possible renaming with the library rename command @FIRST the first residue @LAST the last residue @first the first residue of every fragment with contiguous resi¬due numbers @last the last residue of every fragment with contiguous resi¬due numbers range a residue number range, as defined above

Atom selections can be combined using the following operators: = atoms in the new set (default) ! atoms not in the new set + atoms in the current set or in the new set – atoms in the current set, but not in the new set / atoms in the current set and in the new set Operators are always evaluated from left to right. The current atom set is the set of atoms defined by what precedes the operator. The new atom set is the set of atoms defined by what follows the operator. An empty atom selection selects all atoms.

Additional conditions can be applied to atom selections: random=p Select randomly only a fraction p of all atoms that would normally be selected. shift=min..max Select only atoms with an assigned chemical shift value between min and max ppm in the first chemical shift list. tolerance=min..max Select only assigned atoms with a chemical shift tolerance value between min and max ppm in the first chemical shift list.

Examples of atom selections: HA # atoms called HA "HA HB*" # all atoms called HA or HB "HA @ALA 10..20" # HA in ALA of residues 10–20 "HA @ALA - 10..20" # HA in ALA except residues 10–20 "N CA C + 15 17 - H* Q*" # all N, CA, C, and side-chain heavy at¬oms in # residues 15 and 17 "- C*" # the previous selection except atoms named C… "/ CBOUND" # atoms in the previous selection that are bound to # carbons "CARBON / ASSIGNED" # assigned carbon atoms "H* Q* - HYDROXYL - AMIDE + H HN / &H_* &PSEUD"

1. hydrogens and pseudo atoms, except hydroxyls
2. and side-chain amides, whose atom type either
3. starts with H_ or is PSEUD

The commands atom list and atom select (with the parameter info=full) can be used to check atom selections.