The Build Structure tool builds and modifies atomic structures. The equivalent commands are build and bond. See also: Rotamers, Modeller Comparative, rna, torsion
Build Structure can be opened from the Structure Editing section of the Tools menu and manipulated like other panels (more...). Sections:
The Start Structure section of Build Structure adds new atoms or molecules independent of any pre-existing atoms. See also: open, angle, torsion
Options are to Add:
The peptide can be added to an existing molecule model or generated as a new model with a specified name.
Clicking Apply brings up another dialog for specifying backbone φ (phi) and ψ (psi) angles and other parameters. One or more rows can be chosen with the mouse and Set to values either entered manually or supplied for various types of secondary structure:
|description||φ (°)||ψ (°)|
|antiparallel β strand||-139||135|
|parallel β strand||-119||113|
Sidechain conformations will be taken from the specified Rotamer library (see swapaa for literature citations):
The rotamer at each position will be chosen as described for the command swapaa with the criteria cp, meaning the rotamer with the fewest clashes, and if a tie, then the highest probability according to the rotamer library. The residues are added in N→C order, so only clashes with more N-terminal residues are evaluated. The peptide will be assigned the specified chain ID.
Clicking OK dismisses the dialog and creates the peptide. Hydrogen atoms are not included. Backbone bond lengths and angles are taken from the Amber ff99 parameters. Sidechain bond lengths and angles are taken from the Amber parameter files all*94.lib.
The Modify Structure section of Build Structure can change the element, valence (number of directly attached atoms), and/or geometry (spatial arrangement of attached atoms) of a single selected atom at a time. Hydrogens are appended as needed to fill the valence. Building outward can be done by successive cycles of modifying a hydrogen attached to the previously modified atom.
Clicking Apply changes the selected atom as specified:
There is no checking as to whether the specified number of bonds and geometry are chemically reasonable. Hydrogens will be added to the atom to generate the indicated total number of bonds. They are added to form the idealized bond angles for the specified geometry, or if the atom already has two or more substituents, to maximally avoid those substituents. The default geometry is the same as for the atom's current type. Since the geometry around the atom may be changing, any pre-existing directly attached hydrogens are removed beforehand. No other atoms are removed automatically. If the atom is already bonded to one (and only one) other nonhydrogen atom, the bond will be adjusted to an approximate length depending on the elements involved. No other atoms are moved. Bond lengths for X-H (X = C/N/O/S) are taken from the Amber parm99 parameters.
In PDB format, an atom name can be up to four characters long and should be unique within a residue. An atom name normally starts with the element symbol.
In PDB format, residue names are normally three characters long and chain identifiers a single character.
Clicking the Delete button at the bottom of Modify Structure removes any selected atoms and bonds. See also command: delete, menu: Actions... Delete
The Adjust Bonds section of Build Structure allows adding and deleting covalent bonds. A bond can also be deleted using its selection context menu. See also: bond, delete
Approximate covalent bond radii are used to guess the connectivity of untemplated residues (when not specified in the input structure file) and to generate crude bond lengths for building atomic structures.
|Selected covalent bond radii (Å)|
A complete list, obtained many years ago from documentation from the Cambridge Crystallographic Data Centre, can be found in Table III of:
Determination of molecular topology and atomic hybridization states from heavy atom coordinates. Meng EC, Lewis RA. J Comput Chem 12:891 (1991).