Dihedral angle restraints


Dihedral angle restraints are specified with the following syntax:

ASSIgn atom-selection atom-selection atom-selection atom-selection real real real integer

The atom selections define the atoms to which the dihedral angle restraint will be applied. The four atom selections have to be unique, with each selection selecting exactly one atom. The first real number specifies the energy constant in kcal mole-1 rad-2, the second real number specifies the angle in degrees to which the dihedral angle is restrained, the third real number specifies the range around the restrained angle, and the integer number specifies the exponent.

Example:

 assign (resid  1  and name c ) (resid  2  and name n )
        (resid  2  and name ca) (resid  2  and name c )   1.0  -125.0 25.0 2
 assign (resid  2  and name c ) (resid  3  and name n )
        (resid  3  and name ca) (resid  3  and name c )   1.0  -152.0 20.0 2

The dihedral angle restraints are read into CNS by the restraints dihedral statement:

restraints dihedral
  nassign=300
  assign (resid  1  and name c ) (resid  2  and name n )
         (resid  2  and name ca) (resid  2  and name c )   1.0  -125.0 25.0 2
end

In the distributed task files for NMR structure calculation the user only needs to provide a file with the ASSIGN statements. For example the dihedral angle restraints used in the tutorial trx_dihed_rama.tbl.


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