Distance restraints

NOE distance restraints are specified with the following syntax:

ASSIgn atom-selection atom-selection real real real 

The atom selections define the atoms (or groups of atoms) between which the distance restraint will be applied. The following real numbers determine the parameters of the distance restraint: d (the distance), and dminus, and dplus (the extents either side of this distance) respectively.

Example:

assign (resid 112 and name n)  (resid 74 and name o)  2.8 0.4 0.9
assign (resid 112 and name hn) (resid 74 and name o)  1.8 0.4 0.9
assign (resid 74 and name n)   (resid 112 and name o) 2.8 0.4 0.9
assign (resid 74 and name hn)  (resid 112 and name o) 1.8 0.4 0.9

In CNS, the setup of pseudoatoms is accomplished by the ASSIgn statement, with multiple protons in either atom selection. For the restraining functions, CNS computes either an R-6 averaged distance between the involved protons or the distance between the geometric centers of the two specified atom selections. For distance geometry, CNS automatically applies a pseudoatom correction to the specified distance ranges. Pseudoatoms (multiple atom selections) should be used primarily for unresolved NOE cross peaks, like those of methyl groups, prochiral centers, and aromatic rings. In the case of stereospecific assignments, the distances should be exact.

Example:

assign (resid 4 and name HG#) (resid 4 and name HE2#) 4.0 2.2 1.0
assign (resid 4 and name HG#) (resid 4 and name HE2#) 3.0 1.2 1.0
assign (resid 4 and name HA ) (resid 4 and name HE2#) 4.0 2.2 1.0 

The NOE distance restraints are read into CNS by the NOE statement:

NOE
  class noe
  nrestraints=500
  assign (resid 4 and name HG#) (resid 4 and name HE2#) 4.0 2.2 1.0
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 NOE distance restraints used in the tutorial trx_noe_all.tbl. Hydrogen bond distance restraints have the same format.