Constraints on backbone torsion angles during Monte Carlo polypeptide simulations limit the trial
PHI and PSI angles to a range determined by the secondary structure definition of a particular residue position.
For example: A residue designated as helix (H) is constrained to PHI and PSI values found within the red squares in the first figure below, strand (E) residues are contrained to the area within the green square and residues in polyproline II (P) conformations are constrained to the area within the magenta square. | ||

Residues in turns are always in pairs and the moveset involves both residues in the turn.
In these cases both residue PHI and PSI angles are chosen from the blue squares as appropriate.
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Coil movesets are residue specific. | ||

The possible PHI and PSI angles available
to each residue type designated as coil (C) are indicated in the following plots.
The probability
of selecting paired PHI, PSI values in any one of the colored squares is proportional to the normalized frequency (NF) of finding
angles within a specific square for that residue type in the
protein coil library.
The area representing polyproline II (P) is not a possible move for a coil residue because any residue with torsion angles in this
area would be designated as P. Residues with torsion angles in either helix (H) or strand (E) areas are designated as those respective
secondary structures only if there are five (helix) or three (strand) sequential residues in the same designation. A single residue in
the helix or strand conformation is scored as a coil (C) residue in the protein coil library and therefore these areas are available
for coil Monte Carlo moves.
Grey colored squares (with NF <= 0.05) are not part of the possible moveset. The color scale for the range of normalized frequencies is shown to the right.
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