## alpha() — make an α -helix

`alpha(residues)`

> This makes restraints enforcing an α -helix (mainchain conformation class “A”) for the residue segment specified by residues (which can be created using the model.residue_range() function). The helix is restrained by Φ,Ψ binormal restraints, N-O hydrogen bonds, Cα - Cα distances for , Cα -O distances for , and O-O distances for 6.4. Note that this requires all heavy atoms to be present to work properly, so will not work with the Cα -only topology.

> In many cases (e.g., most comparative modeling runs) you will already have binormal, Cα - Cα , and N-O restraints active (which will conflict with helix restraints), so it is recommended that you first use Restraints.unpick() followed by Restraints.condense() to remove these.

> To actually add the restraints, pass the new object to Restraints.add().

Example: examples/commands/secondary_structure.py

      # Example for model.build_sequence(), secondary_structure.alpha()

      from modeller import *
      from modeller.optimizers import conjugate_gradients

      # Set up environment
      e = environ()
      e.libs.topology.read('${LIB}/top_heav.lib')
      e.libs.parameters.read('${LIB}/par.lib')

      # Build an extended chain model from primary sequence, and write it out
      m = model(e)
      m.build_sequence('GSCASVCGV')
      m.write(file='extended-chain.pdb')

      # Make stereochemical restraints on all atoms
      allatoms = selection(m)
      m.restraints.make(allatoms, restraint_type='STEREO', spline_on_site=False)

      # Constrain all residues to be alpha-helical
      # (Could also use m.residue_range() rather than m.residues here.)
      m.restraints.add(secondary_structure.alpha(m.residues))

      # Get an optimized structure with CG, and write it out
      cg = conjugate_gradients()
      cg.optimize(allatoms, max_iterations=100)
      m.write(file='alpha-helix.pdb')