docs: def tropocollagen(
              cls, aa=28, major_radius=5.0, major_pitch=85.0, auto_build=True):
          """Creates a model of a collagen triple helix.
  
          Parameters
          ----------
          aa : int, optional
              Number of amino acids per minor helix.
          major_radius : float, optional
              Radius of super helix.
          major_pitch : float, optional
              Pitch of super helix.
          auto_build : bool, optional
              If `True`, the model will be built as part of instantiation.
          """
          instance = cls.from_parameters(
              n=3, aa=aa, major_radius=major_radius, major_pitch=major_pitch,
              phi_c_alpha=0.0, minor_helix_type='collagen', auto_build=False)
          instance.major_handedness = ['r'] * 3
          # default z-shifts taken from rise_per_residue of collagen helix
          rpr_collagen = _helix_parameters['collagen'][1]
          instance.z_shifts = [-rpr_collagen * 2, -rpr_collagen, 0.0]
          instance.minor_repeats = [None] * 3
          if auto_build:
              instance.build()
          return instance