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.

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