def tors_zeta(universe, seg, i):
    """Zeta backbone dihedral

    The dihedral is computed based on position atoms for resid `i`.

    Parameters
    ----------
    universe : Universe
         :class:`~MDAnalysis.core.universe.Universe` containing the trajectory
    seg : str
        segment id for base
    i : int
        resid of the first base

    Returns
    -------
    zeta : float
        torsion angle in degrees


    .. versionadded:: 0.7.6
    """
    z = universe.select_atoms(" atom {0!s} {1!s} C3' ".format(seg, i),
        " atom {0!s} {1!s} O3' ".format(seg, i), ' atom {0!s} {1!s} P    '.
        format(seg, i + 1), " atom {0!s} {1!s} O5' ".format(seg, i + 1))
    zeta = z.dihedral.value() % 360
    return zeta
