def tors_beta(universe, seg, i):
    """beta  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
    -------
    beta : float
        torsion angle in degrees


    .. versionadded:: 0.7.6
    """
    b = universe.select_atoms(" atom {0!s} {1!s} P    ".format(seg, i),
                              " atom {0!s} {1!s} O5\' ".format(seg, i),
                              " atom {0!s} {1!s} C5\' ".format(seg, i),
                              " atom {0!s} {1!s} C4\' ".format(seg, i))
    beta = b.dihedral.value() % 360
    return beta