def tors_alpha(universe, seg, i):
    """alpha 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
    -------
    alpha : float
        torsion angle in degrees


    .. versionadded:: 0.7.6
    """
    a = universe.select_atoms(" atom {0!s} {1!s} O3' ".format(seg, i - 1),
        ' 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))
    alpha = a.dihedral.value() % 360
    return alpha
