Angle between CA-CB of knob and CA(h1)-CA(h2). Returns None if knob is GLY.

def packing_angle(self):
        """ Angle between CA-CB of knob and CA(h1)-CA(h2). Returns None if knob is GLY. """
        try:
            knob_vector = self.knob_residue['CB'] - self.knob_residue['CA']
        # exception for GLY residues (with no CB atom).
        except KeyError:
            return None
        hole_vector = self.hole_residues[2]['CA'] - self.hole_residues[1]['CA']
        return angle_between_vectors(knob_vector, hole_vector)