A motif of 3 consecutive residues with dihedral angles as follows: res i: phi -90 bounds -120 to -60, res i: psi -10 bounds -50 to 30, res i+1: phi -75 bounds -100 to -50, res i+1: psi 140 bounds 110 to 170. An extra restriction of the length of the O to O distance would be useful, that it be less than 5 Angstrom. More precisely these two oxygens are the main chain carbonyl oxygen atoms of residues i-1 and i+1.