Up to now, the SBX operator has been presented for unbounded variables, whereas peptide dihedrals are periodic. To restrict the search space to specified lower (l b = -180 • ) and upper (u b = 180 • ) bounds used throughout the GA, the probability distributions are modified so that the probability of creating dihedrals outside of the bounds is equal to zero; without loss of generality in what follows, one can assign the largest value to Θ i k ( Θi k ) and the lowest to Θ j k ( Θj k ). It is straightforward to notice from Eq.5 that a maximum spread allowed for Θi k -Θj k can be chosen as