Fig. 14: Graphical comparison of interactions, based on 0 to 5 stars, of the strength and cooperativity of H-bonds between parallel and antiparallel PPII helices.
From: Orientation affects hydrogen bonding cooperativity in polyproline II helical bundles
The gray PPII helices are parallel to each other and antiparallel to the yellow one. Five stars for strength are assigned to the CO···HN H-bond of a parallel arrangement, three stars to the CO···HαCα inner H-bond, and the one to the CO···HαCα front H-bond based on the second presenting half the HBS of the first and three times that of the third11. Regarding cooperativity, five stars are assigned to the CO···HN H-bond for showing the maximum increase when completing the parallel PPII helical bundle, three stars to CO···HαCα inner H-bond for showing half of that increase, and none to CO···HαCα front H-bond for not showing cooperativity11. Likewise, for the H-bonds of an antiparallel arrangement: five stars for strength and cooperativity for CO···HN H-bonds for presenting more than twice the HBS and interaction energy increases as CO···HαCα proximal H-bonds (Table 4 and Fig. 7), which have been awarded two stars for each category. Meanwhile, only one star of strength for CO···HαCα distal H-bonds for having half the HBS of their proximal counterparts and zero stars of cooperativity for not presenting HBC (Table 4 and Fig. 7). Note that non-canonical distal H-bonds are only present in 2D bundles, but not the 3D bundles.
