Cross‐strand side‐chain interactions versus turn conformation in β‐hairpins

A series of designed peptides has been analyzed by ¹H‐NMR spectroscopy in order to investigate the influence of cross‐strand side‐chain interactions in β‐hairpin formation. The peptides differ in the N‐terminal residues of a previously designed linear decapeptide that folds in aqueous solution into two interconverting β‐hairpin conformations, one with a type I turn (β‐hairpin 4:4) and the other with a type I + G1 β‐bulge turn (β‐hairpin 3:5). Analysis of the conformational behavior of the peptides studied here demonstrates three favorable and two unfavorable cross‐strand side‐chain interactions for β‐hairpin formation. These results are in agreement with statistical data on side‐chain interactions in protein β‐sheets. All the peptides in this study form significant populations of the β‐hairpin 3:5, but only some of them also adopt the β‐hairpin 4:4. The formation of β‐hairpin 4:4 requires the presence of at least two favorable cross‐strand interactions, whereas β‐hairpin 3:5 seems to be less susceptible to side‐chain interactions. A protein database analysis of β‐hairpins 3:5 and β‐hairpins 4:4 indicates that the former occur more frequently than the latter. In both peptides and proteins, β‐hairpins 3:5 have a larger right‐handed twist than β‐hairpins 4:4, so that a factor contributing to the higher stability of β‐hairpin 3:5 relative to β‐hairpin 4:4 is due to an appropriate backbone conformation of the type I + G1 β‐bulge turn toward the right‐handed twist usually observed in protein β‐sheets. In contrast, as suggested previously, backbone geometry of the type I turn is not adequate for the right‐handed twist. Because analysis of buried hydrophobic surface areas on protein β‐hairpins reveals that β‐hairpins 3:5 bury more hydrophobic surface area than β‐hairpins 4:4, we suggest that the right‐handed twist observed in β‐hairpin 3:5 allows a better packing of side chains and that this may also contribute to its higher intrinsic stability.