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

Abstract

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 buffed 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.