Disulfide Bonds versus Trp•••Trp Pairs in Irregular β-Hairpins: NMR Structure of Vammin Loop 3-Derived Peptides as a Case Study

Structural studies on model peptides have led to a good understanding of the rules behind the formation and stability of regular beta-hairpins. To test their applicability to the successful design of irregular beta-hairpins with long loops and/or beta-bulges at the strands, we mimicked loop 3 of vammin, a 4:6 beta-hairpin with a non-Gly beta-bulge. The most stabilising cross-strand pairs, disulfide bonds or/and Trp center dot center dot center dot Trp pairs, were incorporated at non-hydrogen-bonded sites in peptides spanning the 69-80 region of vammin. According to NMR data, these modified peptides adopt beta-hairpin conformations as intended by design. The Trp-containing peptides reproduce even the unusual positive phi angle for the GIn residue, with the indole rings in the preferred edge-to-face orientation. For the first time the beta-hairpin-stabilising capacities of a disulfide bond and a Trp center dot center dot center dot Trp pair are compared in the same model system. We found that the contribution to stability of the noncovalent indole-indole interaction is larger than that of the covalent disulfide bond, and that their combination gives rise to an even more stable beta-hairpin.