Disulfide bonds versus Trp···Trp pairs in irregular β-hairpins: NMR structure of vammin loop 3-derived peptides as a case study

Abstract

Where a noncovalent interaction is better than a covalent bond: The most stabilising cross-strand pairs were incorporated into an irregular β-hairpin, loop 3 of vammin. 1H and 13C NMR conformational analyses of these designed peptides indicated that an edge-to-face Trp···Trp interaction leads to a β-hairpin that is more stable than a disulfide bond.

Structural studies on model peptides have led to a good understanding of the rules behind the formation and stability of regular β-hairpins. To test their applicability to the successful design of irregular β-hairpins with long loops and/or β-bulges at the strands, we mimicked loop 3 of vammin, a 4:6 β-hairpin with a non-Gly β-bulge. The most stabilising cross-strand pairs, disulfide bonds or/and Trp···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 β-hairpin conformations as intended by design. The Trp-containing peptides reproduce even the unusual positive φ angle for the Gln residue, with the indole rings in the preferred edge-to-face orientation. For the first time the β-hairpin-stabilising capacities of a disulfide bond and a Trp···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 β-hairpin.