Stability of cyclic beta-hairpins: asymmetric contributions from side chains of a hydrogen-bonded cross-strand residue pair

Amino acid structural propensities measured in “host−guest” model studies are often used in protein structure prediction or to choose appropriate residues in de novo protein design. While this concept has proven useful for helical structures, it is more difficult to apply successfully to β-sheets. We have developed a cyclic β-hairpin scaffold as a host for measurement of individual residue contributions to hairpin structural stability. Previously, we have characterized substitutions in non-backbone-hydrogen-bonded strand sites; relative stability differences measured in the cyclic host are highly predictive of changes in folding free energy for linear β-hairpin peptides. Here, we examine the hydrogen-bonded strand positions of our host. Surprisingly, we find a large favorable contribution to stability from a valine (or isoleucine) substitution immediately preceding the C-terminal cysteine of the host peptide, but not at the cross-strand position of the host or in either strand of a folded linear β-hairpin...