Geometry and efficacy of cross-strand Trp/Trp, Trp/Tyr, and Tyr/Tyr aromatic interaction in a beta-hairpin peptide.

The Trpzip2 peptide (WTWENGKWTWK-NH(2)), designed by Cochran and co-workers, contains two pairs of Trp's having cross-strand interaction and forms a stable antiparallel beta-hairpin. In order to study the geometries and effects on the structure and stability of different aromatic interactions, selected tryptophan residues were substituted with Tyr to get three Trpzip2 mutants with different Trp/Trp, Trp/Tyr, and Tyr/Tyr interacting pairs. Their native-state structures were determined using two-dimensional (2D) NMR and shown to have the same cross-strand edge-to-face Trp/Trp interaction as that in Trpzip2 for the Trp/Trp pair. The analogous Trp/Tyr and Tyr/Tyr pairs also tended to have an edge-to-face geometry. The effects of specific Trp/Trp, Trp/Tyr, and Tyr/Tyr interactions on hairpin stability were studied by varying temperature and monitoring structure with electronic circular dichroism (CD) and infrared (IR) absorption spectra. IR and CD temperature variations were fit to a two-state model that yielded lower T(m) values for Tyr containing mutants, indicating that Trp/Tyr and Tyr/Tyr interactions have less contribution to hairpin stability than the Trp/Trp interaction. Trp/Tyr interactions can provide significant stabilization, much greater than the Trp/aliphatic interaction, but Tyr/Tyr interactions are not as significant. Cross-strand interacting residues involving Trp with an edge-to-face orientation with Trp or Tyr had the strongest impact on hairpin stability.