Molecular carpentry: Piecing together helices and hairpins in designed peptides

Abstract

The design of a peptide that contains two distinct elements of secondary structure, helix and β-hairpin, is described. Two designed 17-residue peptides: Boc-Val-Ala-Leu-Aib-Val-Ala-Leu-Gly-Gly-Leu-Phe-Val-d-Pro-Gly-Leu-Phe-Val-OMe (I) and Boc-Leu-Aib-Val-Ala-Leu-Aib-Val-Gly-Gly-Leu-Val-Val-d-Pro-Gly-Leu-Val-Val-OMe (II) have been conformationally characterized by NMR spectroscopy. Peptides I and II contain a seven-residue helical module at the N terminus and a eight-residue β-hairpin module at the C terminus, which are connected by a conformationally flexible Gly-Gly segment. The choice of the secondary-structure modules is based upon prior crystallographic and spectroscopic analysis of the individual modules. Analysis of 500 MHz ¹H NMR data, recorded as solutions in methanol, suggests that the observed pattern of chemical shifts, ³J_HN-Cα_H values, temperature coefficients of the NH chemical shifts, and backbone inter-residue nuclear Overhauser effects favor helical structures for residues 1-7 and β-hairpin structures for residues 10-17. The spectroscopic data are compatible with termination of the helical segment by formation of a Schellman motif; this restricts Gly(8) to a left-handed -helical conformation. Gly(9) is the only residue with multiple conformational possibilities in φ,ψ space. Possible orientations of the two secondary-structure modules are considered. This study validates the use of stereochemically rigid peptide modules as prefabricated elements in the construction of synthetic protein mimics.