Molecular carpentry: piecing together helices and hairpins in designed peptides

The design of a peptide that contains two distinct elements of secondary structure, helix and beta-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 beta-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, 3JHN CalphaH values, temperature coefficients of the NH chemical shifts, and backbone inter-residue nuclear Overhauser effects favor helical structures for residues 1-7 and beta-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 alpha-helical conformation. Gly(9) is the only residue with multiple conformational possibilities in phi,psi 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.