Conformationally constrained macrocycles that mimic tripeptide β-strands in water and aprotic solvents

The β-strand conformation is unknown for short peptides in aqueous solution, yet it is a fundamental building block in proteins and the crucial recognition motif for proteolytic enzymes that enable formation and turnover of all proteins. To create a generalized scaffold as a peptidomimetic that is pre-organized in a β-strand, we individually synthesized a series of 15-22-membered macrocyclic analogues of tripeptides and analyzed their structures. Each cycle is highly constrained by two trans amide bonds and a planar aromatic ring with a short nonpeptidic linker between them. A measure of this ring strain is the restricted rotation of the component tyrosinyl aromatic ring (ΔG_rot 76.7 kJ mol^-1 (16-membered ring), 46.1 kJ mol^-1 (17-membered ring)) evidenced by variable temperature proton NMR spectra (DMF-d₇, 200-400 K). Unusually large amide coupling constants (³J_NH-CHα 9-10 Hz) corresponding to large dihedral angles were detected in both protic and aprotic solvents for these macrocycles, consistent with a high degree of structure in solution. The temperature dependence of all amide NH chemical shifts (Δδ/T7-12 ppb/deg) precluded the presence of transannular hydrogen bonds that define alternative turn structures. Whereas similar sized conventional cyclic peptides usually exist in solution as an equilibrium mixture of multiple conformers, these macrocycles adopt a well-defined β-strand structure even in water as revealed by 2-D NMR spectral data and by a structure calculation for the smallest (15-membered) and most constrained macrocycle. Macrocycles that are sufficiently constrained to exclusively adopt a β-strand-mimicking structure in water may be useful pre-organized and generic templates for the design of compounds that interfere with β-strand recognition in biology.