A designed three-stranded β-sheet in an α/β hybrid peptide.

The incorporation of β-amino acid residues into the antiparallel β-strand segments of a multi-stranded β-sheet peptide is demonstrated for a 19-residue peptide, Boc-LV(β)FV(D)PGL(β)FVVL(D)PGLVL(β)FVV-OMe (BBH19). Two centrally positioned (D)Pro-Gly segments facilitate formation of a stable three-stranded β-sheet, in which β-phenylalanine ((β)Phe) residues occur at facing positions 3, 8 and 17. Structure determination in methanol solution is accomplished by using NMR-derived restraints obtained from NOEs, temperature dependence of amide NH chemical shifts, rates of H/D exchange of amide protons and vicinal coupling constants. The data are consistent with a conformationally well-defined three-stranded β-sheet structure in solution. Cross-strand interactions between (β)Phe3/(β)Phe17 and (β)Phe3/Val15 residues define orientations of these side-chains. The observation of close contact distances between the side-chains on the N- and C-terminal strands of the three-stranded β-sheet provides strong support for the designed structure. Evidence is presented for multiple side-chain conformations from an analysis of NOE data. An unusual observation of the disappearance of the Gly NH resonances upon prolonged storage in methanol is rationalised on the basis of a slow aggregation step, resulting in stacking of three-stranded β-sheet structures, which in turn influences the conformational interconversion between type I' and type II' β-turns at the two (D)Pro-Gly segments. Experimental evidence for these processes is presented. The decapeptide fragment Boc-LV(β)FV(D)PGL(β)FVV-OMe (BBH10), which has been previously characterized as a type I' β-turn nucleated hairpin, is shown to favour a type II' β-turn conformation in solution, supporting the occurrence of conformational interconversion at the turn segments in these hairpin and sheet structures.