Multiple conformational states in crystals and in solution in αγ hybrid peptides. Fragility of the c₁₂ helix in short sequences

Abstract

The conformational properties of foldamers generated from αγ hybrid peptide sequences have been probed in the model sequence Boc-Aib-Gpn-Aib-Gpn-NHMe. The choice of α-aminoisobutyryl (Aib) and gabapentin (Gpn) residues greatly restricts sterically accessible conformational space. This model sequence was anticipated to be a short segment of the αγ C₁₂ helix, stabilized by three successive 4→1 hydrogen bonds, corresponding to a backbone-expanded analogue of the α polypeptide 3₁₀-helix. Unexpectedly, three distinct crystalline polymorphs were characterized in the solid state by X-ray diffraction. In one form, two successive C₁₂ hydrogen bonds were obtained at the N-terminus, while a novel C₁₇ hydrogen-bonded γαγ turn was observed at the C-terminus. In the other two polymorphs, isolated C₉ and C₇ hydrogen-bonded turns were observed at Gpn (2) and Gpn (4). Isolated C₁₂ and C₉ turns were also crystallographically established in the peptides Boc-Aib-Gpn-Aib-OMe and Boc-Gpn-Aib-NHMe, respectively. Selective line broadening of NH resonances and the observation of medium range NH(i) ↔NH(i+2) NOEs established the presence of conformational heterogeneity for the tetrapeptide in CDCl₃ solution. The NMR results are consistent with the limited population of the continuous C₁₂ helix conformation. Lengthening of the (αγ)_n sequences in the nonapeptides Boc-Aib-Gpn-Aib-Gpn-Aib-Gpn-Aib-Gpn-Xxx (Xxx = Aib, Leu) resulted in the observation of all of the sequential NOEs characteristic of an αγ C₁₂ helix. These results establish that conformational fragility is manifested in short hybrid αγ sequences despite the choice of conformationally constrained residues, while stable helices are formed on chain extension.