Multiple Conformational States in Crystals and in Solution in rγ Hybrid Peptides. Fragility of the C12 Helix in Short Sequences

The conformational properties of foldamers generated from Rγ hybrid peptide sequences have been probed in the model sequence Boc-Aib-Gpn-Aib-Gpn-NHMe. The choice of R-aminoisobutyryl (Aib) and gabapentin (Gpn) residues greatly restricts sterically accessible conformational space. This model sequence was anticipated to be a short segment of the Rγ C12 helix, stabilized by three successive 4f1 hydrogen bonds, corresponding to a backbone-expanded analogue of the R 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 hydrogenbonded γRγ 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) T 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 (Rγ)n sequences in the nonapeptides Boc-Aib-Gpn-Aib-Gpn-Aib-GpnAib-Gpn-Xxx (Xxx ) Aib, Leu) resulted in the observation of all of the sequential NOEs characteristic of an Rγ C12 helix. These results establish that conformational fragility is manifested in short hybrid Rγ sequences despite the choice of conformationally constrained residues, while stable helices are formed on chain extension.