Nuclear Overhauser effects and circular dichroism as probes of .beta.-turn conformations in acyclic and cyclic peptides with pro-X sequences

Abstract

Nuclear Overhauser effects (NOE) and (CD) techniques have been used to probe \beta-turn conformations in acyclic and cyclic peptides containing Pro-X sequences. The model peptides studied are of the type Piv-Pro-X-NHMe (X = Aib, D-Ala, Gly, Val, and Leu) and Boc-Cys-Pro-X-Cys NHMe (X = Aib, L-Ala, D-Ala, Gly, and Leu). In the acyclic series, observation of NOE's between Pro $C^\alphaH$ and X-NH, together with solvent and temperature dependence of NH chemical shifts, establishes a 4 \longrightarrow 1 hydrogen bond stabilized type I1 \beta-turn in the Gly, D-Ala, and Aib peptides, in $CDC1_3$ and $(CD_3)_2S0$. A positive $n-\pi^*$ CD band at \sim 225-230 nm appears to be characteristic of this structure. For the acyclic Pro-Leu peptide the observation of NOE's for both Pro and Leu $C^\alphaH$ resonances on saturation of Leu NH is compatible with a type V bend or consecutive \gamma-turn conformation. In the cyclic disulfide series the Pro-Aib and Pro-D-Ala peptides favor type I1 \beta-turns, whereas all other peptides adopt type I (111) conformations. All the cyclic disulfides exhibit an intense negative CD band at \sim 228-230 nm. The results suggest that general correlations between CD spectral type and specific \beta-turn conformations may not be obtained. Evidence for solvent-dependent structural changes in the Pro-Aib sequence in both cyclic and acyclic peptides is presented.