The distinct conformational landscapes of 4 S-substituted prolines that promote an endo ring pucker

Abstract

4-Substitution on proline directly impacts protein main chain conformational preferences. The structural effects of N-acyl substitution and of 4-substitution were examined by NMR spectroscopy and X-ray crystallography on minimal molecules with a proline 4 S-nitrobenzoate. The effects of N-acyl substitution on conformation were attenuated in the 4 S-nitrobenzoate context, due to the minimal role of the n→π* interaction in stabilizing extended conformations. By X-ray crystallography, an extended conformation was observed for most molecules. The formyl derivative adopted a δ conformation that is observed at the i+2 position of β-turns. Computational analysis indicated that the structures observed crystallographically represent the inherent conformational preferences of 4 S-substituted prolines with electron-withdrawing 4-position substituents. The divergent conformational preferences of 4 R- and 4 S-substituted prolines suggest their wider structure-specific application in molecular design. In particular, the proline endo ring pucker favored by 4 S-substituted prolines uniquely promotes the δ conformation (( ϕ, ψ) ~ (–80°, 0°)) found in β-turns. In contrast to other acyl capping groups, the pivaloyl group strongly promoted trans amide bond and polyproline II helix conformation, with a close n→π* interaction in the crystalline state, despite the endo ring pucker, suggesting its special capabilities in promoting compact conformations in ϕ due to its strongly electron-donating character.

Graphical Abstract

Conformational control: 4 S-substituted prolines with electron-withdrawing 4-substitutents strongly promote the endo ring pucker and the δ conformation central to β-turns ( ϕ, ψ = −80°, 0°), as determined by X-ray crystallography and calculations. This work suggests their broader application in protein design, medicinal chemistry, and catalysis.