Chiral Sulfoxide-Induced Single Turn Peptide α-Helicity

Abstract

Inducing α-helicity through side-chain cross-linking is a strategy that has been pursued to improve peptide conformational rigidity and bio-availability. Here we describe the preparation of small peptides tethered to chiral sulfoxide-containing macrocyclic rings. Furthermore, a study of structure-activity relationships (SARs) disclosed properties with respect to ring size, sulfur position, oxidation state, and stereochemistry that show a propensity to induce α-helicity. Supporting data include circular dichroism spectroscopy (CD), NMR spectroscopy, and a single crystal X-ray structure for one such stabilized peptide. Finally, theoretical studies are presented to elucidate the effect of chiral sulfoxides in inducing backbone α-helicity.

Figure 1. On-tether chiral centres influence the secondary structure of the peptide.

Figure 2

(a) Sequence of peptide 1-16, where A is L-alanine, C is cysteine, c(x, y) signifies cross-link forming amino acid. (b) Oxidation of peptide 7 to (S)-15A and (R)-15B. HPLC spectrum of the oxidation reaction mixture. CD spectroscopy was performed in 10 mM PBS (pH = 7.4) buffer at 25 °C.

Figure 3

(a) CD spectra of (R)-diastereomer 19B-24B. (b) CD spectra of sulfone-containing peptides 25 and 26 show minimal helix contents. (c) CD spectroscopy was performed for (R)-19B at increasing temperatures. Its helicity decreased slightly as temperature increased (d) At 65 °C, (R)-19B retained 75% of the helicity that it showed at 30 °C. CD spectroscopy was performed in 10 mM PBS (pH = 7.4) buffer at 25 °C.

Figure 4. Crystal structure of (R)-19B and calculated structures of (S)-15A, (R)-15B.

(a) X-ray crystal structure of (R)-19B with thermal ellipsoids shown at the 50% probability level with three α-helical hydrogen bonds. (b) Conformers of (R)-diastereomer 15B (R1, R2) and (S)-diastereomer 15A (S1, S2, S3, S4) with increased stability, calculated with density functional theory at PBE1PBE/6-31 + G** level. Relative free energies are given below each structure. (c) Conformations sampled in REMD simulations of (S)-19A and (R)-19B. The x-axis is the backbone RMSD of the conformations, with respect to the X-ray structure of (R)-19B. The y-axis is the radius of gyration of the conformations. The clusters are labelled as R1′(19B), R2′(19B) and S1′-S3′(19A), similar to structures R1(15B), R2(15B), and S1-S3(15A) in (b), see SI Fig. 8 for the structures of R1′(19B), R2′(19B) and S1′-S3′(19A).