Crystal structure analyses of azasulfuryltripeptides reveal potential for γ-turn mimicry

Azasulfurylpeptides feature an amino acid residue in which the CαH and the carbonyl are replaced respectively by a nitrogen atom and a sulfonyl group. Insight into the conformational preferences of azasulfurylpeptides containing an azasulfurylglycine (AsG) residue has been pursued using X-ray analysis in the solid state. Crystals of N-(Boc)-Pro-AsG-Val-OMe (10) and N-(Cbz)-Ala-AsG-D-Phe-Ot-Bu (11) showed tetrahedral geometries about the sulfur atom with the ω torsion angle preferring a staggered conformation. Furthermore, the ϕ and ψ torsion angles of the central azasulfuryl residue were respectively within close proximity to those of ideal inverse and classical γ-turns. In the crystal lattice, azasulfurylpeptide 11 engaged in intermolecular hydrogen bonds between the sulfonyl oxygen and hydrazide hydrogen in an antiparallel orientation.