Crystal structure and solution conformation of S,S′‐bis(Boc‐Cys‐Ala‐OMe): Intramolecular antiparallel β‐sheet conformation of an acyclic cystine peptide

The conformation of the acyclic biscystine peptide S,S′‐bis(Boc‐Cys‐Ala‐OMe) has been studied in the solid state by x‐ray diffraction, and in solution by¹H‐ and¹³C‐nmr, ir, and CD methods. The peptide molecule has a twofold rotation symmetry and adopts an intramolecular antiparallel β‐sheet structure in the solid state. The two antiparallel extended strands are stabilized by two hydrogen bonds between the Boc CO and Ala NH groups [N ⃛O 2.964 (3) Å, O ⃛HN 2.11 (3) Å, and NH ⃛O angle 162 (3)°]. The disulfide bridge has a right‐handed conformation with the torsion angle C^βSSC^β = 95.8 (2)°. In solution the presence of a twofold rotation symmetry in the molecule is evident from the¹H‐ and¹³C‐nmr spectra. ¹H‐nmr studies, using solvent and temperature dependencies of NH chemical shifts, paramagnetic radical induced line broadening, and rate of deuterium‐hydrogen exchange effects on NH resonances, suggest that Ala NH is solvent shielded and intramolecularly hydrogen bonded in CDCl₃ and in (CD₃)₂SO. Nuclear Overhauser effects observed between Cys C^αH and Ala NH protons and ir studies provide evidence of the occurrence of antiparallel β‐sheet structure in these solvents. The CD spectra of the peptide in organic solvents are characteristic of those observed for cystine peptides that have been shown to adopt antiparallel β‐sheet structures.