Cystine peptides antiparallel β-sheet conformation of the cyclic biscystine pep tide [Boc-Cys-Ala-Cys-NHCH₃]₂

Abstract

The crystal structure analysis of the cyclic biscystine peptide [Boc-Cys¹-Ala²-Cys³-NHCH₃]₂ with two disulfide bridges confirms the antiparallel β-sheet conformation for the molecule as proposed for the conformation in solution. The molecule has exact twofold rotation symmetry. The 22-membered ring contains two transannular NH = OC hydrogen bonds and two additional NH = OC bonds are formed at both ends of the molecule between the terminal (CH₃)₃COCO and NHCH₃ groups. The antiparallel peptide strands are distorted from a regularly pleated sheet, caused mainly by the L-Ala residue in which φ = -155° and ◖ = 162°. In the disulfide bridge C^α (1)-C^β (1)-S(1)-(3′)-C^β(3′)-C^α(3′), S-S = 2.030 Å, angles C^β SS = 107° and 105°, and the torsional angles are -49, -104, +99, -81, -61°, respectively. The biscystine peptide crystallizes in space group C2 with a = 14.555(2) Å, b = 10.854(2) Å, c = 16.512(2)Å, and β = 101.34(1) with one-half formula unit of C₃₀H₅₂N₈O₁₀S₄· 2(CH₃)₂SO per asymmetric unit. Least-squares refinement of 1375 reflections observed with |F| > 3σ(F) yielded an R factor of 7.2%.