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

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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 ψ= 162°. In the disulfide bridge Cα (1)‐Cβ (1)‐S(1)‐(3′)‐Cβ(3′)‐Cα(3′), S—S = 2.030 Å, angles Cβ 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%.