# Cystine peptides: The intramolecular antiparallel \beta sheet conformation of a 20-membered cyclic peptide disulfide

Kishore, R and Raghothama, S and Balaram, P (1987) Cystine peptides: The intramolecular antiparallel \beta sheet conformation of a 20-membered cyclic peptide disulfide. In: Biopolymers, 26 (6). pp. 873-891.

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A 20-membered cyclic peptide disulfide has been synthesized as a conformational model for disulfide loops of limited ring size. 1H-nmr studies at 270 MHz establish the presence of three intramolecular hydrogen bonds involving the Leu, Val, and methylamide NH groups in $CDCl_3$. Evidence for peptide aggregation in $CDCl_3$ is also presented. A structural transition involving loosening of the hydrogen bond formed by the Val NH group is observed upon the measured addition of $(CD_3)_2SO$ to $CDCl_3$. Hydrogen-bonding studies, together with unusually low field positions of the Cys(1) and Cys(6) C H resonances and high $J_{HNC.H}$ values provide support for an intramolecular antiparallel \beta-sheet conformation, facilitated by a chain reversal at the Aib-Ala segment. Extensive nuclear Overhauser effect studies provide compelling evidence for the proposed conformation and also establish a type I'\beta-turn at the Aib-Ala residues, the site of the chain reversal.