# Stabilization of \beta -Turn Conformations in Pro-X Sequences by Disulphide Bridging. Synthesis and Solution Conformations of Five Cyclic Cystine Peptides

Ravi, A and Balaram, P (1984) Stabilization of \beta -Turn Conformations in Pro-X Sequences by Disulphide Bridging. Synthesis and Solution Conformations of Five Cyclic Cystine Peptides. In: Tetrahedron, 40 (13). pp. 2577-2583.

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Five cyclic peptide disulphides of the type Boc-Cys-Pro-X-Cys-NHMe have been synthesized, where X = Gly (1), L-Ala (2), D-Ala (3), Aib (4) and L-Leu (5). 1H NMR studies at 270 MHz in $CD{Cl}_3$ and ${({CD}_3)}_2SO$ provide evidence of a Pro-X \beta-turn conformation, stabilized by a transannular 4 \rightarrow 1 hydrogen bond involving the Cys(4) NH, in all the peptides. In addition peptides 2, 4 and 5 also possess a second intramolecular hydrogen bond involving the -NHMe group. The spectroscopic data are consistent with a consecutive type III \beta-turn conformation for peptides 2, 4 and 5, a type I(III) \beta-turn structure for 1 and a type II turn for 3.