STABILIZATION OF j? -TURN CONFORMATIONS IN PRO-X SEQUENCES BY DISULPHIDE BRIDGING. SYNTHESIS AND SOLUTION CONFORMATIONS OF FIVE CYCLIC CYSTINE PEPTIDES

T 9 S synthesized, where X = Gly (l), L-Ala (2), D-Ala (3), Aib (4) and L&U (5). ‘H NMR studies at 270 MHz in CDCI, and (CD@0 provide evidence of a Pro-X /?-turn conformation, stabilized by a transannular 4-l 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 b-turn conformation for peptides 2, 4 and 5, a type I(II1) b-turn structure for 1 and a type II turn for 3.