Strained-cyclophane-induced β-turn template:: Design, synthesis, and spectroscopic characterization

Three tetrapeptides incorporating a 14-membered (Ri+1, Si+2) cycloisodityrosine at the i + 1 and i + 2 positions were designed and synthesized. Conformational analysis by H-1 NMR and CD spectra as well as molecular modeling indicated that they all adopt a beta-turn conformation. While the CD spectrum of compound 2 is characteristic of the typical type-II beta-turn (maximum at similar to200 nm and a minimum at similar to220 nm), that of 1a (atropisomer of 2) is opposite in sign to the expected spectrum of the type-II beta-turn.