Conformational preferences of peptides containing reverse-turn mimetic bicyclic lactams: Inverse gamma-turns versus type-II ' beta-turns - Insights into beta-hairpin stability

The solid-phase synthesis and characterization of a series of peptides (3-9), containing reverse-turn mimetic bicyclic lactams (1a, 1b), was reported in the preceding paper. The bicyclic lactams (1a, 1b) possess high structural similarity to the two central residues of a beta-turn. The conformational preferences of the constrained peptides have been investigated by NMR spectroscopy and IR spectroscopy. Our experimental results have been complemented by computer modelling studies and show that the constrained peptides (3-9) form an inverse gamma-turn or a type-II' beta-turn through intramolecular hydrogen bonding, depending on the nature of the reverse-turn mimic. In N-acetylated tetrapeptide mimics incorporating the two different bicyclic lactams (a series and b series), H-5 is available for either a gamma-turn (7-membered ring with the carbonyl group of the bicyclic lactam) or a beta-turn (10-membered ring with the carbonyl group of residue 2), as shown in Figures 7 and 9. The a series incorporating the (5,7)-bicyclic lactam predominantly induces the gamma-turn conformation, while the b series incorporating the (5,6)-bicyclic lactam can promote either a gamma-turn or a beta-turn conformation, with the beta-turn usually being preferred and with varying degrees of beta-hairpin formation.