Positional screening and NMR structure determination of side-chain-to-side-chain cyclized beta(3)-peptides

Many beta-peptides fold in a 14-helical secondary structure in organic solvents, but similar 14-helix formation in water requires additional stabilizing elements. Especially the 14-helix stabilization of short beta-peptides in aqueous solution is critical, due to the limited freedom for incorporating stabilizing elements. Here we show how a single lactam bridge, connecting two beta-amino acid side-chains, can lead to high 14-helix character in short beta(3)-peptides in water. A comparative study, using CD and NMR spectroscopy and structure calculations, revealed the strong 14-helix inducing power of a side-chain-to-side-chain cyclization and its optimal position on the beta(3)-peptide scaffold with respect to pH and ionic strength effects. The lactam bridge is ideally incorporated in the N-terminal region of the beta(3)-peptide, where it limits the conformational flexibility of the peptide backbone. The lactam bridge induces a 14-helical conformation in methanol and water to a similar extent. Based on the presented first high resolution NMR 3D structure of a lactam bridged beta(3)-peptide, the fold shows a large degree of high order, both in the backbone and in the side-chains, leading to a highly compact and stable folded structure.