Fig. 2.

Peptides with φ,ψ-angles in the bridge. (A) A clash-free amide-water hydrogen bond. (B) A Ramachandran dipeptide with φ,ψ-angles in the bridge causes unavoidable steric clashes between acetyl carbons (green spheres) and water (aqua sphere), rendering the N-methyl hydrogen solvent-inaccessible and depriving it of a hydrogen-bond acceptor. Clashing atoms are shown as overlapping van der Waals spheres; the backbone chain is shown as a stick model. (C) A type I turn, illustrating how a longer range, intramolecular hydrogen-bond acceptor can satisfy the amide hydrogen without steric interference. (D) A local side chain (serine)—backbone hydrogen bond can also satisfy the amide hydrogen without steric interference. Atom color code: carbon: green; peptide oxygen: red; nitrogen: blue; hydrogen: white; water: aqua.