Modeling favorable and unfavorable helix flanking conformations. Stereochemical constraints on flanking positions of a model αL helix (φ = 65°, ψ = 42°). (A) Unfavorable flanking conformations. Placing an Ncap residue in the α-R conformation occludes solvation of the N2 amide by the N' sidechain. A C' residue in the βR conformation causes a steric clash between the C' and C3 carbonyls. (B) Favorable flanking conformations. An N' in the βR conformation removes any desolvation of the Ncap-N2 amides (shown as spheres). An αR C' replaces the carbonyl clash to C3 with a bivalent hydrogen bond from the C' and C" amides. Carbons of residues in the αL conformation are colored orange.