Hydrogen Bonds Determining the Conformation of Isolated Amino Acid Residues in Proteins: Re-Examination of the Fully Extended C5 Structures.

Hydrogen bonds contribute substantially to the stabilisation of specific three-dimensional structures of peptides. The C5 hydrogen bond is an intraresidue interaction, which appears to contribute to the stabilisation of fully extended residue conformations, with backbone torsion angles (φ, ψ) lying close to ±180°. The present study revisits the occurrence of C5 conformations in proteins, using a high-resolution, non-homologous dataset of 6924 x-ray diffraction structures (sequence identity < 30%, resolution < 1.6 Å). Gly emerges as the residue with the highest propensity to adopt C5 conformations, followed by Ser. Residues with branched sidechains Leu, Val and Ile are strongly disfavoured. Specific examples in protein structures that have contiguous residues in C5 conformations are highlighted.