Peptide bond isosteres: ester or (E)-alkene in the backbone of the collagen triple helix.

[structure: see text] Collagen is the most abundant protein in animals. Interstrand N-H...O=C hydrogen bonds between backbone amide groups form a ladder in the middle of the collagen triple helix. Isosteric replacement of the hydrogen-bond-donating amide with an ester or (E)-alkene markedly decreases the conformational stability of the triple helix. Thus, this recurring hydrogen bond is critical to the structural integrity of collagen. In this context, an ester isostere confers more stability than does an (E)-alkene.