Structure of a cyclic dimer of amino undecanoic acid as a model of folding and hydration in polyamides, polypeptides, and related substances.

Aliphatic amides are often used in the synthesis of peptidomimetic compounds. Here we present the structure of two cyclic dimers of aminoundecanoic acid as determined by x-ray diffraction. Each dimer contains two peptide groups and twenty methylene units. In one of the crystal structures, water is associated with the peptide groups, forming a chain of hexagons similar to those found in crambin, and in other protein and nucleic acid crystals. The aminoundecanoic rings show a fold at the peptide group, similar to either beta-turn type III or V found in proteins. Such folds are an adequate model for the peptide bond structure in nylon crystallites and peptidomimetic compounds.