Structure of a Cyclic Trimer as a Model of Nylon 11. Folding at Lamellar Surfaces

We have studied by single-crystal X-ray diffraction the structure of a cyclic trimer model of nylon 11 (IUPAC name: 1,13,25-triazacyclohexatriacontane-2,14,26-trione). The crystal is organized in layers of hydrogen-bonded rings similar to nylon lamellar crystals. The rings fold both at the amide and methylene regions. Neighbor layers show van der Waals contacts among similar turns, either methylene/methylene or amide/amide. A new type of fold has been found, which includes an amide group and a methylene unit. Such folds may coexist in nylon lamellar crystals with conventional turns similar to the β-turn found in proteins. The orientation of up and down chains in the crystal is different from the orientation usually assumed for nylon 11, which suggests an alternative unit cell for nylon 11. We also include an analysis of the vertical displacement of hydrogen bonded layers found by different authors in nylon 11, which shows that it may vary between zero and three methylene units. In the trimer crystal we f...