Nanofibers from Oxazolidi-2-one Containing Hybrid Foldamers: What is the Right Molecular Size?

A series of oligomers Boc-(L-Phe-D-Oxd)n-OBn consisting of the L-Phe-D-Oxd moiety as basic unit was prepared for n = 2-5 under liquid phase conditions (Oxd = 4-methyl-5-carboxy oxazolidin-2-one). The shortest oligomer Boc-(L-Phe-D-Oxd)2-OBn aggregates into a solid state and forms a fiber-like material with an anti-parallel beta-sheet structure, where the oligopeptide units are connected with each other by only one intermolecular hydrogen bond. The longer oligomers exhibit structural heterogeneity. They start to get organized in secondary structures by formation of intramolecular hydrogen bonds at the pentamer level. Microscopy and diffraction of the oligomers indicate a crystalline character only for the shorter ones.