Amyloid-like Fibrils from a Diphenylalanine Capped with an Aromatic Fluorenyl

The self-assembly behavior of a diphenylalanine amphiphile blocked at the C-terminus with a 9-fluorenylmethyl ester and stabilized at the N-terminus with a trifluoroacetate (TFA) anion, TFA·FF-OFm, has been examined. At low peptide concentration (0.5 mg/mL), long amyloid-like fibrils, which come from the fusion of two or more helical ribbons and/or thinner fibrils, organized in bundles or as individual entities are detected. Microbeam synchrotron radiation infrared spectroscopy has shown that TFA·FF-OFm molecules in amyloid-like fibrils arrange, forming antiparallel β-sheets. Alteration of the experimental conditions to prioritize the thermodynamic contribution with respect to the kinetic one in the self-assembly process inhibits the organization of amyloid-like structures in favor of the formation of conventional fibrous structures. On the basis of experimental observations, a structural model where the individual antiparallel β-sheets are oriented in parallel has been proposed for TFA·FF-OFm amyloid-like fibrils.