Modulation of fluorescence through coassembly of molecules in organic nanostructures

This paper describes the fluorescence of bimolecular coassemblies that form one-dimensional nanostructures. One molecule is a fluorescent peptide amphiphile containing its branched stilbene chromophore covalently linked to the hydrophilic end of the amphiphile, and the second molecule is a shorter, nonfluorescent peptide amphiphile of complementary charge. Using circular dichroism we observe that mixing both molecules results in coassemblies that exhibit a beta-sheet signature in the peptide region indicative of these types of nanostructures. The nature of the coassembly is dependent on the molar ratio of each component, and the changing CD spectra suggest the formation of domains along the length of the nanofibers with decreasing concentrations of the fluorescent component. In coassemblies with dilute concentrations of the fluorophore, we observe an increase in fluorescence intensity and quantum yield, as well as chiral transfer to the achiral segment of the fluorescent peptide amphiphile. The coassemblies studied containing a fluorescent component at a low molar ratio exhibit fluorescence resonance energy transfer to fluorescent acceptors in solution. When the nonfluorescent peptide amphiphile component is designed to bind the important bioactive polysaccharide heparin, a selective transfer of energy is observed between fluorescein-tagged heparin and the coassemblies in both dilute solution and in macroscopic gels.