Sculpting nanofibers of a bolaamphiphilic peptide through sonication, and quantifying its interaction with anions

Abstract

We report ultrasonication-induced fibril-formation by a bolaform peptide, 1. While undisturbed cooling of hot THF solution of 1 led to the formation of microcrystals, bath-sonication of a similar solution resulted in predominant fibrillation with concomitant gelation. Spectroscopic studies indicated that 1 assembled predominantly by H-bonding interactions between the amide groups. Interestingly, we did not find any evidence of conformational change in 1 during sonication through a variety of techniques employed. However, the morphology of assemblies of 1 could be readily modified in a controlled fashion via sonication at early crystal growth stage. Exposing the resulting nanofibrillar assemblies to certain anions dismantled them, and induced macroscopic gel-to-sol transition. The relative rates of this disassembly were governed by the size of anions and their binding constants with 1. ¹H NMR titration experiment revealed strong interaction of fluoride and chloride anions with the amide protons of 1. The binding constants of the fluoride and chloride ions with 1 were calculated for the first time in this class of molecules, and were found to be 120 and 33 M⁻¹ respectively, correlating with the basicity of these ions. Deprotonation of 1 and evidence of HF₂⁻ formation was also observed at high fluoride concentrations. We further demonstrated that the fibers disassembled by fluoride ions could be readily reassembled upon sonication in presence of proton donor such as trifluoroacetic acid.