pH-sensitive morphological transition from nanowire to nanovesicle of a single amino acid-based water soluble molecule

Employment of single amino acid-based molecules for the construction of various nanostructures is a challenging issue. Moreover, it is fascinating to see the controlled fabrication of specific nanostructures from the self-assembly of molecular building blocks through the fine tuning of pH of the solution. The present study demonstrates pH-responsive nanostructural transformation of single amino acid-based nanostructures from nanowires to nanovesicles. The molecules have been developed by coupling the carboxyl group of natural amino acids with the amino group of unnatural m-aminobenzoic acid (m-ABA), such as NH2–Xx–m-ABA–CO2H, where Xx are amino acids Phe, Tyr, Gly, and Pro. The presence of m-aminobenzoic acid helps in self-assembly through insertion of conformational constraints in the peptide backbone and also through aromatic π–π interactions. The insertion of m-aminobenzoic acid also induces proteolytic stability in the nanostructures. The formation of nanowires has been observed at acidic pH (pH 4.2–6.0), while both nanowires and nanovesicles are formed simultaneously at nearly neutral pH (pH 6.4). With an increase in the pH of the solution, only one nanoscopic species, i.e., nanovesicles have been formed exclusively, and these nanovesicles are stable within the range of pH 7.0–9.1. A further increase in pH (pH 10) triggers the disruption of nanovesicular structures that starts at the peripheral wall of the vesicles and is completed after total disintegration at pH 11. Studies show that alkali metal salt KCl can also disrupt these nanovesicles efficiently. These peptide-based nanovesicles can encapsulate a potent drug curcumin, which can be effectively released through the disruption of the vesicles either in presence of KCl or alkaline pH. Single crystal X-ray diffraction analysis indicates the sheet-mediated self-assembly in the formation of different nanostructures. Studies also show that the tyrosine-based peptide nanovesicles are elegant hosts for in situ preparation and stabilization of silver nanoparticles.