Since peptide-based nanostructures are formed from the arrangement of individual molecules due to reversible noncovalent interactions, they provide ideal systems to develop stimuli-responsive materials. Inspired by self-directed biological systems, self-assembled nanostructures with controllable properties may be useful in designing materials that independently respond to changes in their environments or may be tuned to promote specific interactions with other molecules to form more complex multicomponent materials. Towards these goals, a pH-responsive self-assembled system was developed from a coumarin-appended tetrapeptide. The molecule was designed to contain multiple ionizable functional groups, enabling a range of protonation states and charges in response to changes in pH. Since electrostatic repulsive and attractive interactions can play a factor in how molecules stack together, the resulting system was shown by transmission electron microscopy and circular dichroism experiments to display differences in self-assembly morphology and helical chirality according to the pH of the environment. Three distinct self-assembly regimes were defined based on pH regions: positively charged left-handed helical nanotubes, negatively charged right-handed twisted nanoribbons, and negatively charged dispersed monomers. Because the nanotube and nanoribbon structures arose from differences in stacking on the molecular level, interconversion between the two forms was greatly facilitated by dispersing the structures to monomeric form. The conversion between these competing pathways could be initiated primarily by pH, but also with heat or ultra-sonication. In further work, when an oppositely charged dipeptide molecule was assembled into thin, flexible nanofibers in the presence of a more rigid nanotube template, a nanofiber-wrapped nanotube composite was formed, providing an example of the intricate structural control that can be obtained through stimulus-responsive self-assembly.

Controllable self-assembly pathway of a pH-responsive coumarin tetrapeptide towards directed assembly of multicomponent nanostructures

ACS Spring Meeting 2021

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