The self-assembly of synthetic biomaterials, such as collagen peptides, can be harnessed for a range of biomedical applications. In an effort to obtain collagen-based macromolecular assemblies with temporal control, we designed a system that assembled only in the presence of external stimuli. We report a collagen triple helical peptide that is modified with a His_2 moiety on its C-terminus and a nitrilotriacetic acid unit on its N-terminus that rapidly and reversibly assembles in the presence of metal ions. Dynamic light scattering and turbidity experiments confirmed the presence of higher order aggregates in solution upon the introduction of Zn~(2+), Cu~(2+), Ni~(2+), and Co~(2+). This assembly process was found to be fully reversible using EDTA as a metal ion chelator. Control peptides that contain only a single ligand-modified terminus were not responsive to the same metal ions, thus demonstrating the requirement of both ligand modifications for peptide assembly. Scanning electron microscopy imaging of the peptide-metal assemblies revealed micrometer-sized florettes in addition to curved, stacked sheets. More detailed analysis of the Zn~(2+)-generated microflorettes showed that the surface of these particles contains ruffled structures with a highly dense surface area. Potential folding intermediates in the formation of the microflorettes were observed at lower temperatures and at early time points in the assembly that are composed of curved layered sheets. Significantly, the assembly process proceeded under mild conditions using neutrally buffered aqueous solution at room temperature. These microscopic structures offer opportunities in many areas, including drug delivery, tissue engineering, and regenerative medicine.
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