Extremely tough cyclic peptide nanopolymers
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MRS Advances © 2019 Materials Research Society DOI: 10.1557/adv.2019.363
Extremely tough cyclic peptide nanopolymers Manoj K. Kolel-Veetil, LCDR Luis Estrella, Christopher R. So and Kenan P. Fears Chemistry Division, US Naval Research Laboratory, Washington, DC 20375.
ABSTRACT: We present a new class of bioinspired nanomaterials that are stabilized by a combination of covalent and hydrogen bonds. Prior work by others has shown that cyclic peptides can self-assemble to form supramolecular assemblies through backbone-backbone hydrogen bonding. To improve upon this molecular architecture, we develop a synthesis route to polymerize cyclic peptides and form a linear polymer chain that can transition between a rigid nanorod and an unfolded conformation. For a cyclic peptide polymer containing amine-terminated side chains on each ring, we demonstrate self-assembly can be triggered in aqueous solutions by varying the pH. We measure the elastic modulus of the rigid nanorods to be ca. 50 GPa, which is comparable to our molecular dynamics (MD) prediction (ca. 64 GPa). Our results highlight the uniqueness of our molecular architecture, namely their exemplary toughness (up to 3 GJ m-3), in comparison to other cyclic peptide-based assemblies. Finally, we demonstrate amphiphilic cyclic β-peptides are capable of inhibiting the growth of gram-negative and gram-positive bacteria.
INTRODUCTION: Nature is replete with examples of nanostructured biopolymers that comprise unique molecular motifs and architectures leading to exceptional material properties. Generally, these biopolymers consist of a linear aggregation of proteins that provide mechanical strength and functionality for numerous intracellular and extracellular functions [1-5]. While we contemplate and admire nature’s creativity, we are also keenly cognizant of the evolutionary time scale that nature had at its disposal to exhibit such artistry. Thus, as practitioners of synthetic methodologies, for us to create manufactured materials that surpass the performance of natural materials, we must borrow and combine beneficial and disparate ideas from nature. Specifically, in quantifying the mechanical properties of natural biopolymers, there appears to be a broad bifurcation in the properties of such systems: a group comprising stiff polymers of low extensibility [6] and the other, comprising supple polymers with high extensibility [7]. Similarly, while manufactured materials in general have excelled in either stiffness (carbon nanotubes) [8] or extensibility (rubber) [9], the quest to combine the two properties has obviously lead researchers to look for inspiration from nature. In the early 90s, Ghadiri et al. cyclized linear chains of amino acid residues, and demonstrated cyclic peptides can spontaneously self-assemble into nanotubes wherein the assemblies are stabilized by intermolecular hydrogen bonds between the backbone of adjacent peptides [10,11]. Additionally, it was determined that the positive summation of hydrogen bonds in such peptide nanotubes bestows an enhanced
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