D,L-Cyclic Peptides as Structural Materials

Abstract

The bioengineer has a choice of building with proteins, peptides, polymers, nucleic acids, lipids, metals and minerals, each class containing tremendous diversity within its category. While the platforms are diverse, they can be unified by a common goal: to engineer nano- and micro-scale order to improve functionality. In doing so, self-assembling systems aim to bring the lessons learned from the order in natural systems83 into the therapeutics, materials, and electronics that society uses every day. The rigid geometry and tunable chemistry of D,L-cyclic peptides make them an intriguing building-block for the rational design of nano- and microscale hierarchically structured materials. Herein, we utilize a combination of electron microscopy, nanomechanical characterization including depth sensing-based bending experiments, and molecular modeling methods to obtain the structural and mechanical characteristics of cyclo-[(Gln-D-Leu)4] (QL4) assemblies. QL4 monomers assemble to form large, rod-like structures with diameters up to 2 μm and lengths of 10s to 100s of μm. Image analysis suggests that large assemblies are hierarchically organized from individual tubes that undergo bundling to form larger structures. With an elastic modulus of 11.3 ± 3.3 GPa, hardness of 387 ± 136 MPa and strength (bending) of 98 ± 19 MPa the peptide crystals are among the most robust known proteinaceous micro- and nano-fibers. The measured bending modulus of micron-scale fibers (10.5 ± 0.9 GPa) is in the same range as the Young’s modulus measured by nanoindentation indicating that the robust nanoscale network from which the assembly derives its properties is preserved at larger length-scales. Materials selection charts are used to demonstrate the particularly robust properties of QL4 including its specific flexural modulus in which it outperforms a number of biological proteinaceous and non-proteinaceous materials including collagen and enamel. We then demonstrate a composite approach to mechanical reinforcement of polymeric systems by incorporating synthetic D,L-cyclic peptide nanotube bundles as a structural filler in electrospun poly D-, L-lactic acid fibers. With 8 wt% peptide loading, the composite fibers are >5-fold stiffer than fibers composed of the polymer alone, according to AFM-based indentation experiments. The facile synthesis, high modulus, and low density, and reinforcing capabilities of QL4 fibers indicate that they may find utility as a filler material in a variety of high efficiency, biocompatible composite materials. This study represents the first experimental mechanical characterization of D,L-cyclic peptide assemblies or composites.