Tuning Assembly Size in Peptide-Based Supramolecular Polymers by Modulation of Subunit Association Affinity

Biomaterials via peptide assembly: Design, characterization, and application in tissue engineering

A core challenge in biomaterials, with both fundamental significance and technological relevance, concerns the rational design of bioactive microenvironments. Designed properly, peptides can undergo supramolecular assembly into dynamic, physical hydrogels that mimic the mechanical, topological, and biochemical features of native tissue microenvironments. The relatively facile, inexpensive, and automatable preparation of peptides, coupled with low batch-to-batch variability, motivates the expanded use of assembling peptide hydrogels for biomedical applications. Integral to realizing dynamic peptide assemblies as functional biomaterials for tissue engineering is an understanding of the molecular and macroscopic features that govern assembly, morphology, and biological interactions. In this review, we first discuss the design of assembling peptides, including primary structure (sequence), secondary structure (e.g., α-helix and β-sheets), and molecular interactions that facilitate assembly into multiscale materials with desired properties. Next, we describe characterization tools for elucidating molecular structure and interactions, morphology, bulk properties, and biological functionality. Understanding of these characterization methods enables researchers to access a variety of approaches in this ever-expanding field. Finally, we discuss the biological properties and applications of peptide-based biomaterials for engineering several important tissues. By connecting molecular features and mechanisms of assembling peptides to the material and biological properties, we aim to guide the design and characterization of peptide-based biomaterials for tissue engineering and regenerative medicine. STATEMENT OF SIGNIFICANCE: Engineering peptide-based biomaterials that mimic the topological and mechanical properties of natural extracellular matrices provide excellent opportunities to direct cell behavior for regenerative medicine and tissue engineering. Here we review the molecular-scale features of assembling peptides that result in biomaterials that exhibit a variety of relevant extracellular matrix-mimetic properties and promote beneficial cell-biomaterial interactions. Aiming to inspire and guide researchers approaching this challenge from both the peptide biomaterial design and tissue engineering perspectives, we also present characterization tools for understanding the connection between peptide structure and properties and highlight the use of peptide-based biomaterials in neural, orthopedic, cardiac, muscular, and immune engineering applications.

Engineering soluble artificial epidermal growth factor receptor mimics capable of spontaneous in vitro dimerization

Epidermal growth factor receptor (EGFR) is a clinically validated target for a multitude of human cancers. The receptor is activated upon ligand binding through a critical dimerization step. Dimerization can be replicated in vitro by locally concentrating the receptor kinase domains on the surface of lipid-based vesicles. In this study we investigated the use of coiled coils to induce spontaneous receptor kinase domain dimerization in vitro to form non-membrane-bound artificial receptor mimics in solution. Two engineered forms of EGFR kinase domain fused to coiled coil complementary peptides were designed to self-associate upon mixing. Two fusion protein species (P3-EGFR and P4-EGFR) independently showed the same activity and polymerization profile known to exist with EGFR kinase domains. Upon mixing the two species, coiled coil heterodimers were formed that induced EGFR association to form dimers of the kinase domains. This was accompanied by 11.5-fold increase in the phosphorylation rate indicative of kinase domain activation equivalent to the levels achieved using vesicle localization and mimicking in vivo ligand-induced activation. This study presents a soluble tyrosine kinase receptor mimic capable of spontaneous in vitro activation that can facilitate functional and drug discovery studies for this clinically important receptor class.

Polymers with controlled assembly and rigidity made with click-functional peptide bundles

The engineering of biological molecules is a key concept in the design of highly functional, sophisticated soft materials. Biomolecules exhibit a wide range of functions and structures, including chemical recognition (of enzyme substrates or adhesive ligands¹, for instance), exquisite nanostructures (composed of peptides², proteins³ or nucleic acids⁴), and unusual mechanical properties (such as silk-like strength³, stiffness⁵, viscoelasticity⁶ and resiliency⁷). Here we combine the computational design of physical (noncovalent) interactions with pathway-dependent, hierarchical 'click' covalent assembly to produce hybrid synthetic peptide-based polymers. The nanometre-scale monomeric units of these polymers are homotetrameric, α-helical bundles of low-molecular-weight peptides. These bundled monomers, or 'bundlemers', can be designed to provide complete control of the stability, size and spatial display of chemical functionalities. The protein-like structure of the bundle allows precise positioning of covalent linkages between the ends of distinct bundlemers, resulting in polymers with interesting and controllable physical characteristics, such as rigid rods, semiflexible or kinked chains, and thermally responsive hydrogel networks. Chain stiffness can be controlled by varying only the linkage. Furthermore, by controlling the amino acid sequence along the bundlemer periphery, we use specific amino acid side chains, including non-natural 'click' chemistry functionalities, to conjugate moieties into a desired pattern, enabling the creation of a wide variety of hybrid nanomaterials.

Redox controlled reversible transformation of a supramolecular alternating copolymer to a radical cation containing homo-polymer

Viologen-tryptophan ternary complexation inside CB[8] is used to prepare a supramolecular polymer which show redox controlled transformation from homo polymer to hetero polymer.

Self-sorting heterodimeric coiled coil peptides with defined and tuneable self-assembly properties

Coiled coils with defined assembly properties and dissociation constants are highly attractive components in synthetic biology and for fabrication of peptide-based hybrid nanomaterials and nanostructures. Complex assemblies based on multiple different peptides typically require orthogonal peptides obtained by negative design. Negative design does not necessarily exclude formation of undesired species and may eventually compromise the stability of the desired coiled coils. This work describe a set of four promiscuous 28-residue de novo designed peptides that heterodimerize and fold into parallel coiled coils. The peptides are non-orthogonal and can form four different heterodimers albeit with large differences in affinities. The peptides display dissociation constants for dimerization spanning from the micromolar to the picomolar range. The significant differences in affinities for dimerization make the peptides prone to thermodynamic social self-sorting as shown by thermal unfolding and fluorescence experiments, and confirmed by simulations. The peptides self-sort with high fidelity to form the two coiled coils with the highest and lowest affinities for heterodimerization. The possibility to exploit self-sorting of mutually complementary peptides could hence be a viable approach to guide the assembly of higher order architectures and a powerful strategy for fabrication of dynamic and tuneable nanostructured materials.

Supramolecular association of 2D alumino-siloxane aquagel building blocks to 3D porous cages and its efficacy for topical and injectable delivery of fluconazole, an antifungal drug

A fascinating 2D architecture of an alumino-siloxane gel self-assembled into a well-defined 3D porous aquagel cage. It is identified to be an excellently mechanically stable, injectable and non-cytotoxic medium for drug delivery applications.

Origins of structural flexibility in protein-based supramolecular polymers revealed by DEER spectroscopy

Modular assembly of bio-inspired supramolecular polymers is a powerful technique to develop new soft nanomaterials, and protein folding is a versatile basis for preparing such materials. Previous work demonstrated a significant difference in the physical properties of closely related supramolecular polymers composed of building blocks in which identical coiled-coil-forming peptides are cross-linked by one of two subtly different organic linkers (one flexible and the other rigid). Herein, we investigate the molecular basis for this observation by isolating a single subunit of the supramolecular polymer chain and probing its structure and conformational flexibility by double electron-electron resonance (DEER) spectroscopy. Experimental spin-spin distance distributions for two different labeling sites coupled with molecular dynamics simulations provide insights into how the linker structure impacts chain dynamics in the coiled-coil supramolecular polymer.