Objectives: Controlling interactions in synthetic polymers as precisely as in proteins would have a strong impact on polymer science. Advanced structural and functional control can lead to rational design of, integrated nano- and microstructures. To achieve this, properties of oligopeptides were exploited. By incorporating these as monodisperse segments into synthetic polymers we show how to program structure formation in polymers, control inorganic-organic interfaces in fiber composites, induce structure in biomacromolecules for biomedical applications and generate bioactive surfaces to control biological systems (Fig. 1).
I. Functional hybrid polymers (bioconjugates)
The precise control of (dynamic) nano- and microstructures in synthetic polymer materials was identified as one key-factor to achieve advanced functional control. However if the structural and functional diversity existing in biology is compared with that observed in polymer science, it is evident that the capabilities of polymer chemists are still limited.
Concepts present in biological systems demonstrate that an enormous structural variability can be accessed via rather simple and practicable means, i.e. by combining 20 simple building blocks (native α-amino acids) in linear chains of polypeptides. In contrast to this, polymer chemists were progressively increasing the complexity of synthetic polymers. However this development is contrary to the minimalist concept of biology that - instead of increasing the complexity - increases the information content encoded in a linear polymer chain.
Polypeptides are a good example because they have a defined monomer sequence, combined with the absence of both chemical and molecular weight distributions. This enables one to precisely control secondary interactions along the peptide chain, allowing to encode distinct organization processes in the monomer sequence. This highlights the differences between biomacromolecules and „common” block copolymers, since the latter exhibit usually elementary block-block interactions e.g. based on selective solubility, only.
Inspired by the simple, molecular concepts present in structural biology the integration of bio-segments into the existing world of synthetic polymers is investigated and exploited. For that we are focusing on sequence-defined oligopeptides because this class possesses a stable amide backbone, and the synthesis proceeds on fully automated synthesizer platforms in up to gram scales.
In order to selectively introduce peptides into synthetic polymers, new routes had to be developed, applying two main strategies: i. The polymerization strategy, in which the polymer segment is synthesized in the presence of the peptide.2-4 This approach includes the sequence specific introduction of an initiator or chain transfer functionality to a peptide. The resulting macroinitiator or macrotransfer agent was applied in controlled radical polymerization (CRP) processes, such as atom transfer radical polymerization (ATRP) or reversible addition-fragmentation chain transfer polymerization (RAFT). This allows the synthesis of well-defined conjugates with controllable molecular weights and polydispersities as low as 1.1. ii. The coupling approach, including the regio-selective coupling of a polymer that exhibits a defined chain-end functionality with a complementary functionality in a peptide segment.
II. Bioconjugates as models to program macromolecular interaction capabilities
AB-Block copolymers composed of a common synthetic polymer part and a monodisperse, monomer sequence-defined segment are considered belonging to the class of „Precision Polymers” (cf. Figure 2a). Due to the freely programmable monomer sequence and the discrete nature of the peptide segment in those polymers, the possibility arises to study and program macromolecular interaction capabilities (cf. Figure 2b). This can be exploited to i. specifically solubilize and transport low molecular weight drugs, ii. to pack DNA, iii. to direct crystallization or iv. to modulate adhesion behavior of cells on material surfaces. The systematic variation of a well-defined functional segment in these precision polymer might contribute to the fundamental understanding of both sequence-property relationships of functional polymers and the behavior of functional macromolecules in complex environments e.g. in biological systems.
The class of „precision polymers” has been inspired by proteins, but it is the advantage of polymer chemistry to further expand the set of building blocks toward fully synthetic monomers. New monomer sets can be established in order to meet requirements, which result from non-aqueous environments, higher temperatures or established processing/production pathways. As evident from biological macromolecules e. g. from proteins, the room to improve the preciseness of synthetic macromolecules will offer plenty of opportunities.
II. Bioconjugates to program structure and function
Peptides combine self-assembly properties with the potential to actively interact with biological systems. Hence, the resulting peptide-polymer conjugates can be used to program structure formation in polymeric materials, allowing for the rather direct realization of bio-inspired polymer science.
We exploited the biological concept of peptide-guided structure formation for the organization of synthetic polymers, using different peptide-based organizer units (Fig. 3). Particularly, the peptide organization in form of the β-sheet secondary structure motif was investigated. Thus, highly attractive, anisometric tape, fibrillar or fiber-like nanostructures can be accessed. These are important structural and functional elements in both native and synthetic materials that i.e. provide anisotropic strength and elasticity or directed transport.
As outlined in Figure 3 the peptide organizer segment in a peptide-polymer conjugate induces and controls the microstructure formation. Thus, different peptide organizers result in different structures ranging from macro- to nanotapes and nanotubes (cf. Fig. 3 a,b,c).
Linear peptide organizers: The synthesis of extended and robust nanofibers, interesting for material science, requires peptides with strong tendencies to form stable aggregates. These, however, are usually difficult to access. Recently, the SWITCH-strategy of integrating defined defects into the peptide backbone was developed to overcome these obstacles. The defects, referred to as „switch”-segments, temporarily suppress the aggregation tendency of a peptide. However, the native peptide can be reestablished via a selective rearrangement in the switch segments, restoring the aggregation tendency.
Such switch segments have been shown to be highly useful for the peptide-guided organization of synthetic polymers (Fig. 4a), as the rate of switching can be adjusted to control the aggregation kinetics. Using the switch-strategy, PEO-peptide conjugates organized in water into macro-tapes with up to several millimeters in length (Fig. 2a and 4b).
Mimicking adaptive biomaterials
Hierarchical purpose-adapted composites: Biological inorganic-organic materials from bones to glass sponge fibers are high performance, nanofiber-directed composites with tailored properties. Modern supramolecular chemistry offers -probably for the first time- the possibility to mimic such complex systems. This can lead on the one hand to an increased insight into interactions of organic and inorganic building blocks during such co-assembly processes. On the other hand, new adaptive materials might be realizable, allowing organic chemistry to design functional systems that control the generation of integrated composites.
The development of defined peptide-polymer conjugates allows addressing pharmacological and biomedical issues. However, to avoid the inherent immunogenicity of peptides, a novel synthesis route to linear poly(amido amines) (PAMAMs) was developed. This enables the synthesis of monodisperse, sequence defined PAMAMs. The cationic character (balance of tert., sec., and prim. amine groups) of the PAMAM segment can be fine-tuned with monomer resolution, making the PAMAMs - if conjugated to PEO - highly interesting for gene delivery applications. PEO-PAMAMs are compounds with sharp property profiles, allowing the correlation of e.g. the cationic balance with DNA complexation and compression properties (Fig. 7) as well as membrane translocation and transfection activities.
It is predictable that polymer chemistry with its inherent molecular weight distributions will evolve to macromolecular chemistry with precisely defined molecules. Hence, the synthesis of fully synthetic, monodisperse polymers with defined monomer sequences will be the upcoming challenge in polymer science. Completely unnatural polymer classes might be developed, which combine novel units capable of specific molecular recognition with new monomer alphabets to fine-tune secondary interactions along linear polymer chains. Applying such high precision polymers in niche design and as protein mimics allows envisioning progress in the fundamental understanding of the interface between synthetic materials and biological systems.