Functional polypeptide and hybrid materials: Precision synthesis via alpha-amino acid N-carboxyanhydride polymerization and emerging biomedical applications

Polypeptides derived from naturally occurring a-amino acids have emerged as a unique and versatile family of bio-inspired biomaterials that can be tailor-made for varying biomedical applications such as controlled drug release, gene delivery, tissue engineering and regenerative medicine. In contrast to traditional biodegradable polymers such as aliphatic polyesters and polycarbonates, polypeptides are inherently functional, allow precise control over polarity and charges, show excellent stability against hydrolysis, and are prone to rapid biodegradation in vivo by specific enzymes. Ring-opening polymerization (ROP) of alpha-amino acid N-carboxyanhydrides (NCAs) is the most straightforward and practical approach for large-scale production of high molecular weight polypeptides. In the past decade, a remarkable progress has been made in controlled NCA polymerization, which offers an unprecedented access to precision polypeptide and hybrid materials by combining with living radical polymerization, click chemistry, and/or post-polymerization modification. Notably, several micellar anti-cancer drugs based on poly(ethylene glycol)-polypeptides have been already advanced to the clinical evaluation. In this review paper, we give an overview on de novo design, controlled synthesis and emerging biomedical applications of functional polypeptide and hybrid materials. (C) 2013 Elsevier Ltd. All rights reserved.

Solution behavior of peptide and peptide-hybrid materials

Harald Nuhn

Peptide based and peptide hybrid materials represent two interesting and challenging classes of biomaterials. Both classes consist of and contain, respectively, one or more polypeptide blocks, endowing the resulting polymers with enhanced self-assembly properties, biocompatibility or novel characteristics. Peptide based materials, solely comprised of amino acids, benefit from their advanced self-assembly properties. In the case of peptide hybrid materials, being conjugates of peptides and synthetic polymers, either the polypeptide or the synthetic polymer block can drive the self-assembly process. Furthermore, the latter class benefits from combining the advantages of peptides and synthetic polymers, thus overcoming limitations related to the use of the individual components. The application of these materials is still challenging because it is not possible to predict their final self-assembly behavior based on their design. In order to facilitate a target-oriented design of novel materials for application such as drug delivery, tissue engineering, or self-healing materials, a detailed understanding of the complex interactions within and between these molecules is essential. The work presented in this thesis addresses these fundamental questions. Over the last decades, a plethora of peptide and peptide hybrid materials has been reported. Chapter 1 gives an overview of selected articles describing different synthesis routes for peptide as well as peptide hybrid materials, showing examples of molecules and their self-assembly into superstructures, and highlighting recent applications. Chapter 2 describes the synthesis and characterization of different peptide hybrid diblock oligomers composed of two classes of peptide sequences covalently attached to a poly(ethylene glycol) (PEG) block. The first class of peptide sequences contains the intrinsic propensity to form coiled coils, whereas the second class is composed of "switch" peptides and is able to adopt both amphiphilic α-helical and β-strand conformations. Upon dissolution, the first class yielded superstructures comprised of dimeric and tetrameric coiled coil aggregates, whereas the second class formed fibrillar assemblies. In both cases, the self-assembly properties were driven by the peptide block, and the final superstructure consisted of a peptidic core wrapped by the synthetic polymer block. Chapter 3 presents a detailed study on the aqueous solution self-assembly of two poly(styrene)n-b-poly(L-lysine)m peptide hybrid oligomers. The aim was to clarify whether the observed rod-like micelles, obtained by direct dissolution of the diblock oligomers in aqueous solution, reflect intrinsic self-assembly properties of the hybrid diblock oligomers or whether they present structures that are energetically trapped due to the glassy nature of the polystyrene block. To address this question, the influence of a variety of sample preparation techniques, including the use of organic solvents, dialysis from an organic solvent and several thermal treatments, was investigated. All treatments resulted in solution aggregates that differed in size and shape from samples prepared by direct dissolution in aqueous solution. Thus, it seems likely that direct dissolution of polystyrene-b-poly(L-lysine) diblock oligomers in aqueous solution results in kinetically trapped, rod-like aggregates, which can be transformed into non-spherical micelles by the appropriate treatment. Chapter 4 contains a comprehensive study on peptide-based materials, investigating four different series of short elastin-like peptides (ELPs) based on the GVGVP motif. The objective was to test whether and how defined changes in the primary structure of short ELPs affect the secondary structure and the lower critical solution temperature (LCST) behavior. Therefore, the number of repeats was altered, and within one peptide of constant length, all valines were successively substituted by the more hydrophobic amino acids isoleucine, leucine and phenylalanine, respectively. In parallel, a theoretical approach based on the partition coefficient log P was used to predict the shift of the LCST as function of chain length and composition. For short ELPs, the LCST is strongly affected by changes in the primary structure. This has also been predicted by calculating log P. An increase in the hydrophobicity resulted in a shift of the LCST towards lower temperatures. Furthermore, it was shown for the first time that the sensitivity of short ELPs towards external factors, such as salt concentration, is determined by the intrinsic propensity of the incorporated amino acids to form either α-helices or β-strands. In summary, the obtained results on peptide based and peptide-hybrid materials provides deeper insights into their self-assembly characteristics and highlights the potential of these materials to be used for technical and medical applications.

EPFL2008

Polymer–protein conjugates: an enzymatic activity perspective

Marc Gauthier, Harm-Anton Klok

Proteins have been modified with polymers in diverse manners over the past 30 years. However, while proteins have been used to prepare many functional constructs, they are sensitive biomolecules and their bioactivity can be either positively or negatively influenced by many different aspects of polymer modification. The primary focus of this review article is to highlight the opportunities offered by new trends in protein modification, and specifically how they influence the overall biological activity of the conjugate, including its dependence on temperature and pH. We survey the effect of polymer molecular weight, number of conjugated polymer chains, polymer coupling strategy (including random versus site-specific coupling, "grafting from", and multi-point covalent attachment), polymer architecture (including branched and comb-type), polymer interactions with the protein (including electrostatic and host-guest interactions), polymer interactions with enzyme substrate, and polymer biodegradability. We have selected six enzymes, which have been extensively modified with polymers in diverse fashions in the literature, as basis for this discussion. These proteins are L-asparaginase, alpha-chymotrypsin, trypsin, lysozyme, bovine serum albumin, and papain. This review includes polymers such as poly(ethylene glycol) (PEG), polysaccharides, polypeptides, and other synthetic (vinyl) polymers. From the discussed literature we attempt to extract tentative general trends observed between state-of-the-art methods of preparing protein-polymer conjugates and the activity of the conjugate.

2010

Synthesis of Functional Polyesters via Baylis-Hillman Polymerization

Sanhao Ji

Aliphatic polyesters receive increasing attention over the last years driven by their application as biodegradable substitutes for conventional commodity thermoplastics and applications in the biomedical field, where amongst the family of biodegradable polymers, aliphatic polyesters possess the leading position as a consequence of the ready metabolization of the degradation products in most cases. This has led to a wide spread use of polyester homo- and copolymers in drug delivery, surgical implants, and functional materials in tissue engineering. In general, polyesters have a high level of commercial importance, and a variety of well known processing technologies are available. To tune the properties of these materials, e.g. their degradation behavior and to add further functionality, there is significant interest in synthetic strategies which can be used to prepare side-chain functionalized polyesters with variable architectures. This Thesis explores the feasibility of a novel approach towards functional polyesters that is based on the Baylis-Hillman reaction, which involves the base catalyzed condensation of an aldehyde and an acrylate building block to produce an α-methylene-β-hydroxycarbonyl compound. As the Baylis-Hillman polymerization does not require the use of side chain protected monomers, this route may represent an interesting alternative strategy for the preparation of functional polyesters. An attractive feature of the Baylis-Hillman polymerization is that it generate polymer with reactive hydroxyl and vinyl groups, which can be used in a subsequent step for post-modification or prepare functional crosslinked scaffolds. This Thesis consists of four chapters. The different synthetic strategies that have been used for the preparation of multi-functional polyesters and the Baylis-Hillman reaction are reviewed in Chapter 1. Chapter 2 investigates the feasibility of the Baylis-Hillman polymerization of diacrylates and dialdehydes to prepare side-chain functional polyesters and explores the subsequent post-polymerization modification of these polymers to further enhance their functionality. Using 1,3-butanediol acrylate and 2,6-pyridinecarboxaldehyde as monomers and DABCO as catalyst, polymers with a degree of polymerization of up to 25 were prepared. These polymers are attractive as they contain chemically orthogonal side-chain hydroxyl and vinyl groups that can be further modified. In first experiments, it was demonstrated that the side-chain hydroxyl and vinyl groups can be quantitatively post-modified with phenyl isocyanate, respectively, methyl-3-mercaptopropionate. Chapter 3 explores the feasibility of the Baylis-Hillman reaction for the synthesis of hyperbranched polyesters. This Chapter describes an efficient and novel approach to functional hyperbranched polyesters via an A2 + B3 approach based on the Baylis-Hillman reaction. Using trimethylolpropane triacrylate and 2,6-pyridinedicarboxaldehyde as monomers and 3-quinuclidinol as the catalyst, polymer swith a degree of branching between 0.36 and 0.8, and a molecular weight of up to 28,100 were prepared. The effects of monomer ratio and addition mode on the final molecular weight, polydispersity index and degree of branching were studied. In order to demonstrate the potential of this polyester as a reactive polymer, the hydroxyl, vinyl groups and pyridine residues were post-modified with phenyl isocyanate, methyl-3-mercaptopropionate and methyl iodide. Chapter 4 aims to further explore the scope and versatility of the Baylis-Hillman polymerization and studies the preparation of side-chain functional polyesters by polymerization of 1,3-butanediol diacrylate with meta- and para-phthalaldehyde, which are two commercially available dialdehyde building blocks.