A new class of water-soluble, amphiphilic star block copolymers with a large number of arms was prepared by sequential atom transfer radical polymerization (ATRP) of n-butyl methacrylate (BMA) and poly(ethylene glycol) methyl ether methacrylate (PEGMA). As the macroinitiator for the ATRP, a 2-bromoisobutyric acid functionalized fourth-generation hyperbranched polyester (Boltorn H40) was used, which allowed the preparation of star polymers that contained on average 20 diblock copolymer arms. The synthetic concept was validated by AFM experiments, which allowed direct visualization of single molecules of the multiarm star block copolymers. DSC and SAXS experiments on bulk samples suggested a microphase-separated structure, in agreement with the core−shell architecture of the polymers. SAXS experiments on aqueous solutions indicated that the star block copolymers can be regarded as unimolecular micelles composed of a PBMA core and a diffuse PPEGMA corona. The ability of the polymers to encapsulate and release hydrophobic guests was evaluated using 1H NMR spectroscopy. In dilute aqueous solution, these polymers act as unimolecular containers that can be loaded with up to 27 wt % hydrophobic guest molecules.
The site-specific surface modification of colloidal substrates, yielding "patchy" nanoparticles, is a rapidly expanding area of research as a result of the new complex structural hierarchies that are becoming accessible to chemists and materials scientists through colloidal self-assembly. The inherent directionality of cellulose chains, which feature a non-reducing and a reducing end, within individual cellulose nanocrystals (CNCs) renders them an interesting experimental platform for the synthesis of asymmetric nanorods with end-tethered polymer chains. Here, we present water-tolerant reaction pathways toward patchy and uniformly modified CNC hybrids based on atom transfer radical polymerization (ATRP) and initiators that were linked to the CNCs with carbodiimide-mediated coupling and Fischer esterification, respectively. Various monomers, including Nisopropylacrylamide (NIPAM), [2-(methacryloyloxy)ethyl]trimethylammonium chloride (METAC), and sodium 4vinylbenzenesulfonate (4-SS) were polymerized from both types of initiator-modified CNCs, yielding chemically patchy and uniform CNC hybrids, via surface-initiated ATRP (SI-ATRP). Interestingly, the stereochemistry of tethered PNIPAM was affected by the precise location of ATRP initiating sites, as evidenced by 1 H NMR and circular dichroism (CD) spectroscopy. This effect may be related to the inherent right-handed chirality of CNCs. CNC/PMETAC hybrids were labeled with gold nanoparticles (AuNPs) in order to visualize the precise location of polymer tethers via cryo-electron microscopy. In some instances, the AuNPs were indeed concentrated at the end groups of the patchy CNC hybrids.The site-specific surface modification of colloidal substrates, yielding "patchy" (nano)particles, is receiving rapidly expanding interest, as a result of the new complex structural hierarchies that are becoming accessible to chemists and materials scientists through colloidal self-assembly. 1 Such patchy particles include Janus colloids, 2 genetically engineered bacteriophages, 3,4 cylindrical block copolymer micelles, 5 and polymer-tethered nanorods. 6 Particle shape anisotropy provides additional building block parameters, which can broaden the complexity of their assemblies. 7 The lyotropic liquid crystal formation capability of rod-shaped colloidal building blocks is an excellent example of such a self-assembly process. 8 In special cases, these liquid crystal (LC) phases exhibit longrange chirality, displaying so-called cholesteric or chiral nematic arrangements, which are not only fundamentally intriguing for condensed matter investigations, 9 but also useful as chiral building blocks for the creation of advanced functional materials. 10 Relatively few anisotropic colloids that exhibit such chiral phases are known; they include filamentous viruses, 11 collagen, 12 chitin, 13 and cellulose nanocrystals (CNCs). 14 CNCs are a unique class of colloidal liquid crystals in that their cholesteric LC phases are preserved upon evaporation-induced self-assembly (EISA), which together with ...
Squaric acid diesters can be applied as reagents to couple two amino-functional compounds. Consecutive coupling of two amines allows the synthesis of asymmetric squaric acid bisamides with either low molecular weight compounds but also biomolecules or polymers. The key feature of the squaric acid diester mediated coupling is the reduced reactivity of the resulting ester-amide after the first amidation step of the diester. This allows the sequential amidation and generation of asymmetric squaramides with high selectivity and in high yields. This article gives an overview of the well-established squaric acid diester mediated coupling reactions for glycoconjugates and presents recent advances that aim to expand this very versatile reaction protocol to the modification of peptides and proteins.
Polymer-protein conjugates generated from side chain functional synthetic polymers are attractive because they can be easily further modified with, for example, labeling groups or targeting ligands. The residue specific modification of proteins with side chain functional synthetic polymers using the traditional coupling strategies may be compromised due to the nonorthogonality of the side-chain and chain-end functional groups of the synthetic polymer, which may lead to side reactions. This study explores the feasibility of the squaric acid diethyl ester mediated coupling as an amine selective, hydroxyl tolerant, and hydrolysis insensitive route for the preparation of side-chain functional, hydroxyl-containing, polymer-protein conjugates. The hydroxyl side chain functional polymers selected for this study are a library of amine end-functional, linear, midfunctional, hyperbranched, and linear-block-hyperbranched polyglycerol (PG) copolymers. These synthetic polymers have been used to prepare a diverse library of BSA and lysozyme polymer conjugates. In addition to exploring the scope and limitations of the squaric acid diethylester-mediated coupling strategy, the use of the library of polyglycerol copolymers also allows to systematically study the influence of molecular weight and architecture of the synthetic polymer on the biological activity of the protein. Comparison of the activity of PG-lysozyme conjugates generated from relatively low molecular weight PG copolymers did not reveal any obvious structure-activity relationships. Evaluation of the activity of conjugates composed of PG copolymers with molecular weights of 10000 or 20000 g/mol, however, indicated significantly higher activities of conjugates prepared from midfunctional synthetic polymers as compared to linear polymers of similar molecular weight.
This report describes the synthesis and supramolecular organization of a novel class of linear-dendritic block copolymers. The molecules, which are termed rodcoil dendrons, consist of a cholesterol moiety that is attached to L-lysine dendrons of three different generations via a biodegradable oligo(L-lactic acid)n j spacer. Whereas the molecules exhibit very poor ordering in the dry state, different supramolecular morphologies were observed in the hydrated state. Cholesteryl-(L-lactic acid) 23 -(L-lysine)G1 and cholesteryl-(L-lactic acid) 23 -(L-lysine)G2 self-assemble into lamellar structures with periodicities that depend on degree of hydration. At low degrees of hydration, lamellar ordering was also observed for cholesteryl-(L-lactic acid) 22 -(L-lysine)G3. However, at 50 wt %, water steric hindrance in the highly hydrated L-lysine dendrons no longer allows lamellar ordering, and discrete nanosized aggregates are formed. Evidence for the formation of such nanoaggregates was obtained from dynamic light scattering, electron microscopy, and atomic force microscopy. These rodcoil dendron biomaterials could be of potential interest as temporary molecular scaffolds for cell and tissue engineering.
Biologically-inspired peptide sequences have been explored as auxiliaries to mediate self-assembly of synthetic macromolecules into hierarchically organized solution and solid state nanostructures. Peptide sequences inspired by the coiled coil motif and "switch" peptides, which can adopt both amphiphilic alpha-helical and beta-strand conformations, were conjugated to poly(ethylene glycol) (PEG). The solution and solid state self-assembly of these materials was investigated using a variety of spectroscopic, scattering and microscopic techniques. These experiments revealed that the folding and organization properties of the peptide sequences are retained upon conjugation of PEG and that they provide the driving force for the formation of the different nanoscale structures which were observed. The possibility of using defined peptide sequences to direct structure formation of synthetic polymers together with the potential of peptide sequences to induce a specific biological response offers interesting prospects for the development of novel self-assembled and biologically active materials.
The synthesis and properties of a novel set of building blocks for the preparation of selfassembling biomaterials are described. These molecules consist of a relatively short oligo(L-lactic acid)n j segment (n j ) 10-40) that is substituted with a cholesterol moiety as an end group and in some cases has a second biofunctional substituent at its other terminus. The cholesterol moiety not only serves to induce liquid crystalline properties and drive the self-assembly of these oligomers, but also is expected to have an effect on the interaction of cells with these materials. The cholesteryl-(L-lactic acid)n j (CLAn j) oligomers form thermotropic liquid crystals and self-assemble into lamellar structures consisting of interdigitated bilayers. In addition, the CLAn j oligomers can be homogeneously blended with poly(L-lactic acid), thereby offering the possibility to improve the cell interaction properties of this common surgical biomaterial. Furthermore, the secondary alcohol terminus of the CLAn j oligomers allows the opportunity to introduce additional bioactive substituents such as cholesterol, indomethacin (an antiinflammatory drug), and pyrene and rhodamine B (which can act as fluorescent labels for bioimaging purposes) and various R-amino acids. These biofunctional CLAn j oligomers can also be homogeneously mixed with the unsubstituted CLAn j oligomers and thus could enable a further noncovalent functionalization of selfassembling biomaterials.
This contribution describes the synthesis and ring‐opening (co)polymerization of several L‐lysine N‐carboxyanhydrides (NCAs) that contain labile protective groups at the ϵ‐NH2 position. Four of the following L‐lysine NCAs were investigated: Nϵ‐trifluoroacetyl‐L‐lysine N‐carboxyanhydride, Nϵ‐(tert‐butoxycarbonyl)‐L‐lysine N‐carboxyanhydride, Nϵ‐(9‐fluorenylmethoxycarbonyl)‐L‐lysine N‐carboxyanhydride, and Nϵ‐(6‐nitroveratryloxycarbonyl)‐L‐lysine N‐carboxyanhydride. In contrast to the harsh conditions that are required for acidolysis of benzyl carbamate moieties, which are usually used to protect the ϵ‐NH2 position of L‐lysine during NCA polymerization, the protective groups of the L‐lysine NCAs presented here can be removed under mildly acidic or basic conditions or by photolysis. As a consequence, these monomers may allow access to novel peptide hybrid materials that cannot be prepared from ϵ‐benzyloxycarbonyl‐L‐lysine N‐carboxyanhydride (Z‐Lys NCA) because of side reactions that accompany the removal of the Z groups. By copolymerization of these L‐lysine NCAs with labile protective groups, either with each other or with γ‐benzyl‐L‐glutamate N‐carboxyanhydride or Z‐Lys NCA, orthogonally side‐chain‐protected copolypeptides with number‐average degrees of polymerization ≤20 were obtained. Such copolypeptides, which contain different side‐chain protective groups that can be removed independently, are interesting for the synthesis of complex polypeptide architectures or can be used as scaffolds for the preparation of synthetic antigens or protein mimetics. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1167–1187, 2003
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