Matthew I. Gibson

Synthesis of Functional Polymers by Post-Polymerization Modification

Post-polymerization modification is based on the direct polymerization or copolymerization of monomers bearing chemoselective handles that are inert towards the polymerization conditions but can be quantitatively converted in a subsequent step into a broad range of other functional groups. The success of this method is based on the excellent conversions achievable under mild conditions, the excellent functional-group tolerance, and the orthogonality of the post-polymerization modification reactions. This Review surveys different classes of reactive polymer precursors bearing chemoselective handles and discusses issues related to the preparation of these reactive polymers by direct polymerization of appropriately functionalized monomers as well as the post-polymerization modification of these precursors into functional polymers.

High-Affinity Glycopolymer Binding to Human DC-SIGN and Disruption of DC-SIGN Interactions with HIV Envelope Glycoprotein

Noncovalent interactions between complex carbohydrates and proteins drive many fundamental processes within biological systems, including human immunity. In this report we aimed to investigate the potential of mannose-containing glycopolymers to interact with human DC-SIGN and the ability of these glycopolymers to inhibit the interactions between DC-SIGN and the HIV envelope glycoprotein gp120. We used a library of glycopolymers that are prepared via combination of copper-mediated living radical polymerization and azide−alkyne [3+2] Huisgen cycloaddition reaction. We demonstrate that a relatively simple glycopolymer can effectively prevent the interactions between a human dendritic cell associated lectin (DC-SIGN) and the viral envelope glycoprotein gp120. This approach may give rise to novel insights into the mechanisms of HIV infection and provide potential new therapeutics.

Polymeric Dibromomaleimides As Extremely Efficient Disulfide Bridging Bioconjugation and Pegylation Agents

A series of dibromomaleimides have been shown to be very efficacious at insertion into peptidic disulfide bonds. This conjugation proceeds with a stoichiometric balance of reagents in buffered solutions in less than 15 min to give discrete products while maintaining the disulfide bridge and thus peptide conformation. The insertion is initiated by disulfide reduction using a water-soluble phosphine, tris(2-carboxyethyl)phosphine (TCEP) which allows for subsequent substitution of the two maleimide bromides by the generated thiols. Reaction of salmon calcitonin (sCT) with 2,3-dibromomaleimide (1.1 excess) in the presence of TCEP (1.1 equiv) in aqueous solution at pH 6.2 gives complete production of a single conjugate which requires no workup. A linear methoxy poly(ethylene glycol) (PEG) was functionalized via a Mitsunobu reaction and used for the successful site-specific and rapid pegylation of sCT. This reaction occurs in 15 min with a small stoichiometry excess of the pegylating agent to give insertion at the disulfide with HPLC showing a single product and MALDI-ToF confirming conjugation. Attempts to use the group in a functional ATRP polymerization initiator led to polymerization inhibition. Thus, in order to prepare a range of functional polymers an indirect route was chosen via both azide and aniline functional initiators which were converted to 2,3-dibromomaleimides via appropriate reactions. For example, the azide functional polymer was reacted via a Huisgen CuAAC click reaction to an alkyne functional 2,3-dibromomaleimide. This new reagent allowed for the synthesis of conjugates of sCT with comb polymers derived from PEG methacrylic monomers which in addition gave appropriate cloud points. This reaction represents a highly efficient polymer conjugation method which circumvents problems of purification which normally arise from having to use large excesses of the conjugate. In addition, the tertiary structure of the peptide is efficiently maintained.

Size- and Coating-Dependent Uptake of Polymer-Coated Gold Nanoparticles in Primary Human Dermal Microvascular Endothelial Cells

A library-orientated approach is used to gain understanding of the interactions of well-defined nanoparticles with primary human endothelial cells, which are a key component of the vasculature. Fifteen sequentially modified gold nanoparticles (AuNPs) based on three different core sizes (18, 35, 65 nm) and five polymeric coatings were prepared. The synthetic methodology ensured homogeneity across each series of particles to allow sequential investigation of the chemical features on cellular interactions. The toxicity of these nanoparticles, their uptake behavior in primary human dermal microvascular endothelial cells (HDMECs), and quantification of uptake were all investigated. The results of our studies indicated that high concentrations of gold nanoparticles (250 μg/mL) were nontoxic and that the number of internalized nanoparticles was related to nanoparticle size and surface chemistry. In summary, the positive-charged ethanediamine-coated AuNPs were internalized to a greater extent than the negative- or neutral-charged AuNPs. Moreover, differences in the amounts of internalized AuNPs could be shown for the three neutral-charged AuNPs, whereas the uptake of hydroxypropylamine-coated particles was preferred compared with glucosamine-coated or PEGylated AuNPs. Hydroxypropylamine-coated AuNPs were found to be the most efficient neutral-charged particles in overcoming the endothelial cell barrier and entering the cell.

Tunable thermo-responsive polymer–protein conjugates via a combination of nucleophilic thiol–ene “click” and SET-LRP

Herein we report the synthesis of a protein macroinitiator in a one-pot strategy using phosphine-mediated thiol–ene “click”, the macroinitiator was used to polymerise ethylene glycol containing monomers to yield polymer–protein conjugates with tunable thermoresponsive behaviour.

Probing Bacterial-Toxin Inhibition with Synthetic Glycopolymers Prepared by Tandem Post-Polymerization Modification: Role of Linker Length and Carbohydrate Density†

Probing the depths: A tandem post-polymerization modification strategy was used to systematically probe the multivalent inhibition of a bacterial toxin as a function of linker length (see scheme), carbohydrate density, and glycopolymer chain length. Guided by structural-biology information, the binding-pocket depth of the toxin was probed and used as a means to specifically improve inhibition of the toxin by the glycopolymer.

Synthetic polymers enable non-vitreous cellular cryopreservation by reducing ice crystal growth during thawing

The cryopreservation of cells, tissue and organs is fundamental to modern biotechnology, transplantation medicine and chemical biology. The current state-of-the-art method of cryopreservation is the addition of large amounts of organic solvents such as glycerol or dimethyl sulfoxide, to promote vitrification and prevent ice formation. Here we employ a synthetic, biomimetic, polymer, which is capable of slowing the growth of ice crystals in a manner similar to antifreeze (glyco)proteins to enhance the cryopreservation of sheep and human red blood cells. We find that only 0.1 wt% of the polymer is required to attain significant cell recovery post freezing, compared with over 20 wt% required for solvent-based strategies. These results demonstrate that synthetic antifreeze (glyco)protein mimics could have a crucial role in modern regenerative medicine to improve the storage and distribution of biological material for transplantation.

Inhibition of ice crystal growth by synthetic glycopolymers: implications for the rational design of antifreeze glycoprotein mimics.

A series of structurally diverse polymers, containing either peptide or vinyl-derived backbones, was tested for ice recrystallization inhibition activity, which is commonly associated with antifreeze (glyco)proteins. It was revealed that only polymers bearing hydroxyl groups in the side chain could inhibit ice growth. Furthermore, well-defined glycopolymers were shown to have a small but significant recrystallization inhibition effect, showing that it may be possible to design antifreeze glycoprotein mimics based upon polymers derived from vinyl monomers.

Size-dependent LCST transitions of polymer-coated gold nanoparticles : cooperative aggregation and surface assembly

Nanoparticles working together. Thermoresponsive polymer-coated nanoparticles are shown to display size dependent, cooperative, lower critical solution temperature behavior. This is exploited to direct the assembly of appropriate nanoparticles onto complementary polymer brush coated surfaces.

Thermoresponsive behaviour of poly[(oligo(ethyleneglycol methacrylate)]s and their protein conjugates: importance of concentration and solvent system

Thermoresponsive poly[oligo(ethyleneglycol) methacrylate]s with a variety of different oligo(ethyleneglycol) graft lengths were synthesised by reversible-addition fragmentation chain transfer (RAFT) polymerisation. The lower critical solution temperature (LCST) behaviour of these polymers was evaluated as a function of the polymer concentration and the concentration of dissolved solutes, in order to understand their applicability for in vitro and in vivo applications. It was observed that in the relevant dilute (<1 mg mL−1) concentration range the observed LCSTs increased by approximately 6 °C compared to higher concentrations. This was confirmed by complimentary dynamic light scattering and differential scanning calorimetry measurements. The impact of biological solutions on the LCST was determined using bovine blood plasma, which resulted in observed LCSTs lower than what is found in traditional buffer or pure aqueous solutions. Finally, a well-defined polymer–protein conjugate was synthesised by ‘grafting from’ using single-electron transfer (SET) polymerisation. This model polymer–protein therapeutic also displayed similar concentration dependant behaviour, highlighting the importance of testing novel ‘smart’ materials and conjugates at both relevant concentration ranges and in appropriate solvent systems in order to use them in biotechnological applications.