Paul D. Thornton

Highly Specific Dual Enzyme-Mediated Payload Release from Peptide-Coated Silica Particles

Stimuli-responsive gate mechanisms offer potential for the controlled passage of payload molecules from a porous carrier vehicle on-demand. We describe a method for the enzyme-mediated release of macromolecular guest molecules from inorganic silica particles coated with a bioactive peptide shell, synthesized precisely by Fmoc chemistry. Specific enzymatic hydrolysis of the peptide shell removes the bulky peptide-terminated Fmoc groups, permitting the selective release of previously entrapped guest molecules.

Block Copolypeptides Prepared by N-Carboxyanhydride Ring-Opening Polymerization

N-Carboxyanhydride ring-opening polymerization (NCA ROP) is a synthetically straightforward methodology to generate homopolypeptides. Extensive control over the polymerization permits the production of highly monodisperse synthetic polypeptides to a targeted molecular weight in the absence of unfavorable side reactions. Sequential NCA ROP permits the creation of block copolypeptides composed of individual polypeptide blocks boasting different functionalities, secondary structures, and desirable chemical properties. Consequently, a plethora of novel materials have been generated that have found wide-range applicability. This review offers an insight into contemporary synthetic approaches toward NCA ROP before highlighting a number of block copolypeptide architectures generated.

Polypeptide core-shell silica nanoparticles with high grafting density by N-carboxyanhydride (NCA) ring opening polymerization as responsive materials and for bioconjugation

Silica nanoparticles were furnished with a functional polypeptide shell to create a pH-responsive inorganic–organic hybrid material. Free amine groups present on silica nanoparticles initiated the N-carboxyanhydride (NCA) polymerization of particular amino acid NCAs directly onto the inorganic support, offering a convenient method to functionalize silica core nanoparticles with a uniformly dense polypeptide shell. Poly(γ-benzyl-L-glutamate) (PBLG), poly(e-carbobenzyoxy-L-lysine) (PZLL), and S-tert-butyl protected polycysteine (PtBLC) were grafted from the silica core both independently as homopolypeptides and simultaneously to form a copolypeptide shell, highlighting the versatility that the grafting mechanism possesses. The grafting of PBLG was investigated in detail at 0 °C and 20 °C to determine any differences in the size and uniformity of the polypeptide shell formed. Dynamic light scattering (DLS) analysis revealed a correlation between the thickness of the uniform organic shell and the amount of amino acid in the monomer feed, with higher linearity at the lower polymerization temperature. Size Exclusion Chromatography (SEC) analyses of degrafted PBLG confirmed the DLS results. A high grafting density of around 0.4 PBLG chains per nm2 was calculated highlighting the control afforded in this approach to polypeptide grafting. Subsequent deprotection of the PBLG homopolypeptide shell afforded pH-sensitive poly(glutamic acid) (PGA) coupled silica nanoparticles. The selective release of a model rhodamine B dye was demonstrated to emphasize the potential that these hybrid nanomaterials have for the on-demand release of payload molecules in response to a targeted pH trigger. Moreover, covalent bioconjugation was successfully shown by attachment of green fluorescent protein.

Side-chain functionalisation of unsaturated polyesters from ring-opening polymerisation of macrolactones by thiol–ene click chemistry

A novel method for the modification of polyglobalide, produced by enzymatic ring-opening polymerisation, with pendant side-chains viathiol–ene click chemistry is disclosed. This methodology may offer a chemically simplistic route to the (bio)functionalisation of polyesters.

Selective enzymatic degradation of self-assembled particles from amphiphilic block copolymers obtained by the combination of N-carboxyanhydride and nitroxide-mediated polymerization.

Combining controlled radical polymerizations and a controlled polypeptide synthetic technique, such as N-carboxyanhydride (NCA) ring-opening polymerization, enables the generation of well-defined block copolymers to be easily accessible. Here we combine NCA polymerization with the nitroxide-mediated radical polymerization of poly(n-butyl acrylate) (PBA) and polystyrene (PS), using a TIPNO and SG1-based bifunctional initiator to create a hybrid block copolymer. The polypeptide block consists of (block) copolymers of poly(L-glutamic acid) embedded with various quantities of L-alanine. The formed superstructures (vesicles and micelles) of the block copolymers possessed varying degrees of enzyme responsiveness when exposed to elastase and thermolysin, resulting in controlled enzymatic degradation dictated by the polypeptide composition. The PBA containing block copolymers possessing 50% L-alanine in the polypeptide block showed a high degradation response compared to polymers containing lower L-alanine quantities. The particles stabilized by copolypeptides with L-alanine near the hydrophobic block showed full degradation within 4 days. Particles containing polystyrene blocks revealed no appreciable degradation under the same conditions, highlighting the specificity of the system and the importance of synthetic polymer selection. However, when the degradation temperature was increased to 70 °C, degradation could be achieved due to the higher block copolymer exchange between the particle and the solution. A number of novel biohybrid structures are disclosed that show promise as enzyme-responsive materials with potential use as payload release vehicles, following their controlled degradation by specific, target, enzymes.

Enzyme responsive polymer hydrogel beads

We report on a new class of enzyme responsive polymer hydrogels, the molecular accessibility of which can be changed selectively by enzymes present in a sample fluid.

Star polypeptides by NCA polymerisation from dendritic initiators: synthesis and enzyme controlled payload release

Well-defined star polypeptides were successfully synthesised by initiation of γ-benzyl-L-glutamate N-carboxyanhydride (NCA) from polypropylene imine (PPI) dendrimers. The dendrimer generation and the dendrimer to NCA ratio were systematically varied to afford a range of star shaped architectures with a maximum of 8 to 64 poly(γ-benzyl-L-glutamate) (PBLG) arms. High molar masses up to 500 000 g mol−1 were achieved that were otherwise unobtainable for the analogous linear polypeptides in the absence of the dendrimer core. By deprotection the PBLG star polypeptides were converted into poly(L-glutamic acid) (PGA) star polypeptides. Various concentrations of rhodamine B could be loaded into the polypeptide star architectures dependent on the number of PGA arms and the length of the grafted polypeptide chain produced. Furthermore, the polypeptidic nature of PGA-grafted dendrimers ensures their responsiveness, through controlled degradation, to the target enzyme thermolysin. An enzyme-responsive release mechanism was devised and demonstrated in which rhodamine B payload was released upon incubation with thermolysin but not the control enzyme chymotrypsin. The rate and extent of rhodamine B release was dependent on the composition of the hybrid material, which can be readily tuned to provide highly specific temporal and spatial controlled release.

Enzyme-degradable self-assembled hydrogels from polyalanine-modified poly(ethylene glycol) star polymers.

The generation of a range of star-shaped block copolymers composed of a biocompatible poly(ethylene glycol) (PEG) core tethered to a polyalanine (PAla) shell that possesses the capability to (reversibly) self-assemble in water is described. The hydrogels formed offer a hydrophilic environment ideal for biological processes involving proteins and are able to withhold albumin for prolonged periods before its triggered release following the targeted material degradation by the proteolytic enzyme elastase. Consequently, the materials formed offer significant promise for the delivery of proteins, and possibly inhibitors, in response to a proteolytic enzyme overexpressed in chronic wounds.

Synthesis and photochemical properties of spiropyran graft and star polymers obtained by ‘click’ chemistry

The synthesis of photo-responsive polymers with high density of spiropyran (SP) groups has been investigated. Initially the ATRP synthesis of star-shaped polymer using acrylated benzospiropyran (BSP) was attempted but results suggested an incompatibility of ATRP with BSP preventing the formation of polymers or polymer architectures with a higher chain length or density of spiropyran side chains. Alternatively, copper catalyzed cycloaddition (click chemistry) was successfully employed to obtain spiropyran functionalized polymers. Well-defined poly(6-benzospiropyran hexylmethacrylate)s bearing a BSP moiety on the side chain of each unit of the polymer with linear, star-like and molecular brush architectures and narrow molecular weight distributions were successfully synthesized combining ATRP and click chemistry. All polymers exhibited conventional photochromism as confirmed by absorption and emission spectroscopy. The linear and star-like polymers showed similar behavior with comparable ring-closure kinetics and photostability, while the kinetics of the molecular brush architecture was two times slower and its photostability significantly greater.

Bio-functionalisation to enzymatically control the solution properties of a self-supporting polymeric material

Molecular self-assembly permits the spontaneous aggregation of a variety of low molecular weight amino acid subunits into highly ordered aggregates. Functionalisation of water soluble poly(allylamine) with acetyl protected dialanine enables the formation of a biopolymer self-supporting material (SSM). The presence of an enzyme cleavable dipeptide linker renders the SSM responsive to disruption by a targeted proteolytic enzyme.