Roey J. Amir

Enzymatically Triggered Self-Assembly of Block Copolymers

The polymerization of vinyl monomers with cleavable enzymatic substrates has been shown to lead to water-soluble double-hydrophilic block copolymers which, upon enzymatic activation of the diblock copolymers, become amphiphilic and undergo self-assembly into colloidal nanostructures. The ability to change the chemical and physical characteristics of polymeric materials by an enzymatic reaction opens the way for novel and exciting applications such as enzymatic-triggered activation of surfaces and formation of nanostructures in vivo in a highly controlled manner.

Enzyme-responsive amphiphilic PEG-dendron hybrids and their assembly into smart micellar nanocarriers.

Enzyme-responsive micelles have great potential as drug delivery platforms due to the high selectivity of the activating enzymes. Here we report a highly modular design for the efficient and simple synthesis of amphiphilic block copolymers based on a linear hydrophilic polyethyleneglycol (PEG) and an enzyme-responsive hydrophobic dendron. These amphiphilic hybrids self-assemble in water into micellar nanocontainers that can disassemble and release encapsulated molecular cargo upon enzymatic activation. The utilization of monodisperse dendrons as the stimuli-responsive block enabled a detailed kinetic study of the molecular mechanism of the enzymatically triggered disassembly. The modularity of these PEG-dendron hybrids allows control over the disassembly rate of the formed micelles by simply tuning the PEG length. Such smart amphiphilic hybrids could potentially be applied for the fabrication of nanocarriers with adjustable release rates for delivery applications.

Stimuli-Responsive Azulene-Based Conjugated Oligomers with Polyaniline-like Properties

Novel azulene building blocks, prepared via the cycloaddition of thiophene-S,S-dioxides and fulvenes, allow for incorporation of the seven-membered ring of the azulene nucleus directly into the backbone of conjugated materials. This unique mode of incorporation gives remarkably stable, stimuli-responsive materials upon exposure to acid. This simple doping/dedoping strategy provides for effective optical band gap control and on/off fluorescence switching, reminiscent of polyaniline.

Multifunctional Trackable Dendritic Scaffolds and Delivery Agents

Dendrimers and other 3-D molecular assemblies are attractive scaffolds for biological delivery agents and diagnostic probes[1,2] due to their globular shape, modular structure, monodispersity and plurality of functional end groups.[3] To address this potential, a number of strategies and related dendritic architectures have been developed for delivery of bioactive molecules to desired cells or tissue,[4] with encapsulation[5] and covalent attachment to the dendritic chain ends being two major approaches.[6] While the encapsulation of drugs or dyes within the inner cavities of the dendrimer is promising,[5] in most cases only a limited number of guest molecules can be encapsulated even with dendrimers of high generations.[4d] Moreover, the non-covalent nature of the encapsulation makes it a challenge to control the stability of the loaded carrier and subsequent release of the payload.[4e] An alternative strategy exploits the large number of dendritic chain ends to carry the cargo molecules.[6] However, loading of large amounts of hydrophobic drugs or dyes can alter the dendrimer surface properties and decrease its solubility and bio-compatibility.[7] Partial functionalization[8] alleviates this issue but results in random chain end modification leading to a dispersity in loading, variable bio-performance and in many cases only low degrees of surface functionalization can be achieved without significantly changing the surface properties.[9]

Breakdown of interference rules in azulene, a nonalternant hydrocarbon.

We have designed and synthesized five azulene derivatives containing gold-binding groups at different points of connectivity within the azulene core to probe the effects of quantum interference through single-molecule conductance measurements. We compare conducting paths through the 5-membered ring, 7-membered ring, and across the long axis of azulene. We find that changing the points of connectivity in the azulene impacts the optical properties (as determined from UV-vis absorption spectra) and the conductivity. Importantly, we show here that simple models cannot be used to predict quantum interference characteristics of nonalternant hydrocarbons. As an exemplary case, we show that azulene derivatives that are predicted to exhibit destructive interference based on widely accepted atom-counting models show a significant conductance at low biases. Although simple models to predict the low-bias conductance do not hold with all azulene derivatives, we demonstrate that the measured conductance trend for all molecules studied actually agrees with predictions based on the more complete GW calculations for model systems.

Enhanced bioactivity of internally functionalized cationic dendrimers with PEG cores.

Hybrid dendritic-linear block copolymers based on a 4-arm poly(ethylene glycol) (PEG) core were synthesized using an accelerated AB2/CD2 dendritic growth approach through orthogonal amine/epoxy and thiol-yne chemistries. The biological activity of these 4-arm and the corresponding 2-arm hybrid dendrimers revealed an enhanced, dendritic effect with an exponential increase in cell internalization concomitant with increasing amine end groups and low cytotoxicity. Furthermore, the ability of these hybrid dendrimers to induce endosomal escape combined with their facile and efficient synthesis makes them attractive platforms for gene transfection. The 4-arm-based dendrimer showed significantly improved DNA binding and gene transfection capabilities in comparison with the 2-arm derivative. These results combined with the MD simulation indicate a significant effect of both the topology of the PEG core and the multivalency of these hybrid macromolecules on their DNA binding and delivery capablities.

Free radical polymers with tunable and selective bio- and chemical degradability.

A versatile synthetic strategy has been developed which enables the facile incorporation of cleavable functional groups, i.e., esters, thioesters, and disulfides, into the carbon-carbon backbone of vinyl-based polymers. Through the synthesis of novel cyclic monomers, RAFT-mediated radical ring-opening copolymerizations with traditional vinyl monomers such as methyl methacrylate, N,N-dimethylaminoethyl methacrylate, and 2-hydroxyethyl methacrylate lead to the introduction of controlled degradability into these widely used vinyl copolymer systems. An additional benefit of this strategy is the inherent versatility available through the incorporation of cyclic monomers containing diverse functional groups such as esters, thioesters, disulfides, and silyl ether units that allow degradation under basic/acidic, reductive, or enzymatic conditions. By integrating multiple, orthogonal cyclic monomers into linear copolymer backbones, well-defined systems with programmable degradation profiles are obtained which allows for tunable, selective, and stepwise degradation of the vinyl polymer backbones.

Azulene-based conjugated polymers: unique seven-membered ring connectivity leading to stimuli-responsiveness

A novel family of stimuli-responsible polyazulenes connected through the seven-membered ring, in contrast to traditional five-membered connectivity, have been prepared by Yamamoto-, Sonogashira-, and Stille-coupling of 4,7-dibromoazulenes. Polymer absorption can be tuned by structural modification of the conjugated backbones with UV-vis and EPR studies indicating that the seven-membered ring connection of the azulene unit is effective for the generation and stabilization of azulenium cations. The degree of conjugation and the reversibility of optical properties upon protonation and deprotonation is sensitive to the steric environment of the seven-membered ring and can be tuned by both substitution and incorporation of linkers between the azulene units.

Conjugated oligomers incorporating azulene building blocks-seven- vs. Five-membered ring connectivity

The properties of isomeric azulene derivatives based on 7- versus 5-membered ring substitution were examined by the synthesis and characterization of well-defined electroactive oligomers. The substitution pattern was shown to dramatically influence solid-state, electronic and optical properties of the oligomers with acid-responsive materials only being observed when the azulenium cation could be directly stabilized by substituents in the 7-membered ring. Protonation was accompanied by a reversible color change and a strong red-shift of the absorption maximum as indicated by UV-vis studies. In addition, we show that the absorption maxima and optical band-gaps of azulenium cations can be tuned by the nature of the chromophore connected to the seven-membered ring of the azulene nucleus.

Encapsulation and covalent binding of molecular payload in enzymatically activated micellar nanocarriers.

The high selectivity and often-observed overexpression of specific disease-associated enzymes make them extremely attractive for triggering the release of hydrophobic drug or probe molecules from stimuli-responsive micellar nanocarriers. Here we utilized highly modular amphiphilic polymeric hybrids, composed of a linear hydrophilic polyethylene glycol (PEG) and an esterase-responsive hydrophobic dendron, to prepare and study two diverse strategies for loading of enzyme-responsive micelles. In the first type of micelles, hydrophobic coumarin-derived dyes were encapsulated noncovalently inside the hydrophobic core of the micelle, which was composed of lipophilic enzyme-responsive dendrons. In the second type of micellar nanocarrier the hydrophobic molecular cargo was covalently linked to the end-groups of the dendron through enzyme-cleavable bonds. These amphiphilic hybrids self-assembled into micellar nanocarriers with their cargo covalently encapsulated within the hydrophobic core. Both types of micelles were highly responsive toward the activating enzyme and released their molecular cargo upon enzymatic stimulus. Importantly, while faster release was observed with noncovalent encapsulation, higher loading capacity and slower release rate were achieved with covalent encapsulation. Our results clearly indicate the great potential of enzyme-responsive micellar delivery platforms due to the ability to tune their payload capacities and release rates by adjusting the loading strategy.