Activatable Dendritic 19F Probes for Enzyme Detection

Tuning the Reactivity of Micellar Nanoreactors by Precise Adjustments of the Amphiphile and Substrate Hydrophobicity

Polymeric assemblies, such as micelles, are gaining increasing attention due to their ability to serve as nanoreactors for the execution of organic reactions in aqueous media. The ability to conduct organic transformations, which have been traditionally limited to organic media, in water is essential for the further development of important fields ranging from green catalysis to bioorthogonal chemistry. Considering the recent progress that has been made to expand the range of organometallic reactions conducted using nanoreactors, we aimed to gain a deeper understanding of the roles of the hydrophobicity of both the core of micellar nanoreactors and the substrates on the reaction rates in water. Toward this goal, we designed a set of five metal-loaded micelles composed of polyethylene glycol-dendron amphiphiles and studied their ability to serve as nanoreactors for a palladium-mediated depropargylation reaction of four substrates with different log P values. Using dendrons as the hydrophobic block, we could precisely tune the lipophilicity of the nanoreactors, which allowed us to reveal linear correlations between the rate constants and the hydrophobicity of the amphiphiles (estimated by the dendron's cLog P). While exponential dependence was obtained for the lipophilicity of the substrates, a similar degree of rate acceleration was observed due to the increase in the hydrophobicity of the amphiphiles regardless of the effect of the substrate's log P. Our results demonstrate that while increasing the hydrophobicity of the substrates may be used to accelerate reaction rates, tuning the hydrophobicity of the micellar nanoreactors can serve as a vital tool for further optimization of the reactivity and selectivity of nanoreactors.

Multichannel dual protein sensing using amphiphilic supramolecular assemblies

Protein sensing strategies have implications in detection of many human pathologies. Here, a supramolecular strategy for sensing two different proteins using a multichannel readout approach is outlined. Protein-ligand binding or enzymatic cleavage can both be programmed to induce supramolecular disassembly, which leads to fluorescence enhancement via aggregation-induced emission (AIE), protein-induced fluorescence enhancement (PIFE), or disassembly-induced fluorescence enhancement (DIFE). The accompanying signal change from two different fluorophores and their patterns are then used for specific protein sensing.

Catalytic Synthesis of PEGylated EGCG Conjugates that Disaggregate Alzheimer’s Tau

The naturally occurring flavonoid (–)-epigallocatechin gallate (EGCG) is a potent disaggregant of tau fibrils. Guided by the recent cryo-electron microscopy (cryoEM) structure of EGCG bound to fibrils of tau derived from an Alzheimer’s brain donor, methods to site-specifically modify the EGCG D-ring with aminoPEGylated linkers are reported. The resultant molecules inhibit tau fibril seeding by Alzheimer’s brain extracts. Formulations of aminoPEGylated EGCG conjugated to the (quasi)-brain-penetrant nanoparticle Ferumoxytol inhibit seeding by AD-tau with linker length affecting activity. The protecting group-free catalytic cycloaddition of amino azides to mono-propargylated EGCG described here provides a blueprint for access to stable nanoparticulate forms of EGCG potentially useful as therapeutics to eliminate Alzheimer’s-related tau tangles.

Supramolecular Self-Associations of Amphiphilic Dendrons and Their Properties

This review presents precisely defined amphiphilic dendrons, their self‐association properties, and their different uses. Dendrons, also named dendritic wedges, are composed of a core having two different types of functions, of which one type is used for growing or grafting branched arms, generally multiplied by 2 at each layer by using 1→2 branching motifs. A large diversity of structures has been already synthesized. In practically all cases, their synthesis is based on the synthesis of known dendrimers, such as poly(aryl ether), poly(amidoamine) (in particular PAMAM), poly(amide) (in particular poly(L‐lysine)), 1→3 branching motifs (instead of 1→2), poly(alkyl ether) (poly(glycerol) and poly(ethylene glycol)), poly(ester), and those containing main group elements (poly(carbosilane) and poly(phosphorhydrazone)). In most cases, the hydrophilic functions are on the surface of the dendrons, whereas one or two hydrophobic tails are linked to the core. Depending on the structure of the dendrons, and on the experimental conditions used, the amphiphilic dendrons can self‐associate at the air‐water interface, or form micelles (eventually tubular, but most generally spherical), or form vesicles. These associated dendrons are suitable for the encapsulation of low‐molecular or macromolecular bioactive entities to be delivered in cells. This review is organized depending on the nature of the internal structure of the amphiphilic dendrons (aryl ether, amidoamine, amide, quaternary carbon atom, alkyl ether, ester, main group element). The properties issued from their self‐associations are described all along the review.

Protein-Engineered Nanoscale Micelles for Dynamic 19F Magnetic Resonance and Therapeutic Drug Delivery

Engineered proteins provide an interesting template for designing fluorine-19 (19F) magnetic resonance imaging (MRI) contrast agents, yet progress has been hindered by the unpredictable relaxation properties of fluorine. Herein, we present the biosynthesis of a protein block copolymer, termed "fluorinated thermoresponsive assembled protein" (F-TRAP), which assembles into a monodisperse nanoscale micelle with interesting 19F NMR properties and the ability to encapsulate and release small therapeutic molecules, imparting potential as a diagnostic and therapeutic (theranostic) agent. The assembly of the F-TRAP micelle, composed of a coiled-coil pentamer corona and a hydrophobic, thermoresponsive elastin-like polypeptide core, results in a drastic depression in spin-spin relaxation ( T2) times and unaffected spin-lattice relaxation ( T1) times. The nearly unchanging T1 relaxation rates and linearly dependent T2 relaxation rates have allowed for detection via zero echo time 19F MRI, and the in vivo MR potential has been preliminarily explored using 19F magnetic resonance spectroscopy (MRS). This fluorinated micelle has also demonstrated the ability to encapsulate the small-molecule chemotherapeutic doxorubicin and release its cargo in a thermoresponsive manner owing to its inherent stimuli-responsive properties, presenting an interesting avenue for the development of thermoresponsive 19F MRI/MRS-traceable theranostic agents.

19F MRI of Polymer Nanogels Aided by Improved Segmental Mobility of Embedded Fluorine Moieties

Using fluorinated probes for 19F MRI imaging is an emerging field with potential utility in cellular imaging and cell tracking in vivo, which complements conventional 1H MRI. An attractive feature of 19F-based imaging is that this is a bio-orthogonal nucleus and the naturally abundant isotope is NMR active. A significant hurdle however in the 19F MRI arises from the tendency of organic macromolecules, with multiple fluorocarbon substitutions, to aggregate in the aqueous phase. This aggregation results in significant loss of sensitivity, because the T2 relaxation times of these aggregated 19F species tend to be significantly lower. In this report, we have developed a strategy to covalently trap nanoscopic states with an optimal degree of 19F substitutions, followed by significant enhancement in T2 relaxation times through increased segmental mobility of the side chain substituents facilitated by the stimulus-responsive elements in the polymeric nanogel. In addition to NMR relaxation time based evaluations, the ability to obtain such signals are also evaluated in mouse models. The propensity of these nanoscale assemblies to encapsulate hydrophobic drug molecules and the availability of surfaces for convenient introduction of fluorescent labels suggest the potential of these nanoscale architectures for use in multimodal imaging and therapeutic applications.

Sensing caspase-1 activity using activatable 19F MRI nanoprobes with improved turn-on kinetics

Tandemly-repeated peptide design enhanced enzyme accessibility and cleavage efficiency of substrates on the nanoparticle surface, resulting in high MRI signal amplification.

Micellar Stability in Biological Media Dictates Internalization in Living Cells

The dynamic nature of polymeric assemblies makes their stability in biological media a crucial parameter for their potential use as drug delivery systems in vivo. Therefore, it is essential to study and understand the behavior of self-assembled nanocarriers under conditions that will be encountered in vivo such as extreme dilutions and interactions with blood proteins and cells. Herein, using a combination of fluorescence spectroscopy and microscopy, we studied four amphiphilic PEG-dendron hybrids and their self-assembled micelles in order to determine their structure-stability relations. The high molecular precision of the dendritic block enabled us to systematically tune the hydrophobicity and stability of the assembled micelles. Using micelles that change their fluorescent properties upon disassembly, we observed that serum proteins bind to and interact with the polymeric amphiphiles in both their assembled and monomeric states. These interactions strongly affected the stability and enzymatic degradation of the micelles. Finally, using spectrally resolved confocal imaging, we determined the relations between the stability of the polymeric assemblies in biological media and their cell entry. Our results highlight the important interplay between molecular structure, micellar stability, and cell internalization pathways, pinpointing the high sensitivity of stability-activity relations to minor structural changes and the crucial role that these relations play in designing effective polymeric nanostructures for biomedical applications.

Activatable probes for diagnosis and biomarker detection by MRI

Magnetic resonance imaging (MRI) is a non-invasive imaging technique with widespread use in diagnosis. Frequently, contrast in MRI is enhanced with the aid of a contrast agent, among which smart, responsive, OFF/ON or activatable probes are of particular interest. These kinds of probes elicit a response to selective stimuli, evidencing the presence of enzymes or acidic pH, for instance. In this review, we will focus on smart probes that are detectable by both ¹H and ¹⁹F MRI, frequently based on nanomaterials. We will discuss the triggering factors and the strategies employed thus far to activate each probe.

Molecular Precision and Enzymatic Degradation: From Readily to Undegradable Polymeric Micelles by Minor Structural Changes

Studying the enzymatic degradation of synthetic polymers is crucial for the design of suitable materials for biomedical applications ranging from advanced drug delivery systems to tissue engineering. One of the key parameters that governs enzymatic activity is the limited accessibility of the enzyme to its substrates that may be collapsed inside hydrophobic domains. PEG-dendron amphiphiles can serve as powerful tools for the study of enzymatic hydrolysis of polymeric amphiphiles due to the monodispersity and symmetry of the hydrophobic dendritic block, which significantly simplifies kinetic analyses. Using these hybrids, we demonstrate how precise, minor changes in the hydrophobic block are manifested into tremendous changes in the stability of the assembled micelles toward enzymatic degradation. The obtained results emphasize the extreme sensitivity of self-assembly and its great importance in regulating the accessibility of enzymes to their substrates. Furthermore, the demonstration that the structural differences between readily degradable and undegradable micelles are rather minor, points to the critical roles that self-assembly and polydispersity play in designing biodegradable materials.

“DNA–Teflon” sequence-controlled polymers

Efficient automated synthesis of sequence-controlled “DNA–Teflon” polymers with potential drug delivery and bioimaging applications.

Fluorinated smart micelles as enzyme-responsive probes for 19F-magnetic resonance

Labeling smart PEG–dendron hybrids with fluorine-containing groups transform their micelles into enzyme-responsive probes for ¹⁹F-magnetic resonance.