Sequences in dendrons and dendrimers

Polymers for Disrupting Protein-Protein Interactions: Where Are We and Where Should We Be?

Protein-protein interactions (PPIs) are central to the cellular signaling and regulatory networks that underlie many physiological and pathophysiological processes. It is challenging to target PPIs using traditional small molecule or peptide-based approaches due to the frequent lack of well-defined binding pockets at the large and flat PPI interfaces. Synthetic polymers offer an opportunity to circumvent these challenges by providing unparalleled flexibility in tuning their physiochemical properties to achieve the desired binding properties. In this review, we summarize the current state of the field pertaining to polymer-protein interactions in solution, highlighting various polyelectrolyte systems, their tunable parameters, and their characterization. We provide an outlook on how these architectures can be improved by incorporating sequence control, foldability, and machine learning to mimic proteins at every structural level. Advances in these directions will enable the design of more specific protein-binding polymers and provide an effective strategy for targeting dynamic proteins, such as intrinsically disordered proteins.

Activatable Dendritic 19F Probes for Enzyme Detection

We describe a novel activatable probe for fluorine-19 NMR based on self-assembling amphiphilic dendrons. The dendron probe has been designed to be spectroscopically silent due to the formation of large aggregates. Upon exposure to the specific target enzyme, the aggregates disassemble to give rise to a sharp ¹⁹F NMR signal. The probe is capable of detecting enzyme concentrations in the low nanomolar range. Response time of the probe was found to be affected by the hydrophilic-lipophilic balance of dendrons. Understanding the structural factors that underlie this design principle provides the pathway for using this strategy for a broad range of enzyme-based imaging.

Protein-triggered supramolecular disassembly: insights based on variations in ligand location in amphiphilic dendrons

We use monodisperse dendrons that allow control over functional group presentation to investigate the influence of the location of a ligand on protein-induced disassembly and release of encapsulated small molecules. Based on both experiments and molecular dynamics simulations, we demonstrate that ligand location greatly influences release of guest molecules from the dendron-based supramolecular assembly. We show that a ligand moiety grafted to the dendron periphery is more accessible for the target protein in aqueous solution. On the other hand, the ligand moiety placed at the focal point or at the intermediate layer within the dendritic scaffold is less accessible, since it is surrounded by an environment rich in PEG chains, which hinders binding and even influences nonspecific interactions. We also demonstrate that the specific binding between one ligand and the target protein can destabilize the dendritic assembly. Furthermore, if more ligands are available, multivalent interactions are also possible with extravidin, which speed up disassembly and trigger the release of hydrophobic guests.

Dendrimers terminated with dichlorotriazine groups provide a route to compositional diversity

Triazine dendrimers terminated with either four or eight dichlorotriazines can be prepared in high yields by reacting an amine-terminated dendrimer with cyanuric chloride. These materials exist as white powders and are stable to storage at room temperature. Sequential nucleophilic aromatic substitution with two different amine nucleophiles yields compounds that display the desired compositional diversity. Reaction conditions for the substitution were developed using a model dichlorotriazine with amine nucleophiles at -20, 0, and 25 °C. Selective substitution is favored at lower temperatures and with more nucleophilic amine groups.

Site-specific installation and study of electroactive units in every layer of dendrons

Whereas encapsulation of functional groups at the core of dendrimers is well-understood, very little is known about their intermediate layers or even the periphery. Here we report on a systematic investigation of every layer of dendrimers by incorporating a single ferrocene unit in well-defined locations in dendrons. Site-specific incorporation of the ferrocene unit was achieved by utilizing the dendrimer sequencing methodology. We show here that the redox potential values of ferrocene at intermediate layers were remarkably different from those at the core and the periphery. Although redox potential values were location-dependent, no significant change in the rate of heterogeneous electron transfer (k(0)) was observed with respect to locations. This was attributed to the possibility that free rotation of dendrimer nullifies the distance between the electrode and ferrocene unit. Finally, we also show that no Faradaic current was observed for the amphiphilic assemblies of these dendrons, whereas the same dendron did exhibit significant Faradaic current in nonassembling solvent environments.

Isoxyl activation is required for bacteriostatic activity against Mycobacterium tuberculosis

Isoxyl (ISO), a thiourea derivative that was successfully used for the clinical treatment of tuberculosis during the 1960s, is an inhibitor of the synthesis of oleic and mycolic acids in Mycobacterium tuberculosis. Its effect on oleic acid synthesis has been shown to be attributable to its inhibitory activity on the stearoyl-coenzyme A desaturase DesA3, but its enzymatic target(s) in the mycolic acid pathway remains to be identified. With the goal of elucidating the mode of action of ISO, we have isolated a number of spontaneous ISO-resistant mutants of M. tuberculosis and undertaken their genotypic characterization. We report here the characterization of a subset of these strains carrying mutations in the monooxygenase gene ethA. Through complementation studies, we demonstrate for the first time that the EthA-mediated oxidation of ISO is absolutely required for this prodrug to inhibit its lethal enzymatic target(s) in M. tuberculosis. An analysis of the metabolites resulting from the in vitro transformation of ISO by purified EthA revealed the occurrence of a formimidamide allowing the formulation of an activation pathway in which the oxidation of ISO catalyzed by EthA is followed by chemical transformations involving extrusion or elimination and, finally, hydrolysis.

Control of Surface Functionality in Poly(phenylenevinylene) Dendritic Architectures

The efficient synthesis of new asymmetric poly(phenylenevinylene) dendritic macromolecules using a stepwise convergent-growth approach is described. By an iterative methodology that made use of the Horner-Wadsworth-Emmons (HWE) reaction, dendrons and dendrimers up to the third generation, with eight different functional groups located at the periphery, were prepared in good yields. Both the number and placement of functionalities can be accurately controlled to afford a large variety of dendritic architectures.

Dendrimer Analogues of Linear Molecules to Evaluate Energy and Charge-Transfer Properties

[reaction: see text] We have designed and synthesized difunctionalized dendrimers containing two donors in the periphery and an acceptor at the core to serve as scaffolds for comparison with linear analogues to investigate the advantage of dendritic scaffolds for energy and charge transfer. Comparison of these dendrimers with the fully decorated dendrimers provides information on the advantage of chromophore density in energy/charge transfer from periphery to the core.

Nonconjugated Red-Emitting Dendrimers with p-Type and/or n-Type Peripheries

[reaction: see text] Dendron-encapsulated materials having a benzo[c][1,2,5]thiadiazole-based red-emitting core and p-type and/or n-type peripheries were synthesized. Intra- and intermolecular energy transfer from the peripheries to the core were found to be peripheries dependent. Red-emitting electroluminescent devices with promising performance were fabricated.

Dendrimeric micelles for controlled drug release and targeted delivery

This review highlights the developments in dendrimer-based micelles for drug delivery. Dendrimers, the perfectly branched monodisperse macromolecules, have certain structural advantages that make them attractive candidates as drug carriers for controlled release or targeted delivery. As polymeric micelle-based approaches precede the work in dendrimers, these are also discussed briefly. The review concludes with a perspective on possible applications of biaryl-based dendrimeric micelles that exhibit environment-dependent conformations, in drug delivery.

Probing Every Layer in Dendrons

Dendrons with a fluorescent probe in a specific location have been synthesized and characterized. Accessibility of guest molecules to each of these layers was then analyzed using an intermolecular photoinduced electron-transfer process. Comparisons of the accessibility among each layer of the dendron and among the generations are provided.

Energy and Electron Transfer in Bifunctional Non-Conjugated Dendrimers

Nonconjugated dendrimers, which are capable of funneling energy from the periphery to the core followed by a charge-transfer process from the core to the periphery, have been synthesized. The energy and electron donors involve a diarylaminopyrene unit and are incorporated at the periphery of these dendrimers. The energy and electron acceptor is at the core of the dendrimer, which involves a chromophore based on a benzthiadiazole moiety. The backbone of the dendrimers is benzyl ether based. A direct electron-transfer quenching of the excited state of the periphery or a sequential energy transfer-electron-transfer pathway are the two limiting mechanisms of the observed photophysical properties. We find that the latter mechanism is prevalent in these dendrimers. The energy transfer occurs on a picosecond time scale, while the charge-transfer process occurs on a nanosecond time scale. The lifetime of the charge separated species was found to be in the range of microseconds. Energy transfer efficiencies ranging from 80% to 90% were determined using both steady-state and time-resolved measurements, while charge-transfer efficiencies ranging from 70% to 80% were deduced from fluorescence quenching of the core chromophore. The dependence of the energy and charge-transfer processes on dendrimer generation is analyzed in terms of the backfolding of the flexible benzyl ether backbone, which leads to a weaker dependence of the energy and charge-transfer efficiencies on dendrimer size than would be expected for a rigid system.

Dendrimers with Both Polar and Apolar Nanocontainer Characteristics

Amphiphilic dendrimers with a custom-designed biaryl repeat unit have been synthesized. These dendrimers afford hydrophobic nanocontainers in polar solvents and hydrophilic nanocontainers in apolar solvents. These container properties were investigated using dye incorporation studies. The aggregation states of these dendrimers are also reported using DLS studies.