Controlling Multivalency and Multimodality: Up to Pentamodal Dendritic Platforms Based on Diethylenetriaminepentaacetic Acid Cores

Heterobivalent Ligand for the Adenosine A2A-Dopamine D2 Receptor Heteromer

A G protein-coupled receptor heteromer that fulfills the established criteria for its existence in vivo is the complex between adenosine A_2A (A_2AR) and dopamine D₂ (D₂R) receptors. Here, we have designed and synthesized heterobivalent ligands for the A_2AR-D₂R heteromer with various spacer lengths. The indispensable simultaneous binding of these ligands to the two different orthosteric sites of the heteromer has been evaluated by radioligand competition-binding assays in the absence and presence of specific peptides that disrupt the formation of the heteromer, label-free dynamic mass redistribution assays in living cells, and molecular dynamic simulations. This combination of techniques has permitted us to identify compound 26 [K_DB1 (A_2AR) = 2.1 nM, K_DB1 (D₂R) = 0.13 nM], with a spacer length of 43-atoms, as a true bivalent ligand that simultaneously binds to the two different orthosteric sites. Moreover, bioluminescence resonance energy transfer experiments indicate that 26 favors the stabilization of the A_2AR-D₂R heteromer.

Radical Dendrimers Based on Biocompatible Oligoethylene Glycol Dendrimers as Contrast Agents for MRI

Finding alternatives to gadolinium (Gd)-based contrast agents (CA) with the same or even better paramagnetic properties is crucial to overcome their established toxicity. Herein we describe the synthesis and characterization of entirely organic metal-free paramagnetic macromolecules based on biocompatible oligoethylene glycol dendrimers fully functionalized with 5 and 20 organic radicals (OEG Gn-PROXYL (n = 0, 1) radical dendrimers) with the aim to be used as magnetic resonance imaging (MRI) contrast agents. Conferring high water solubility on such systems is often a concern, especially in large generation dendrimers. Our approach to overcome such an issue in this study is by synthesizing dendrimers with highly water-soluble branches themselves. In this work, we show that the highly water-soluble OEG Gn-PROXYL (n = 0, 1) radical dendrimers obtained showed properties that convert them in good candidates to be studied as contrast agents for MRI applications like diagnosis and follow-up of infectious diseases, among others. Importantly, with the first generation radical dendrimer, a similar r1 relaxivity value (3.4 mM-1s-1) in comparison to gadolinium-diethylenetriamine pentaacetic acid (Gd-DTPA) used in clinics (3.2 mM-1s-1, r.t. 7T) has been obtained, and it has been shown to not be cytotoxic, avoiding the toxicity risks associated with the unwanted accumulation of Gd in the body.

Strain-Stiffening in Dynamic Supramolecular Fiber Networks

The cytoskeleton is a highly adaptive network of filamentous proteins capable of stiffening under stress even as it dynamically assembles and disassembles with time constants of minutes. Synthetic materials that combine reversibility and strain-stiffening properties remain elusive. Here, strain-stiffening hydrogels that have dynamic fibrous polymers as their main structural components are reported. The fibers form via self-assembly of bolaamphiphiles (BA) in water and have a well-defined cross-section of 9 to 10 molecules. Fiber length recovery after sonication, H/D exchange experiments, and rheology confirm the dynamic nature of the fibers. Cross-linking of the fibers yields strain-stiffening, self-healing hydrogels that closely mimic the mechanics of biological networks, with mechanical properties that can be modulated by chemical modification of the components. Comparison of the supramolecular networks with covalently fixated networks shows that the noncovalent nature of the fibers limits the maximum stress that fibers can bear and, hence, limits the range of stiffening.

Design of a True Bivalent Ligand with Picomolar Binding Affinity for a G Protein-Coupled Receptor Homodimer

Bivalent ligands have emerged as chemical tools to study G protein-coupled receptor dimers. Using a combination of computational, chemical, and biochemical tools, here we describe the design of bivalent ligand 13 with high affinity ( K_DB1 = 21 pM) for the dopamine D₂ receptor (D₂R) homodimer. Bivalent ligand 13 enhances the binding affinity relative to monovalent compound 15 by 37-fold, indicating simultaneous binding at both protomers. Using synthetic peptides with amino acid sequences of transmembrane (TM) domains of D₂R, we provide evidence that TM6 forms the interface of the homodimer. Notably, the disturber peptide TAT-TM6 decreased the binding of bivalent ligand 13 by 52-fold and had no effect on monovalent compound 15, confirming the D₂R homodimer through TM6 ex vivo. In conclusion, by using a versatile multivalent chemical platform, we have developed a precise strategy to generate a true bivalent ligand that simultaneously targets both orthosteric sites of the D₂R homodimer.

Efficient route to pre-organized and linear polyaminopolycarboxylates: Cy-TTHA, Cy-DTPA and mono/di- reactive, tert-butyl protected TTHA/Cy-TTHA

Pre-organized polyaminopolycarboxylate chelators Cy-TTHA and Cy-DTPA were synthesized via modular five-step syntheses from commercially available starting materials in ~ 62% and 47% overall yields, respectively. Furthermore, strategies are reported for the efficient preparation of mono- and di-reactive, tert-butyl-protected TTHA/Cy-TTHA to selectively functionalize central chelators' carboxylic acids.

r1andr2Relaxivities of Dendrons Based on a OEG-DTPA Architecture: Effect of Gd3+Placement and Dendron Functionalization

In magnetic resonance imaging, contrast agents are employed to enhance the signal intensity. However, current commercial contrast agents are hindered by a low relaxivity constant. Dendrimers can be employed to create higher molecular weight contrast agents which have an increased relaxivity due to a lower molecular rotation. In this study, dendrimers containing DTPA derivatives as cores and/or branching units were used to chelate gadolinium ions. Locating the gadolinium ions inside the dendrimers results in higher relaxivity constants, possibly because the paramagnetic center is closer to the rotational axis of the macromolecule. The highest gain in relaxivity was produced by decorating the dendron surface with peptide sequences, which could be explained by the presence of more second-sphere water molecules attracted by the peptides. These findings could contribute to the development of more effective contrast agents, either by placing the paramagnetic gadolinium ion in a strategic position or through functionalization of the dendron surface.

Multivalent dendrimers presenting spatially controlled clusters of binding epitopes in thermoresponsive hyaluronan hydrogels

The controlled presentation of biofunctionality is of key importance for hydrogel applications in cell-based regenerative medicine. Here, a versatile approach was demonstrated to present clustered binding epitopes in an injectable, thermoresponsive hydrogel. Well-defined multivalent dendrimers bearing four integrin binding sequences and an azido moiety were covalently grafted to propargylamine-derived hyaluronic acid (Hyal-pa) using copper-catalyzed alkyne-azide cycloaddition (CuAAC), and then combined with pN-modified hyaluronan (Hyal-pN). The dendrimers were prepared by synthesizing a bifunctional diethylenetriamine pentaacetic acid core with azido and NHBoc oligo(ethylene glycol) aminoethyl branches, then further conjugated with solid-phase synthesized RGDS and DGRS peptides. Azido terminated pN was synthesized by reversible addition-fragmentation chain transfer polymerization and reacted to Hyal-pa via CuAAC. Nuclear magnetic resonance (NMR), high performance liquid chromatography, size exclusion chromatography and mass spectroscopy proved that the dendrimers had well-defined size and were disubstituted. NMR and atomic absorption analysis confirmed the hyaluronan was affixed with dendrimers or pN. Rheological measurements demonstrated that dendrimers do not influence the elastic or viscous moduli of thermoresponsive hyaluronan compositions at a relevant biological concentration. Finally, human mesenchymal stromal cells were encapsulated in the biomaterial and cultured for 21days, demonstrating the faculty of this dendrimer-modified hydrogel as a molecular toolbox for tailoring the biofunctionality of thermoresponsive hyaluronan carriers for biomedical applications.

A hydrazide-anchored dendron scaffold for chemoselective ligation strategies

We report the design and synthesis of a dendron scaffold, enabling the chemoselective decoration of target molecules with multiple copies of functional species, such as peptides, via a hydrazone bond.