Design, synthesis and evaluation of multifunctional dithienothiophene monomers, dimers and trimers

Inspired by the color tuning of carotenoids pigments in lobster, our group highlighted how the interplay of chromophore polarization and planarization is the key point for the design of environment sensitive fluorescent probes, named “flippers”. They are constituted by an electron rich DTT (dithieno[3,2-b;20,30-d]thiophene) moiety connected through a bond to an electron poor DTT-S,S-dioxide to achieve a fully operative push-pull system. A polar head group is linked to the hydrophobic core to ensure right orientation inside the lipid membrane. Sensitivity comes from the ability to adopt a twisted or planar conformation in response to the packing of the environment. The difference observed in excitation spectrum of the two conformation can be used to detect and study membrane tension. To screen the effect of further chemical modifications, chromophore length and oxidation, steric hindrance and extended conjugation are the main factors that can be evaluated in order to affect the spectroscopic properties of these powerful probes

Planarizable push-pull probes : overtwisted flipper mechanophores

Planarizable push–pull fluorescent probes, also referred to as flipper probes, have been introduced as conceptually innovative mechanophores that report on forces in their local environment in lipid bilayer membranes. The best flipper probes respond to a change from liquid disordered to solid ordered membranes with a red shift in excitation of 50–90 nm. A simultaneous increase in fluorescence lifetime and negligible background fluorescence from the aqueous phase qualifies these fluorescent probes for meaningful imaging in live cells. Here, we report that the replacement of methyl with isobutyl substituents along the scaffold of a dithienothiophene dimer strongly reduces fluorescence intensity but increases solvatochromism slightly. These trends imply that the large substituents in “leucine flippers” hinder the planarization in the first excited state to result in twisted intramolecular charge transfer (TICT). As a result of this overtwisting, the leucine flippers form interesting fluorescent micelles in water but fail to respond to changes in membrane order. These dramatic changes in function provide one of the most impressive illustrations for the hypersensitivity of fluorescent membrane probes toward small changes in their structure

Signal responsive transient coacervation in complex coacervate core micelles

Triggered coacervate phase (de)stabilisation in complex coacervate core micelles (C3Ms) has traditionally been limited to changes in pH and salt concentration, limiting options in responsive C3M material design. To expand this toolbox, we have developed C3Ms, that, at constant physiological pH, assemble and disassemble by coupling to a chemical reaction network (CRN) driven by the conversion of electron deficient allyl acetates and thiol or amine nucleophiles. This CRN produces transient quaternization of tertiary amine-functionalised block copolymers, which can then form the complex coacervate phase. We demonstrate triggered C3M assembly using two different allyl acetates, resulting in dramatically different assembly rates from hours to days. These are applied in various combinations with selected nucleophiles, demonstrating sequential signal induced C3M formation and deformation, as well as transient non-equilibrium (de)formation. We expect that timed and signal-responsive control over coacervate phase formation at physiological pH will find application in nucleic acid delivery, nano reactors and protocell research

Anion Transport with Chalcogen Bonds

In this report, we introduce synthetic anion transporters that operate with chalcogen bonds. Electron-deficient dithieno[3,2-b;2′,3′-d]thiophenes (DTTs) are identified as ideal to bind anions in the focal point of the σ holes on the cofacial endocyclic sulfur atoms. Anion binding in solution and anion transport across lipid bilayers are found to increase with the depth of the σ holes of the DTT anionophores. These results introduce DTTs and related architectures as a privileged motif to engineer chalcogen bonds into functional systems, complementary in scope to classics such as 2,2′-bipyrroles or 2,2′-bipyridines that operate with hydrogen bonds and lone pairs, respectively

Organocatalytic Control over a Fuel-Driven Transient Esterification Network

Signal transduction in living systems is the conversion of information into a chemical change and the principal process by which cells communicate. This process enables phenomena such as time-keeping and signal amplification. In nature, these functions are encoded in non-equilibrium (bio)chemical reaction networks (CRNs) controlled by enzymes. While these catalytically controlled processes are an integral part of biocatalytic pathways, man-made analogs are rare. Here, we incorporate catalysis in an artificial fuel driven out-of-equilibrium CRN. The study entails the design of an organocatalytically controlled fuel driven esterification CRN, where the forward (ester formation) and backward reaction (ester hydrolysis) are controlled by varying the ratio of two different organocatalysts: pyridine and imidazole. This catalytic regulation enables full control over ester yield and lifetime. The fuel-driven strategy is subsequently used in the design of a responsive polymer system, where transient polymer conformation and aggregation can be controlled through variation of fuel and catalysts levels. Altogether, we show how organocatalysis is an important tool to exert control over a man-made fuel driven system and induce a change in a macromolecular superstructure, as ubiquitously found in natural non-equilibrium systems. </p

TORC2 controls endocytosis through plasma membrane tension

Target of rapamycin complex 2 (TORC2) is a conserved protein kinase that regulates multiple plasma membrane (PM)–related processes, including endocytosis. Direct, chemical inhibition of TORC2 arrests endocytosis but with kinetics that is relatively slow and therefore inconsistent with signaling being mediated solely through simple phosphorylation cascades. Here, we show that in addition to and independently from regulation of the phosphorylation of endocytic proteins, TORC2 also controls endocytosis by modulating PM tension. Elevated PM tension, upon TORC2 inhibition, impinges on endocytosis at two different levels by (1) severing the bonds between the PM adaptor proteins Sla2 and Ent1 and the actin cytoskeleton and (2) hindering recruitment of Rvs167, an N-BAR–containing protein important for vesicle fission to endocytosis sites. These results underline the importance of biophysical cues in the regulation of cellular and molecular processes

Mechanosensitive Oligodithienothiophenes: Transmembrane Anion Transport Along Chalcogen-Bonding Cascades

The design, synthesis, and evaluation of multifunctional dithieno[3,2‐b;2′,3′‐d]thiophene (DTT) trimers is described. Twisted push‐push‐pull or donor‐donor‐acceptor (DDA) trimers composed of one DTT acceptor and two DTT donors show strong mechanochromism in lipid bilayer membranes. Red shifts in excitation rather than emission and fluorescence recovery with increasing membrane order are consistent with planarization of the twisted, extra‐long mechanophores in the ground state. The complementary pull‐pull‐pull or AAA trimers with deep σ holes all along the scaffold are not mechanochromic in membranes but excel with submicromolar anion transport activity. Anion transport along membrane‐spanning strings of chalcogen‐bond donors is unprecedented and completes previous results on transmembrane cascades that operate with equally unorthodox interactions such as halogen bonds and anion‐π interactions

A Chalcogen-Bonding Cascade Switch for Planarizable Push-Pull Probes

Planarizable push–pull probes have been introduced to demonstrate physical forces in biology. However, the donors and acceptors needed to polarize mechanically planarized probes are incompatible with their twisted resting state. The objective of this study was to overcome this “flipper dilemma” with chalcogen‐bonding cascade switches that turn on donors and acceptors only in response to mechanical planarization of the probe. This concept is explored by molecular dynamics simulations as well as chemical double‐mutant cycle analysis. Cascade switched flipper probes turn out to excel with chemical stability, red shifts adding up to high significance, and focused mechanosensitivity. Most important, however, is the introduction of a new, general and fundamental concept that operates with non‐trivial supramolecular chemistry, solves an important practical problem and opens a wide chemical space