Cells and organelles are delimited by lipid bilayers in which high deformability is essential to many cell processes, including motility, endocytosis and cell division. Membrane tension is therefore a major regulator of the cell processes that remodel membranes, albeit one that is very hard to measure in vivo. Here we show that a planarizable push-pull fluorescent probe called FliptR (fluorescent lipid tension reporter) can monitor changes in membrane tension by changing its fluorescence lifetime as a function of the twist between its fluorescent groups. The fluorescence lifetime depends linearly on membrane tension within cells, enabling an easy quantification of membrane tension by fluorescence lifetime imaging microscopy. We further show, using model membranes, that this linear dependency between lifetime of the probe and membrane tension relies on a membrane-tension-dependent lipid phase separation. We also provide calibration curves that enable accurate measurement of membrane tension using fluorescence lifetime imaging microscopy.

To measure the chemical-mechanic states of lipid membranes, once needs various tools, many of which being incompatible with cell biology protocols. Applying lessons from nature, we developed a mechanosensitive fluorescent probe, the twisted dithienothiophene. This push-pull probe, change planarization state in function of his environment, and we have taken full advantage of this mechano-probe potential and we calibrated based on membrane tension, fluidity and different lipid composition by measuring the push-pull fluorescence lifetime. Likewise, we are able to use this fluorescent probe on life cells, for visualize differences between organelles, as well as to distinguish lipids properties among cells cultured on classic plates or in extracellular matrix.

Planarizable and polarizable dithieno[3,2‐b;2′,3′‐d]thiophene (DTT) dimers have been introduced recently as fluorescent probes that report on membrane fluidity with red shifts in excitation, i.e. planarization in the ground state. In this study, we elaborate on the hypothesis that twisted push‐pull probes could perform best in the presence of one unorthodox substituent that acts as a weak acceptor with electron‐rich and as a strong donor with electron‐poor aromatics. According to Hammett constants, we thought that sulfides could provide access to such a conceptually innovative donor‐acceptor switch. To elaborate on this hypothesis, we here describe the design, synthesis and evaluation of a comprehensive series of twisted push‐pull probes with turn‐on sulfide donors. Their planarization is explored in lipid bilayer membranes of different thickness and fluidity from liquid‐disordered to liquid‐ordered and solid‐ordered phases. Results from membranes are compared to the planarization of turn‐on mechanophores in crystals, proteins, and cyclodextrin macrocycles of varied diameter.

It is currently not possible to directly measure the lateral pressure of a biomembrane. Mechanoresponsive fluorescent probes are an elegant solution to this problem but it requires first the establishment of a direct correlation between the membrane surface pressure and the induced color change of the probe. Here, we analyze planarizable dithienothiophene push-pull probes in a monolayer at the air/water interface using fluorescence microscopy, grazing-incidence angle X-ray diffraction, and infrared reflection-absorption spectroscopy. An increase of the lateral membrane pressure leads to a well-packed layer of the 'flipper' mechanophores and a clear change in hue above 18 mN/m. The fluorescent probes had no influence on the measured isotherm of the natural phospholipid DPPC suggesting that the flippers probe the lateral membrane pressure without physically changing it. This makes the flipper probes a truly useful addition to the membrane probe toolbox.

In this report, "fluorescent flippers" are introduced to create planarizable push-pull probes with the mechanosensitivity and fluorescence lifetime needed for practical use in biology. Twisted push-pull scaffolds with large and bright dithienothiophenes and their S,S-dioxides as the first "fluorescent flippers" are shown to report on the lateral organization of lipid bilayers with quantum yields above 80% and lifetimes above 4 ns. Their planarization in liquid-ordered (Lo) and solid-ordered (So) membranes results in red shifts in excitation of up to +80 nm that can be transcribed into red shifts in emission of up to +140 nm by Förster resonance energy transfer (FRET). These unique properties are compatible with multidomain imaging in giant unilamellar vesicles (GUVs) and cells by confocal laser scanning or fluorescence lifetime imaging microscopy. Controls indicate that strong push-pull macrodipoles are important, operational probes do not relocate in response to lateral membrane reorganization, and two flippers are indeed needed to "really swim," i.e., achieve high mechanosensitivity.

Monomers with large surface area and high quantum yield, that is fluorescent flippers, have been engineered into twisted push-pull oligomers to create membrane probes with high mechanosensitivity and long fluorescence lifetime. Here, the synthesis and characterization of thieno[3,4]pyrazines and 2,1,3-benzothiadiazoles are described in comparison with the original dithienothiophene S,S-dioxides. Dithienothiophene S,S-dioxide flippers are confirmed as the best reported so far, and poor results with single flipper probes support that two flippers are needed for the probe to really "swim", that is, for high mechanosensitivity.

This article assembles pertinent insights behind the concept of planarizable push-pull probes. As a response to the planarization of their polarized ground state, a red shift of their excitation maximum is expected to report on either the disorder, the tension, or the potential of biomembranes. The combination of chromophore planarization and polarization contributes to various, usually more complex processes in nature. Examples include the color change of crabs or lobsters during cooking or the chemistry of vision, particularly color vision. The summary of lessons from nature is followed by an overview of mechanosensitive organic materials. Although often twisted and sometimes also polarized, their change of color under pressure usually originates from changes in their crystal packing. Intriguing exceptions include the planarization of several elegantly twisted phenylethynyl oligomers and polymers. Also mechanosensitive probes in plastics usually respond to stretching by disassembly. True ground-state planarization in response to molecular recognition is best exemplified with the binding of thoughtfully twisted cationic polythiophenes to single- and double-stranded oligonucleotides. Molecular rotors, en vogue as viscosity sensors in cells, operate by deplanarization of the first excited state. Pertinent recent examples are described, focusing on λ-ratiometry and intracellular targeting. Complementary to planarization of the ground state with twisted push-pull probes, molecular rotors report on environmental changes with quenching or shifts in emission rather than absorption. The labeling of mechanosensitive channels is discussed as a bioengineering approach to bypass the challenge to create molecular mechanosensitivity and use biological systems instead to sense membrane tension. With planarizable push-pull probes, this challenge is met not with twistome screening, but with "fluorescent flippers," a new concept to insert large and bright monomers into oligomeric probes to really feel the environment and also shine when twisted out of conjugation.

The concept to couple fluorophore planarization and fluorophore polarization for the construction of innovative fluorescent membrane probes is elaborated comprehensively in the context of oligothiophenes. Increasing length with different degree of twist from ter- to quinquethiophenes results in increasing extinction coefficients, decreasing quantum yields and relatively minor red shifts. Quaterthiophenes show maximal Stokes shifts and are thus preserved to further elaborate on deplanarization. Increasing quaterthiophene deplanarization results in increasing blue shifts and decreasing quantum yields in solution, whereas planarization in solid-ordered lipid bilayer membranes gives the respective red shifts with fluorescence recovery. An extensive screening reveals that intermediate global deplanarization with strong individual twists near the membrane interface are best. Weaker and stronger global twisting and strong individual twists deeper in the membrane are less convincing because planarization becomes either too easy or too difficult. The best probe reports decreasing membrane fluidity with a red shift of 44 nm and a fluorescence increase of almost 500%. These insights are important because they cover significant chemical space to help improving our understanding of chromophore twisting and promise bright perspectives with regard to biological applications and refined probe design.

We report design, synthesis and evaluation of push-pull quaterthiophene amphiphiles containing one 3,4-ethylenedioxythiophene (EDOT) and a single strong twist in the scaffold. Planarizable push-pull oligothiophene amphiphiles have been introduced recently as conceptually innovative fluorescent probes that sense the fluidity and the potential of lipid bilayer membranes. The "hyper-twisted" EDOT probes respond to planarization and restricted rotational freedom with a red shift and changes in vibrational finestructure in the excitation spectrum, respectively. In solution, comparably weak solvatochromism and significant thermochromism are found. Planarization and restricted rotational freedom afford exquisite sensitivity toward nature and fluidity of lipid bilayer membranes, including ratiometric detection of phase transitions. The sensing of membrane potentials is weakened by these unique properties but remains possible.

Oxidation mechanisms in air nonthermal plasma (NTP) at room temperature and atmospheric pressure were investigated in a corona reactor energized by +dc, -dc, or +pulsed high voltage.. The two bromomethanes CF(2)Br(2) and CH(2)Br(2) were chosen as model organic pollutants because of their very different reactivities with OH radicals. Thus, they served as useful mechanistic probes: they respond differently to the presence of humidity in the air and give different products. By FT-IR analysis of the postdischarge gas the following products were detected and quantified: CO(2) and CO in the case of CH(2)Br(2), CO(2) and F(2)C ═ O in the case of CF(2)Br(2). F(2)C ═ O is a long-lived oxidation intermediate due to its low reactivity with atmospheric radicals. It is however removed from the NTP processed gas by passage through a water scrubber resulting in hydrolysis to CO(2) and HF. Other noncarbon containing products of the discharge were also monitored by FT-IR analysis, including HNO(3) and N(2)O. Ozone, an important product of air NTP, was never detected in experiments with CF(2)Br(2) and CH(2)Br(2) because of the highly efficient ozone depleting cycles catalyzed by BrOx species formed from the bromomethanes. It is concluded that, regardless of the type of corona applied, CF(2)Br(2) reacts in air NTP via a common intermediate, the CF(2)Br radical. The possible reactions leading to this radical are discussed, including, for -dc activation, charge exchange with O(2)(-), a species detected by APCI mass spectrometry.

Catalyst development is a challenging task, caused by the subtle effects that determine whether a catalyst is efficient or not Success is enhanced by using methodology that relies to a smaller extent on rational design. Combinatorial high-throughput approaches allow for a systematic exploration of chemical space, but require an easy synthetic access to structurally diverse catalysts The use of dynamic or reversible chemistry for the construction of catalysts is an attractive option as it allows for self-assembly and self-selection processes. Synthesis is restricted to the building blocks after which diversity is simply generated upon mixing. Self-selection of the best catalyst by the target reaction relieves the burden of rational design Molecular systems exhibiting catalysis as an emerging property due to a cooperative interplay of the molecular components are envisioned for the future.

The development of a serine protease model using a self-selection protocol is described. The developed system mimics the regeneration step of an enzyme involved in covalent enzyme catalysis. A transition-state analogue of a transesterification reaction is used to self-select functional groups able to accelerate ester cleavage. It is shown that the insertion of a tertiary amine substituent flanking the reaction center reinforces transition-state stabilization by directing the reactive center towards the self-selected functionality. In addition, the tertiary amine activates a bland (solvent) nucleophile for attack on an ester bond similar to what occurs in a serine protease. A quantitative correspondence is observed between the amplification factors and catalytic activity, illustrating the potential of the dynamic covalent capture strategy to precisely detect and quantify weak noncovalent interactions.

Oxidation mechanisms in air nonthermal plasma (NTP) at room temperature and atmospheric pressure were investigated in a corona reactor energized by +dc, −dc, or +pulsed high voltage.. The two bromomethanes CF2Br2 and CH2Br2 were chosen as model organic pollutants because of their very different reactivities with OH radicals. Thus, they served as useful mechanistic probes: they respond differently to the presence of humidity in the air and give different products. By FT-IR analysis of the postdischarge gas the following products were detected and quantified: CO2 and CO in the case of CH2Br2, CO2 and F2CO in the case of CF2Br2. F2CO is a long-lived oxidation intermediate due to its low reactivity with atmospheric radicals. It is however removed from the NTP processed gas by passage through a water scrubber resulting in hydrolysis to CO2 and HF. Other noncarbon containing products of the discharge were also monitored by FT-IR analysis, including HNO3 and N2O. Ozone, an important product of air NTP, was never detected in experiments with CF2Br2 and CH2Br2 because of the highly efficient ozone depleting cycles catalyzed by BrOx species formed from the bromomethanes. It is concluded that, regardless of the type of corona applied, CF2Br2 reacts in air NTP via a common intermediate, the CF2Br radical. The possible reactions leading to this radical are discussed, including, for −dc activation, charge exchange with O2 −, a species detected by APCI mass spectrometry.

(13)C-isotope labelling is presented as a novel tool for the study of complex chemical systems. (13)C-isotope labelling permits the quantification of all 26 members of a dynamic library from a single (13)C NMR spectrum without the need for advanced instrumentation or sophisticated experimental protocols.

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