land § These two authors contributed equally ABSTRACT. Measuring forces inside cells is particularly challenging. With the development of quantitative microscopy, fluorophores which allow the measurement of forces became highly desirable. We have previously introduced a mechanosensitive flipper probe, which responds to the change of plasma membrane tension by changing fluorescence lifetime and thus allows tension imaging by FLIM. Herein, we describe the design, synthesis, and evaluation of flipper probes that selectively label intracellular organelles, i.e., lysosomes, mitochondria, and the endoplasmic reticulum. The probes respond uniformly to osmotic shocks applied extracellularly, thus confirming sensitivity toward changes in membrane tension.At rest, different lifetimes found for different organelles relate to known differences in membrane organization rather than membrane tension and allow co-labeling in the same cells. At the organelle scale, lifetime heterogeneity provides unprecedented insights on ER tubules and sheets, and nuclear membranes.Examples on endosomal trafficking or increase of tension at mitochondrial constriction sites outline the potential of intracellularly targeted fluorescent tension probes to address essential questions that were previously beyond reach.The importance of mechanical forces in biological processes is only starting to emerge. 1-3 Plasma membrane tension is a topic of particular current interest because mounting evidence suggests its involvement in regulating various biochemical processes in cells. 2 Although membrane tension should also regulate membranous organelles' functions, standard techniques of force measurements, such as optical tweezers or force microscopes are difficult to apply inside of cells. 3 Therefore, the role of membrane tension in

In this review, the multifunctionality of dithieno[3,2-b:2',3'-d]thiophenes (DTTs) is covered comprehensively. This is of interest because all involved research is very recent, emphasizes timely topics such as mechanochemistry for bioimaging or chalcogen bonds for catalysis and solar cells, and because the newly emerging privileged scaffold is embedded in an inspiring structural space. At the beginning, DTTs are introduced with regard to nomenclature, constitutional isomers and optoelectronic properties. The structural space around DTTs is mapped out next with regard to heteroatom substitution in bridge and core, covering much of the periodic table, eccentric heteroatom doping and bridge expansions. After a brief summary of synthetic approaches to the DTT scaffold, chalcogen bonds are introduced as, together with redox switching and turn-on fluorescence, one of the three conceptual foundations of most multifunctionality. Realized functions cover anion binding, transport (ion carriers, ion channels), catalysis, and the first fluorescent probes to image physical forces in living cells.

Alkyl- and aryl vinyl sulfones were obtained by eosin Y (EY)-mediated visible-light photooxidation of sulfinate salts and the reaction of the resulting S-centered radicals with alkenes. Optimized reaction conditions, the sulfinate and alkene scope, and X-ray structural analyses of several reaction products are provided. A detailed spectroscopic study explains the reaction mechanism, which proceeds through the EY radical cation as key intermediate oxidizing the sulfinate salts.

Tools to image membrane tension in response to mechanical stimuli are badly needed in mechanobiology. We have recently introduced mechanosensitive flipper probes to report quantitatively global membrane tension changes in fluorescence lifetime imaging microscopy (FLIM) images of living cells. However, to address specific questions on physical forces in biology, the probes need to be localized precisely in the membrane of interest (MOI). Herein we present a general strategy to image the tension of the MOI by tagging our newly introduced HaloFlippers to self-labeling HaloTags fused to proteins in this membrane. The critical challenge in the construction of operational HaloFlippers is the tether linking the flipper and the HaloTag: It must be neither too taut nor too loose, be hydrophilic but lipophilic enough to passively diffuse across membranes to reach the HaloTags, and allow partitioning of flippers into the MOI after the reaction. HaloFlippers with the best tether show localized and selective fluorescence after reacting with HaloTags that are close enough to the MOI but remain nonemissive if the MOI cannot be reached. Their fluorescence lifetime in FLIM images varies depending on the nature of the MOI and responds to myriocin-mediated sphingomyelin depletion as well as to osmotic stress. The response to changes in such precisely localized membrane tension follows the validated principles, thus confirming intact mechanosensitivity. Examples covered include HaloTags in the Golgi apparatus, peroxisomes, endolysosomes, and the ER, all thus becoming accessible to the selective fluorescence imaging of membrane tension.

A new application of flavin derivatives in visible light photocatalysis was found. 1-Butyl-7,8-dimethoxy-3-methylalloxazine, when irradiated by visible light, was shown to allow an efficient cyclobutane ring formation via an intramolecular [2+2] cycloaddition of both styrene dienes, considered as electron-rich substrates, and electron-poor bis(arylenones), presumably proceeding via an energy transfer mechanism.

Planarizable push-pull probes have been introduced to demonstrate physical forces in biology.However,the donors and acceptors needed to polarizem echanically planarized probes are incompatible with their twisted resting state.T he 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.T his 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.M ost important, however,i st he introduction of an ew,g eneral and fundamental concept that operates with non-trivial supramolecular chemistry,solves an important practical problem and opens aw ide chemical space.Planarizable push-pull (PP) chromophores have been introduced [1] as mechanosensitive [2] fluorescent membrane probes [1][2][3] to image membrane tension [4] in living cells. [5,6] The current best is constructed around twisted dithienothiophene (DTT) dithienothiophene S,S-dioxide (DTTO2) conjugates ( Figure 1a,D ' = S, A' = SO 2 ). [7] Thet wo "flippers" [7] are twisted out of coplanarity by repulsion between the methyls (light blue circles) and the s holes (dark blue ovals) [8][9][10][11] on the sulfurs next to the connecting bond ( Figure 1b,l eft, 1). The PP system is prepared first with "sulfide" donors and "sulfone" acceptors in the DTT and the DTTO2b ridges, respectively (Figure 1b, 8,9). Conjugation of DTT and DTTO2u pon mechanical co-planarization then turns on this intrinsic PP system and shifts the excitation maximum to the red (Figure 1b,r ight). Thee mission maximum is nearly mechanoinsensitive because the probes emit only from the planar form. [12] To achieve significant red shifts upon planarization in the ground state,a dditional PP donors Da nd acceptors Aa re required (Figure 1a). These exocyclics ubstituents represent atrue dilemma because in the twisted resting state,the DTT donors and DTTO2 acceptors,a tl east partially decoupled from each other and equipped with extra Da nd A, could become too rich and too poor in electron density,respectively, and decompose easily (Figure 1a). Because both DTTs and DTTO2are comparably electron-rich, [13] this problem is more pronounced on the DTT side.T herefore,Dand As hould ideally turn on only in response to flipper planarization. Sulfides,p reviously introduced as covalent PP turn-on donors, [12] failed to afford operational probes. [14] Non-covalent 1,4-chalcogen bonds (1,4-CBs) [8][9][10][11] as in 1 were more successful, also because spontaneous degradation into reactive Figure 1. a) Flipper dilemma and b) CB cascade switch:a)Inplanarizable PP probes, exocyclic donors D D and acceptors A A are needed in planar but incompatiblew ith twisted form. b) Twisting of the central bond (1)turns off PP D D (2,r ed circle), and A A (3,b lue circle) because Lewis base Y( 5)hardly...

We report design, synthesis, and evaluation of fluorescent flipper probes for single-molecule super-resolution imaging of membrane tension in living cells. Reversible switching from bright-state ketones to dark-state hydrates, hemiacetals, and hemithioacetals is demonstrated for twisted and planarized mechanophores in solution and membranes. Broadband femtosecond fluorescence up-conversion spectroscopy evinces ultrafast chalcogen-bonding cascade switching in the excited state in solution. According to fluorescence lifetime imaging microscopy, the new flippers image membrane tension in live cells with record red shifts and photostability. Single-molecule localization microscopy with the new tension probes resolves membranes well below the diffraction limit. The imaging of physical forces in living systems is a central challenge in current biology. 1 To contribute solutions, we have introduced small-molecule chemistry tools to image membrane tension in living cells. 2Based on the concept of planarizable push-pull probes, 3,4 the so-called flipper probes allowed us to image the change of plasma 5 and organellar membrane tension in living cells. 6 Flipper probes are twisted dithienothiophene (DTT) push-pull dimers (Figure 1a). Their mechanical planarization in the ground state

This article describes four fluorescent membrane tension probes that have been designed, synthesized, evaluated, commercialized and applied to current biology challenges in the context of the NCCR Chemical Biology. Their names are Flipper-TR®, ER Flipper-TR®, Lyso Flipper-TR®, and Mito Flipper-TR®. They are available from Spirochrome.