Fluorogenic In Situ Thioacetalization: Expanding the Chemical Space of Fluorescent Probes, Including Unorthodox, Bifurcated, and Mechanosensitive Chalcogen Bonds

Abstract

Progress with fluorescent flippers, small-molecule probes to image membrane tension in living systems, has been limited by the effort needed to synthesize the twisted push−pull mechanophore. Here, we move to a higher oxidation level to introduce a new design paradigm that allows the screening of flipper probes rapidly, at best in situ. Late-stage clicking of thioacetals and acetals allows simultaneous attachment of targeting units and interfacers and exploration of the critical chalcogen-bonding donor at the same time. Initial studies focus on plasma membrane targeting and develop the chemical space of acetals and thioacetals, from acyclic amino acids to cyclic 1,3-heterocycles covering dioxanes as well as dithiolanes, dithianes, and dithiepanes, derived also from classics in biology like cysteine, lipoic acid, asparagusic acid, DTT, and epidithiodiketopiperazines. From the functional point of view, the sensitivity of membrane tension imaging in living cells could be doubled, with lifetime differences in FLIM images increasing from 0.55 to 1.11 ns. From a theoretical point of view, the complexity of mechanically coupled chalcogen bonding is explored, revealing, among others, intriguing bifurcated chalcogen bonds.

A Fluorogenic Substrate for Quinoline Reduction: Pnictogen‐Bonding Catalysis in Aqueous Systems

“…Together with attractive interactions between the acidic hydrogens in ortho position and the substrate in ground‐ and transition‐state, it was thus conceivable that the complementary repulsion between them and the σ hole, the contrary of intramolecular pnictogen bonds, contributes to catalytic activity. Intramolecular repulsion between acidified hydrogens and σ holes has been shown previously to account for function [72] …”

Hydrophobic Interfacing of Fluorescent Membrane Probes

“…The head group triad of the original Flipper‐TR® 1 is composed of an ether, a triazole and a carboxylate (Figure 1). [5] The ether is essential as a non‐covalent turn‐on donor in the probes push‐pull system for reasons that are not important for the topic of this study (Figure 1a, r′ , a ) [5,68] . The triazole serves as a proton scavenger to prevent headgroup elimination [5,69] .…”

“…[5] The ether is essential as a non-covalent turn-on donor in the probes pushpull system for reasons that are not important for the topic of this study (Figure 1a, r', a). [5,68] The triazole serves as a proton scavenger to prevent headgroup elimination. [5,69] The carboxylate assures plasma membrane labeling and suppresses background fluorescence by micelle formation in water.…”

Core‐Alkynylated Fluorescent Flippers: Altered Ultrafast Photophysics to Track Thick Membranes

“…Experimental details, including references; [53,61,70,71,[89][90][91][92][93][94][95][96][97][98][99][100][101][102][103] preliminary results on the topic have been published in two PhD theses. [104,105]…”