Photoactivatable Carbo- and Silicon-Rhodamines and Their Application in MINFLUX Nanoscopy

MINFLUX nanoscopy: Visualising biological matter at the nanoscale level

Since its introduction in 2017, MINFLUX nanoscopy has shown that it can visualise fluorescent molecules with an exceptional localisation precision of a few nanometres. In this overview, we provide a brief insight into technical implementations, fluorescent marker developments and biological studies that have been conducted in connection with MINFLUX imaging and tracking. We also formulate ideas on how MINFLUX nanoscopy and derived technologies could influence bioimaging in the future. This insight is intended as a general starting point for an audience looking for a brief overview of MINFLUX nanoscopy from theory to application.

Singlets-Driven Photoredox Catalysts: Transforming Noncatalytic Red Fluorophores to Efficient Catalysts

Red-light absorbing photoredox catalysts offer potential advantages for large-scale reactions, expanding the range of usable substrates and facilitating bio-orthogonal applications. While many red-light absorbing/emitting fluorophores have been developed recently, functional red-light absorbing photoredox catalysts are scarce. Many photoredox catalysts rely on long-lived triplet excited states (triplets), which can efficiently engage in single electron transfer (SET) reactions with substrates. However, triplets of π-conjugated molecules are often significantly lower in energy than photogenerated singlet excited states (singlets). Combined with the inherent low energy of red light, this could limit the reductive/oxidative powers. Here, we introduce a series of sustainable heavy atom-free photoredox catalysts based on red-light absorbing dibenzo-fused BODIPY. The catalysts consist of two covalently linked units: a dibenzo-fused BODIPY fluorophore and an electron donor, arranged orthogonally. Excitation of the dibenzoBODIPY unit induces charge separation (CS) from the donor to the dibenzoBODIPY unit, forming a radical pair (RP) state. Unlike the regular BODIPY counterparts, these catalysts do not form triplets. Instead, SET occurs from the high-energy singlet-born RP states, preventing energy loss and effectively utilizing the low-energy red light. The proximity of donor molecules allows efficient charge separation despite the CS being uphill in energy. The molecules demonstrate efficient catalysis of Atom Transfer Radical Addition (ATRA) reaction, yielding products with high yields ranging from 70 to 90%, while the molecule without a donor group does not exhibit catalytic activity. The mechanistic studies by transient absorption and electron paramagnetic resonance (EPR) spectroscopy methods support the proposed mechanism. The study presents a new molecular design strategy for converting noncatalytic fluorophores to efficient photoredox catalysts operating in the red spectral region.

β-Galactosidase- and Photo-Activatable Fluorescent Probes for Protein Labeling and Super-Resolution STED Microscopy in Living Cells

We report on the synthesis of two fluorescent probes which can be activated by β-Galactosidase (β-Gal) enzymes and/or light. The probes contained 2-nitro-4-oxybenzyl and 3-nitro-4-oxybenzyl fragments, with β-Gal residues linked to C-4. We performed the enzymatic and photoactivation of the probes in a cuvette and compared them, prior to the labeling of Vimentin-Halo fusion protein in live cells with overexpressed β-galactosidase. The dye fluorescence afforded the observation of enzyme activity by means of confocal and super-resolution optical microscopy based on stimulated emission depletion (STED). The tracing of enzymatic activity with the retention of activated fluorescent products inside cells was combined with super-resolution imaging as a tool for use in biomedicine and life science.

No-wash fluorogenic labeling of proteins for reversible photoswitching in live cells

Photoswitchable fluorescent molecules (PSFMs) are positioned as valuable tools for biomolecule localization tracking and super-resolution imaging technologies due to their unique ability to reversibly control fluorescence intensity upon light irradiation. Despite the high demand for PSFMs that are suitable for live-cell imaging, no general method has been reported that enables reversible fluorescence control on proteins of interest in living cells. Herein, we have established a platform to realize reversible fluorescence switching in living cells by adapting a protein labeling system. We have developed a new PSFM, named HTL-Trp-BODIPY-FF, which exhibits strong fluorogenicity upon recognition of Halo-tag protein and reversible fluorescence photoswitching in living cells. This is the first example of a PSFM that can be applicable to a general-purpose Halo-tag protein labeling system for no-wash live-cell imaging.