Photocaged fluorescent probes for spatiotemporally monitoring G-Quadruplex DNA in live cells

G-quadruplexes (G4s) are important in biological processes such as gene transcription, telomere maintenance, and chromosome stability, and they hold promise as therapeutic targets in cancer research. Current G4 probes face challenges, including high background fluorescence, low regulation efficiency, and lack of spatiotemporal control. Photocaged technology offers precise temporal control and minimal background interference, making it a promising solution to these issues. Herein, we developed two photocaged G4 fluorescent probes, Nv-N-CQ and Nv-O-CQ, which use a photoremovable protecting group to block the fluorescence of coumarin-quinazoline (CQ) and its ability to bind to G4s. Upon UV light activation, Nv-O-CQ efficiently converted to CQ with minimal byproduct formation. It selectively bound to G4 structures, such as c-MYC, and enhanced their thermal stability. In cellular experiments, the probe demonstrated light-controlled fluorescence release and spatiotemporal specificity towards G4s in the cytoplasm. These findings highlight the potential of Nv-O-CQ for biological imaging, probe development, and spatiotemporal studies.

Coumarin-Derived Caging Groups in the Spotlight: Tailoring Physiochemical and Photophysical Properties

The development of light-responsive molecular tools enables spatiotemporal control of biochemical processes with superior precision. Amongst these molecular tools, photolabile caging groups are employed to prevent critical binding interactions between a bioactive molecule and its corresponding target. Only upon irradiation with light, the bioactive is released in its 'active' form and is now readily available to bind to its target. Coumarin-derived caging groups constitute one of the most popular classes of photolabile protecting groups, due to their facile synthetic accessibility, ease of tuning photophysical properties via structural modification and rapid photolysis reactions. Herein, we highlight the recent progress made on the development of coumarin-derived caging groups, in which the red-shifting of absorption spectra, improving aqueous solubility and tailoring sub-cellular localisation has been of particular interest.

Elucidation of the working principle of a gene-directed caged HDAC inhibitor with cell-type selectivity

Histone deacetylases (HDACs) play crucial roles in the epigenetic regulation of gene expression. Here, we report CM-Bhc-SAHA, a novel caged HDAC inhibitor, genetically targeting cells of interest. Mammalian cells expressing porcine liver esterase led to the optochemical inhibition of endogenous HDAC activity when treated with CM-Bhc-SAHA and irradiated with 405 nm light.

Chemo-optogenetic Protein Translocation System Using a Photoactivatable Self-Localizing Ligand

Manipulating subcellular protein localization using light is a powerful approach for controlling signaling processes with high spatiotemporal precision. The most widely used strategy for this is based on light-induced protein heterodimerization. The use of small synthetic molecules that can control the localization of target proteins in response to light without the need for a second protein has several advantages. However, such methods have not been well established. Herein, we present a chemo-optogenetic approach for controlling protein localization using a photoactivatable self-localizing ligand (paSL). We developed a paSL that can recruit tag-fused proteins of interest from the cytoplasm to the plasma membrane within seconds upon light illumination. This paSL-induced protein translocation (paSLIPT) is reversible and enables the spatiotemporal control of signaling processes in living cells, even in a local region. paSLIPT can also be used to implement simultaneous optical stimulation and multiplexed imaging of molecular processes in a single cell, offering an attractive and novel chemo-optogenetic platform for interrogating and engineering dynamic cellular functions.

Design and synthesis of gene-directed caged cyclic nucleotides exhibiting cell type selectivity

We designed a new caging group that can be photoactivated only in the presence of a non-endogenous enzyme when exposed to 405 nm light. Because cells or tissues can be genetically tagged by an exogenously expressed enzyme, this novel method can serve as a strategy for adding targeting abilities to photocaged compounds.

Targeted Cancer Therapy Using Compounds Activated by Light

Cancer chemotherapy is affected by a modest selectivity and toxic side effects of pharmacological interventions. Among novel approaches to overcome this limitation and to bring to therapy more potent and selective agents is the use of light for selective activation of anticancer compounds. In this review, we focus on the anticancer applications of two light-activated approaches still in the experimental phase: photoremovable protecting groups ("photocages") and photoswitches. We describe the structural considerations behind the development of novel compounds and the plethora of assays used to confirm whether the photochemical and pharmacological properties are meeting the stringent criteria for an efficient in vivo light-dependent activation. Despite its immense potential, light activation brings many challenges, and the complexity of the task is very demanding. Currently, we are still deeply in the phase of pharmacological tools, but the vivid research and rapid development bring the light of hope for potential clinical use.

Visible-to-NIR-Light Activated Release: From Small Molecules to Nanomaterials

Photoactivatable (alternatively, photoremovable, photoreleasable, or photocleavable) protecting groups (PPGs), also known as caged or photocaged compounds, are used to enable non-invasive spatiotemporal photochemical control over the release of species of interest. Recent years have seen the development of PPGs activatable by biologically and chemically benign visible and near-infrared (NIR) light. These long-wavelength-absorbing moieties expand the applicability of this powerful method and its accessibility to non-specialist users. This review comprehensively covers organic and transition metal-containing photoactivatable compounds (complexes) that absorb in the visible- and NIR-range to release various leaving groups and gasotransmitters (carbon monoxide, nitric oxide, and hydrogen sulfide). The text also covers visible- and NIR-light-induced photosensitized release using molecular sensitizers, quantum dots, and upconversion and second-harmonic nanoparticles, as well as release via photodynamic (photooxygenation by singlet oxygen) and photothermal effects. Release from photoactivatable polymers, micelles, vesicles, and photoswitches, along with the related emerging field of photopharmacology, is discussed at the end of the review.

Conditionally Activatable Visible-Light Photocages

The proof of concept for conditionally activatable photocages is demonstrated on a new vinyltetrazine-derivatized coumarin. The tetrazine form is disabled in terms of light-induced cargo release, however, bioorthogonal transformation of the modulating tetrazine moiety results in fully restored photoresponsivity. Irradiation of such a “click-armed” photocage with blue light leads to fast and efficient release of a set of caged model species, conjugated via various linkages. Live-cell applicability of the concept was also demonstrated by the conditional release of a fluorogenic probe using mitochondrial pretargeting.

Photocontrollable Release with Coumarin-Based Profragrances

The achievement of controllable and lasting scent on a targeted surface is a long-term goal in the field of flavors and fragrances. Herein, we design a novel series of phototriggered coumarin-based profragrances conjugated with volatile carboxylic fragrances via activatable chemical bridge of ester group, thereby achieving the controllable release of volatile fragrances under ambient conditions. Upon exposure to light, the fragile ester group of profragrances allows the slow release of fragrance molecules, building up a new light-sensitive fragrance delivery system. The incorporated coumarin unit of CM-OH as phototrigger is killing two birds with one stone, that is, precise photocontrollable release of fragrance molecules, and unprecedented fluorescence intensity to monitor the releasing process of fragrance molecules with linear relationship (R² > 0.95). In comparison, the light-induced releasing amount from profragrances of CM-O-EA, CM-O-PEA, CM-O-PA, and CM-O-CA is much lower than corresponding free fragrances by 33-, 8.5-, 13-, and 983-fold, respectively. As demonstrated, the coumarin-based profragrances provide a phototriggered platform to realize the controllable release of volatile fragrances, resulting in a long-lasting headspace concentration on the targeted surface of wallpaper.