Photoremovable protecting groups in chemistry and biology: reaction mechanisms and efficacy

Control of biomedical nanoparticle distribution and drug release in vivo by complex particle design strategies

The utilization of targeted nanoparticles as a selective drug delivery system is a powerful tool to increase the amount of active substance reaching the target site. This can increase therapeutic efficacy while reducing adverse drug effects. However, nanoparticles face several challenges: upon injection, the immediate adhesion of plasma proteins may mask targeting ligands, thereby diminishing the target cell selectivity. In addition, opsonization can lead to premature clearance and the widespread presence of receptors or enzymes limits the accuracy of target cell recognition. Nanoparticles may also suffer from endosomal entrapment, and controlled drug release can be hindered by premature burst release or insufficient particle retention at the target site. Various strategies have been developed to address these adverse events, such as the implementation of switchable particle properties, regulating the composition of the formed protein corona, or using click-chemistry based targeting approaches. This has resulted in increasingly complex particle designs, raising the question of whether this development actually improves the therapeutic efficacy in vivo. This review provides an overview of the challenges in targeted drug delivery and explores potential solutions described in the literature. Subsequently, appropriate strategies for the development of nanoparticular drug delivery concepts are discussed.

Not So Bioorthogonal Chemistry

The advent of bioorthogonal chemistry has transformed scientific research, offering a powerful tool for selective and noninvasive labeling of (bio)molecules within complex biological environments. This innovative approach has facilitated the study of intricate cellular processes, protein dynamics, and interactions. Nevertheless, a number of challenges remain to be addressed, including the need for improved reaction kinetics, enhanced biocompatibility, and the development of a more diverse and orthogonal set of reactions. While scientists continue to search for veritable solutions, bioorthogonal chemistry remains a transformative tool with a vast potential for advancing our understanding of biology and medicine. This Perspective offers insights into reactions commonly classified as "bioorthogonal", which, however, may not always demonstrate the desired selectivity regarding the interactions between their components and the additives or catalysts used under the reaction conditions.

Opto-Epigenetic Regulation of Histone Arginine Asymmetric Dimethylation via Type I Protein Arginine Methyltransferase Inhibition

Histone arginine asymmetric dimethylation, which is mainly catalyzed by type I protein arginine methyltransferases (PRMTs), is involved in broad biological and pathological processes. Recently, several type I PRMT inhibitors, such as MS023, have been developed to reverse the histone arginine dimethylation status in tumor cells, but extensive inhibition of type I PRMTs may cause side effects in normal tissues. Herein, we designed a photoactivatable MS023 prodrug (C-MS023) to achieve spatiotemporal inhibition of histone arginine asymmetric dimethylation. In vitro studies showed that C-MS023 exhibited reduced potency in inhibiting type I PRMTs. Importantly, visible light irradiation at 420 nm could trigger the photolysis of the prodrug, thereby liberating MS023 for effective downregulation of histone arginine asymmetric dimethylation and DNA replication-related transcriptomic activities. This opto-epigenetic small-molecule prodrug potentially aids in further research into the pathophysiological functions of type I PRMTs and the development of targeted epigenetic therapeutics.

Optogenetics with Atomic Precision─A Comprehensive Review of Optical Control of Protein Function through Genetic Code Expansion

Conditional control of protein activity is important in order to elucidate the particular functions and interactions of proteins, their regulators, and their substrates, as well as their impact on the behavior of a cell or organism. Optical control provides a perhaps optimal means of introducing spatiotemporal control over protein function as it allows for tunable, rapid, and noninvasive activation of protein activity in its native environment. One method of introducing optical control over protein activity is through the introduction of photocaged and photoswitchable noncanonical amino acids (ncAAs) through genetic code expansion in cells and animals. Genetic incorporation of photoactive ncAAs at key residues in a protein provides a tool for optical activation, or sometimes deactivation, of protein activity. Importantly, the incorporation site can typically be rationally selected based on structural, mechanistic, or computational information. In this review, we comprehensively summarize the applications of photocaged lysine, tyrosine, cysteine, serine, histidine, glutamate, and aspartate derivatives, as well as photoswitchable phenylalanine analogues. The extensive and diverse list of proteins that have been placed under optical control demonstrates the broad applicability of this methodology.

A near infrared light activated phenothiazine based cancer cell specific phototherapeutic system: a synergistic approach to chemo-photothermal therapy

In the pursuit of more effective cancer therapies, phototherapy has emerged as a promising approach due to its non-invasive nature and high precision. This study presents the development of a near-infrared (NIR) light-responsive phenothiazine (PTZ) based phototherapeutic system designed for targeted cancer treatment. This phototherapeutic system integrates four crucial elements for enhanced therapeutic efficacy: cancer cell-specific activity, mitochondrial targeting, photothermal conversion, and controlled drug release. The PTZ system utilizes the acidochromic 1,3-oxazine ring, which opens in the acidic tumor microenvironment, forming a positive iminium ion (CN+). This ionic species targets cancer cell mitochondria, ensuring precise localization. Under NIR light irradiation (640 nm), the phototherapeutic system undergoes a red shift in the absorption and reduction in the fluorescence intensity, demonstrating a significant photothermal effect that converts light to heat, thereby inducing tumor cell apoptosis. Furthermore, NIR light triggers the controlled release of the anticancer drug chlorambucil, enabling precise spatiotemporal drug delivery. The closed form of the phototherapeutic system also facilitates drug release upon visible light irradiation (≥410 nm) with high photochemical efficiency. This dual-mode photothermal and photocontrolled drug delivery offers a synergistic approach to cancer therapy, maximizing therapeutic outcomes while minimizing side effects. Our findings underscore the potential of this innovative phototherapeutic system to advance cancer treatment through targeted, controlled, and effective drug delivery.

Conditional Control of Benzylguanine Reaction with the Self-Labeling SNAP-tag Protein

SNAP-tag, a mutant of the O6-alkylguanine-DNA-alkyltransferase, self-labels by reacting with benzylguanine (BG) substrates, thereby forming a thioether bond. SNAP-tag has been genetically fused to a wide range of proteins of interest in order to covalently modify them. In the context of both diagnostic and therapeutic applications, as well as use as a biological recording device, precise control in a spatial and temporal fashion over the covalent bond-forming reaction is desired to direct inputs, readouts, or therapeutic actions to specific locations, at specific time points, in cells and organisms. Here, we introduce a comprehensive suite of six caged BG molecules: one light-triggered and five others that can be activated through various chemical and biochemical stimuli, such as small molecules, transition metal catalysts, reactive oxygen species, and enzymes. These molecules are unable to react with SNAP-tag until the trigger is present, which leads to near complete SNAP-tag conjugation, as illustrated both in biochemical assays and on human cell surfaces. This approach holds promise for targeted therapeutic assembly at disease sites, offering the potential to reduce off-target effects and toxicity through precise trigger titration.

Precise photorelease in living cells by high-viscosity activatable coumarin-based photocages

Intracellular viscosity is a critical microenvironmental factor in various biological systems, and its abnormal increase is closely linked to the progression of many diseases. Therefore, precisely controlling the release of bioactive molecules in high-viscosity regions is vital for understanding disease mechanisms and advancing their diagnosis and treatment. However, viscosity alone cannot directly trigger chemical reactions. Inspired by molecular rotor fluorophores, we have developed a series of high-viscosity activated photocages by modifying the C3 position of the coumarin scaffold with electron-withdrawing groups. In low-viscosity environments, both fluorescence and photocleavage of the photocages are inhibited by nonradiative decay caused by intramolecular free rotation. In contrast, in high-viscosity environments, the restriction of this intramolecular rotation restores fluorescence and photocleavage. These unique photolysis properties enable the selective photorelease of these photocages in high-viscosity conditions. As a proof of concept, we have developed a drug delivery system that targets abnormal mitochondria with high viscosity. This system demonstrates enhanced photolysis efficiency in abnormal mitochondria compared to normal ones, allowing for precise drug release in diseased mitochondria while ensuring excellent biological safety in healthy mitochondria. We anticipate that these photocages will serve as convenient and efficient tools for the precise release of active molecules in high-viscosity environments.

Spatio-temporal control of mitosis using light via a Plk1 inhibitor caged for activity and cellular permeability

The ability to control the activity of kinases spatially and temporally is essential to elucidate the role of signalling pathways in development and physiology. Progress in this direction has been hampered by the lack of tools to manipulate kinase activity in a highly controlled manner in vivo. Here we report a strategy to modify BI2536, the well characterized inhibitor of the conserved and essential mitotic kinase Polo-like kinase 1 (Plk1). We introduce the same coumarin photolabile protecting group (PPG) at two positions of the inhibitor. At one position, the coumarin prevents the interaction with Plk1, at the second it masks an added carboxylic acid, important for cellular retention. Exposure to light results in removal of both PPGs, leading to the activation of the inhibitor and its trapping inside cells. We demonstrate the efficacy of the caged inhibitor in three-dimensional spheroid cultures: by uncaging it with a single light pulse, we can inhibit Plk1 and arrest cell division, a highly dynamic process, with spatio-temporal control. Our design can be applied to other small molecules, providing a solution to control their activity in living cells with unprecedented precision.

Design strategies for tetrazine fluorogenic probes for bioorthogonal imaging

Tetrazine fluorogenic probes play a critical role in bioorthogonal chemistry, selectively activating fluorescence upon reaction to enhance precision in imaging and sensing within complex biological environments. Recent structural innovations-such as varied fluorophore choices, spacer optimization, and direct tetrazine integration within a fluorophore's π-conjugated system-have expanded their spectral range from visible to NIR, enhancing adaptability across various applications. This review examines advancements in the rational design and synthesis of these probes. We examine key fluorogenic mechanisms, such as energy transfer, internal conversion, and electron/charge transfer, that significantly influence fluorescence activation. We also highlight representative applications in live-cell imaging, super-resolution microscopy, and therapeutic monitoring, underscoring the expanding role of tetrazine probes in biomedical research and diagnostics. Collectively, these insights provide a strategic foundation for developing next-generation tetrazine probes with tailored properties to address evolving diagnostic and therapeutic challenges.

Thiolate-CLPG (chemiluminescent protecting groups) based on coumaranone thiolcarbamates

The base-induced oxidative release of aromatic and aliphatic thiols from novel coumaranone-thiolurethanes 3a-3e is accompanied by long-lasting blue or green chemiluminescence. Thus, these compounds serve as base/oxidation-labile protecting groups that decompose with strong visible luminescence and subsequent quantitative release of the protected thiolate as leaving group.

Red light excitation: illuminating photocatalysis in a new spectrum

Red-light-activated photocatalysis has become a powerful approach for achieving sustainable chemical transformations, combining high efficiency with energy-saving, mild conditions. By harnessing the deeper penetration and selectivity of red and near-infrared light, this method minimizes the side reactions typical of higher-energy sources, making it particularly suited for large-scale applications. Recent advances highlight the unique advantages of both metal-based and metal-free catalysts under red-light irradiation, broadening the range of possible reactions, from selective oxidations to complex polymerizations. In biological contexts, red-light photocatalysis enables innovative applications in phototherapy and controlled drug release, exploiting its tissue penetration and low cytotoxicity. Together, these developments underscore the versatility and impact of red-light photocatalysis, positioning it as a cornerstone of green organic chemistry with significant potential in synthetic and biomedical fields.

Mechanism-Guided Development of Bifunctional Cyclooctenes as Active, Practical, and Light-Gated Bromination Catalysts

Most molecular catalysts have been developed employing polar functional groups as catalytic sites. However, the use of non-polar functional groups for catalysis has received less attention due to their modest molecular interactions while the bioorthogonal reactivity of non-polar alkenes as substrates is frequently used in click chemistry. In this study, we conducted mechanistic studies on the catalysis of trans-cyclooctene (TCO) derivatives with the strained olefin as the catalytic site using kinetic and computational analyses to aid the design of more active olefin catalysts. The analysis reveals the significant role of the benzyl substituents in accelerating the generation of bromonium species through dispersion interaction in the rate-determining step. Guided by the mechanistic insights, we developed bifunctional TCO catalysts bearing a functionalized benzyl group, taking advantage of the remarkable substituent effects. Experimental studies confirmed the theoretical model and revealed that TCO with a para-hydroxybenzyl group provided excellent catalytic activity. Furthermore, inclusion of the functionalized benzyl groups allowed more readily available and robust cis-cyclooctenes to be used as active catalysts, expanding the practical utility of the olefin catalysts. By using a photochemically labile masking group on the para-hydroxybenzyl substituent, the first light-gated bromination catalyst was developed, enabling spatiotemporal control of the transformation.

Precise photopharmacological eradication of metastatic tumor cells

Owing to their high efficacy, antimitotic chemotherapeutics are the mainstay for most cancer treatments. However, these drugs do not discriminate between tumor and healthy cells, thus show dose-limiting toxicity and severe adverse effects. To improve treatments, rendering chemotherapeutics tumor-cell specific is highly desirable. Although various strategies, such as targeted antibody-drug conjugates, aim to achieve this goal, the identification of a tumor-specific 'Achilles' heel' remains a challenge. Here, we followed an alternative approach, which does not rely on tumor-specific characteristics, but rather uses spatially confined illumination of the light-activatable microtubule inhibitor SBTubA4P to target its cytotoxic activity to tumor cells. We demonstrate that localized illumination of SBTubA4P allows for precise eradication of disseminated sarcoma cells in zebrafish xenografts without inducing systemic toxicity. In addition to the already-described light-dependent inhibition of microtubule dynamics by SBTubA4P, our data indicate that this molecule creates reactive oxygen species upon UV illumination, which significantly increases its cytotoxic effects. SBTubA4P is a valuable addition to the precision oncology toolbox, and zebrafish xenografts constitute a well-suited model to investigate photoactivatable compounds in vivo.

Generation of thiyl radicals in a spatiotemporal controlled manner by light: Applied for the cis to trans isomerization of unsaturated fatty acids/phospholipids

Thiyl radicals are important reactive sulfur species and can cause cis to trans isomerization on unsaturated fatty acids. However, biocompatible strategies for the controlled generation of thiyl radicals are still lacking. In this work, we report the study of a series of naphthacyl-derived thioethers as photo-triggered thiyl radical precursors. Tertiary naphthacyl thioether was identified to be a suitable template that could be used to produce both aryl and alkyl thiyl radicals under ultraviolet (UV) light or sunlight. The effective cis-to trans-isomerization of unsaturated fatty acid models (methyl oleate, methyl linoleate) and a natural phospholipid (cardiolipin) using these photo-triggered substrates was demonstrated. This reaction was also proved to proceed effectively in cells to produce thiyl radicals and subsequent fatty acid isomerization. Additionally, the most promising thiyl radical precursor showed antiviral activity in a pseudotyped virus model, likely due to disrupting viral lipid membranes upon UV activation. These findings highlight the potential of thiyl radicals for both biochemical and antiviral applications.

Water-Compatible Staudinger-Diels-Alder Ligation

The development of bioorthogonal reactions is expected to propel further advances in chemical biology. In this study, we demonstrate Staudinger-Diels-Alder (SDA) ligation as a candidate for a new bioorthogonal reaction. This reaction ligates two molecules via strong C-C bonds at room temperature. We found that the aryl substituent of azide-benzocyclobutene (azide-BCB) had a strong influence on the molecule's tolerance to water. In particular, Cl-substituted azide-BCBs generated the ligated product in high yield, even in the presence of water. Mechanistic investigations using DFT methods revealed that hydrophobic electron-withdrawing substituents suppressed the side reactions of SDA ligation.

Abraham Patchornik: The Contemporary Relevance of His Work for Chemistry and Biology

Abraham Patchornik was born in 1926 in Ness Ziona, a town in Palestine founded by his great-grandfather Reuben Lehrer in 1883. He started to study chemistry as an undergraduate at the Hebrew University. However, this was interrupted by the war, and he completed his studies in various locations in West Jerusalem. From 1952 to 1956 Patchornik completed his PhD at the (new) Weizmann Institute of Science with Ephraim Katchalski. After a postdoc at the NIH, he returned to the Weizmann in 1958, when he joined the Department of Biophysics. In 1972-1979, he became chairman of the new Department of Organic Chemistry at the Weizmann, and his own research was geared toward applying creative chemistry to solve biological problems. Patchornik passed away in his hometown of Ness Ziona in 2014. Patchornik was a conceptual leader in peptide and polymer chemistry. Given the importance of selective functional group protection for the construction of oligomeric molecules, he became interested in using "nonstandard", orthogonal chemistry for this purpose, i.e. photosensitive protecting groups (PPGs) in place of thermal reactions. It was R.B. Woodward who suggested this strategy to Patchornik in 1965, while Patchornik was on sabbatical leave at Harvard. However, it was not until Patchornik returned to the Weizmann that this idea of a versatile PPG to enable multistep synthesis was realized. Here, we provide an account of the early photosensitive protecting groups that Patchornik and co-workers developed, and the immense impact they have had on various fields. In particular, we survey the use of PPGs in live cell physiology (i.e., caged compounds), and the development of gene chips via light-directed solid-phase synthesis. Further, we highlight recent work applying new PPGs for "photochemical delivery" of drugs, otherwise termed photopharmacology. Finally, we discuss the relationship between caged compounds and how contemporary neuroscience uses genetically encoded chromophores to control cell function.

Photoinduced Deprotection of 2-Nitrophenylneopentyl Glycol Boronates Enables Light-Triggered Polycondensation of Siloxanes

Various protecting groups have been developed for boronic acids, mostly based on diols. Alternatives include trifluoroborates and amine complexes, which offer easier synthesis and release under milder conditions. We present here a new strategy involving photocleavable protecting groups for boronic and borinic acids, based on the 2-nitrobenzyl motif. A screening of several 1,2- and 1,3-diols bearing a 2-nitrobenzyl group led us to identify 1-(2-nitrophenyl)neopentyl glycol (npnp) for the protection of boronic acid derivatives. This diol is easily prepared in a single step on gram scale, and the corresponding npnp boronates are stable in wet acetonitrile at 90 °C for several days, and under under acidic conditions. Irradiation at 365 nm in acetonitrile allows for the controlled liberation of boronic acids in good to excellent yields, a method also applied to a dimesitylborinic ester bearing a 2-nitrobenzylalcohol moiety. 1-(2-Nitrophenyl)neopentyl glycol boronates ArB(npnp) demonstrated their utility in light-triggered siloxane crosslinking. We showed that catalysts incorporated into a polymer matrix, irradiated, and then incubated at 50 °C for 7 days resulted in efficient polymerization, forming solid materials in some cases.

Surfing the limits of cyanine photocages one step at a time

Near-infrared light-activated photocages enable controlling molecules with tissue penetrating light. Understanding the structural aspects that govern the photouncaging process is essential to enhancing their efficacy, crucial for practical applications. Here we explore the impact of thermodynamic stabilization on contact ion pairs in cyanine photocages by quaternarization of the carbon reaction centers. This strategy enables the first direct uncaging of carboxylate payloads independent of oxygen, resulting in a remarkable two-orders-of-magnitude enhancement in uncaging efficiency. Our computational analyses reveal that these modifications confer a kinetic instead of thermodynamic effect, reducing ion-ion interactions and allowing complete separation of free ions while inhibiting recombination. We demonstrate that, while thermodynamic stabilization is effective in traditional chromophores operating at shorter wavelengths, it rapidly reaches its thermodynamic limitations in NIR photocages by compromising the photocage stability in the dark. Thanks to these findings, we establish that activation of cyanine photocages is limited to wavelengths of light below 1000 nm. Our work illuminates the path to improving uncaging cross-sections in NIR photocages by prioritizing kinetic trapping and separation of ions.

Impact of Transition-State Aromaticity on Selective Radical-Radical Coupling of Triarylimidazolyl Radicals

Radical coupling reactions are generally known to have a low selectivity due to the high reactivity of radicals. In this study, high regio and substrate selectivity was discovered in the dimerization of triarylimidazolyl radicals (TAIR), a versatile photochromic reaction. When two different radicals, 2-(4-cyanophenyl)-4,5-diphenyl-1H-imidazolyl radical (CN-TAIR) and 2-(4-methoxyphenyl)-4,5-diphenyl-1H-imidazolyl radical (OMe-TAIR), were simultaneously generated in situ, a hexaarylbiimidazole, formed by selective coupling at the nitrogen atom at position 1 of CN-TAIR and the carbon atom at position 2 of OMe-TAIR, was isolated with high selectivity as the main product among 24 possible radical dimer hexaarylbiimidazole derivatives. This high regio and substrate selectivity cannot be explained solely by the stability of the product and/or the electrophilicity and nucleophilicity of the radicals but originates from the aromaticity of the transition state in the radical-radical coupling reaction. To date, the selectivity of radical coupling reactions has been thought to be controlled by steric hindrance and radical spin density, but this study revealed a new factor for controlling radical coupling, that is, transition-state aromaticity. Aromaticity has been reported to have an important effect not only in the reactivity and structure of ground-state molecules but also on the electronically excited states and transition states in pericyclic reactions such as the Diels-Alder reaction and the Cope-Claisen rearrangement. This study demonstrated for the first time that radical coupling reactions can also be controlled by transition-state aromaticity.

Light-Activated Gene Expression System Using a Caging-Group-Free Photoactivatable Dye

Optical regulation of transcription using chemical compounds is an effective strategy to manipulate gene expression spatiotemporally. Conventional caging approaches with photoremovable protecting groups may require intense UV-light exposure and release potentially toxic byproducts. To address these problems, here we developed a light-mediated transcriptional regulation system by combining a caging-group-free photoactivatable dye PaX560 and a multidrug-binding transcriptional regulator QacR. The cationic dye generated from PaX560 through traceless photoconversion bound QacR and reduced its repressor function, resulting in transcriptional activation. Importantly, this system allowed transcriptional activation with a large dynamic range under mild visible light exposure and simultaneous detection of the state of the photoactivated effector. This module was integrated into the T7 RNA polymerase expression system to demonstrate light-activated transcription in vitro and in living cells.

Light-Controlled Intracellular Synthesis of Poly(luciferin) Polymers Induces Cell Paraptosis

Accumulation of misfolded proteins challenges cellular proteostasis and is implicated in aging and chronic disorders. Cancer cells, moreover, face an elevated level of basal proteotoxic stress; hence, exacerbating endoplasmic reticulum (ER) stress has been shown to induce programmed cell death while enhancing anticancer immunogenicity. We hypothesize that hydrophobic abiotic macromolecules can trigger a similar stress response. Most polymers and nanoparticles, however, are sequestered in endo/lysosomes after endocytosis, which prevents their interaction with the proteostasis machinery. We adopted an in situ polymerization approach to synthesize polymers in cells with cell-permeable monomers. Specifically, we developed a biocompatible polycondensation between l-cysteine and 2-cyanobenzothiazole (CBT) with photochemical control to form insoluble poly(luciferin) aggregates. We identified that in situ polymerization activates the BiP-PERK-CHOP pathway of the unfolded protein response and that the unresolved ER stress initiates a form of regulated cell death consistent with paraptosis. In addition, the dying cells emit damage-associated molecular patterns (DAMPs), indicating an immunogenic cell death that could potentiate antitumor immunity. Our results show that in situ polymerization mimics misfolded protein aggregates to induce proteotoxic stress and cancer cell death, offering a novel therapeutic strategy to exploit cancer vulnerability.

Competing Photocleavage on Boron and at the meso-Position in BODIPY Photocages

BODIPY photocages (photocleavable protective groups) have stirred interest because they can release biologically active cargo upon visible light excitation. We conducted combined theoretical and experimental investigations on selected BODIPY photocages to elucidate the mechanism of the competing photocleavage at the boron and meso-position. Based on the computations, the former reaction involves elongation of the B-C bond, yielding a tight borenium cation and methyl anion. These ions are intercepted by CH3OH, enabling an efficient proton-coupled electron transfer (PCET) to produce the methane and isolated ether photoproducts. Singlet and triplet excited-state lifetimes were measured in CH3OH and CD3OD to probe the kinetic isotope effects (KIEs). The resulting KIEs are small, implying that the kinetic bottleneck is due to the C-B bond scission rather than the subsequent PCET. The introduction of a methoxy group in the meso-phenoxy substituent redirects the photosubstitution toward the meso-position. The corresponding regiochemistry was explained computationally. On elongating the C-O bonds in the S1 state, it is found that the unproductive conical intersection is encountered much earlier for the alkyl-O bond than for the phenyl-O bond. The current findings are valuable for the rational design of new BODIPY photocages with tailored biological applications.

Distinguishing the Bimodal Interaction of a Squaraine Dye with a Protein by a Functional Group Photodeprotection Strategy

In this study, we present a protecting group photocleavage method to investigate both covalent and noncovalent interactions between a squaraine dye (PSq) and Bovine Serum Albumin (BSA). This approach allows for the photoinduced activation and deactivation of PSq fluorescence, providing valuable insights into the dual-mode interaction of the dye with the protein.

Bioorthogonal Activation of Deep Red Photoredox Catalysis Inducing Pyroptosis

The revolutionary impact of photoredox catalytic processes has ignited novel avenues for exploration, empowering us to delve into nature in unprecedented ways and to pioneer innovative biotechnologies for therapy and diagnosis. However, integrating artificial photoredox catalysis into living systems presents significant challenges, primarily due to concerns over low targetability, low compatibility with complex biological environments, and the safety risks associated with photocatalyst toxicity. To address these challenges, herein, we present a novel bioorthogonally activatable photoredox catalysis approach. In this approach, potent photocatalyst selection via atom replacement of the rhodamine core yielded the bioorthogonally activatable photocatalyst (PC-Tz). The introduction of 1,2,4,5-tetrazine quenched its photocatalytic properties, which were restored upon an intracellular inverse electron-demand Diels-Alder (iEDDA) reaction with trans-cyclooctene (TCO) localized in mitochondria. This reaction led to remarkable photocatalytic oxidation of nicotinamide adenine dinucleotide (NADH), effectively manipulating the mitochondrial electron transport chain (ETC) under hypoxic conditions in cancer cells. Additionally, photocatalytic pyroptotic cell death was observed through a caspase-3/gasdermin E (GSDME) pathway, achieving notable antitumor efficacy and adenosine triphosphate (ATP) reduction in tumor cells. To the best of our knowledge, this represents the first example of bioorthogonally activatable photoredox catalysis, opening new avenues for chemists to spatiotemporally control activity in specific cell organelles without disrupting other native biological processes.

Delocalized Nonbonding Orbitals of Thioxanthone in Polycyclic Aromatic Hydrocarbons for Reduced Energy Gap and Narrowband Emission

Chalcogen-containing carbonyls, specifically thioxanthone (TX), hold great potential in organic light-emitting diodes (OLEDs). While the development of narrowband OLEDs with chalcogen-containing carbonyls remains challenging due to difficulties in achieving both high device efficiency and narrow emission spectra. Herein, via a strategic incorporation of the TX moiety, two orange-red narrowband emitters, 2TXBN and BNTXBN, are designed and synthesized for the first time. Both 2TXBN and BNTXBN exhibit bright orange-red emissions with peaks at 582 and 585 nm, respectively, along with narrow full widths at half maxima of 30 and 32 nm. Notably, 2TXBN demonstrates delocalization of the nonbonding orbital within the TX segment, which raises the first triplet energy level and reduces the singlet-triplet energy gap. This electronic structural adjustment effectively shortens the delayed fluorescence lifetime, leading to enhanced device performance. Accordingly, OLED employing 2TXBN as the emitter achieves remarkable performance, with a maximum external quantum efficiency of 31.0 %, a current efficiency of 69.0 cd A-1, and a power efficiency of 76.0 lm W-1, highlighting the efficacy of the nonbonding orbital delocalization strategy in achieving bathochromic-shifted narrowband OLED materials.

Glycosylated AIE-active Red Light-triggered Photocage with Precisely Tumor Targeting Capability for Synergistic Type I Photodynamic Therapy and CPT Chemotherapy

Photocaging is an emerging protocol for precisely manipulating spatial and temporal behaviors over biological activity. However, the red/near-infrared light-triggered photolysis process of current photocage is largely singlet oxygen (1O2)-dependent and lack of compatibility with other reactive oxygen species (ROS)-activated techniques, which has proven to be the major bottleneck in achieving efficient and precise treatment. Herein, we reported a lactosylated photocage BT-LRC by covalently incorporating camptothecin (CPT) into hybrid BODIPY-TPE fluorophore via the superoxide anion radical (O2 -⋅)-cleavable thioketal bond for type I photodynamic therapy (PDT) and anticancer drug release. Amphiphilic BT-LRC could be self-assembled into aggregation-induced emission (AIE)-active nanoparticles (BT-LRCs) owing to the regulation of carbohydrate-carbohydrate interactions (CCIs) among neighboring lactose units in the nanoaggregates. BT-LRCs could simultaneously generate abundant O2 -⋅ through the aggregation modulated by lactose interactions, and DNA-damaging agent CPT was subsequently and effectively released. Notably, the type I PDT and CPT chemotherapy collaboratively amplified the therapeutic efficacy in HepG2 cells and tumor-bearing mice. Furthermore, the inherent AIE property of BT-LRCs endowed the photocaged prodrug with superior bioimaging capability, which provided a powerful tool for real-time tracking and finely tuning the PDT and photoactivated drug release behavior in tumor therapy.

Compact Molecular Conformation of Prodrugs Enhances Photocleaving Performance for Tumor Vascular Growth Inhibition

Highly spatiotemporal-resolved photomodulation demonstrates promise for investigating key biological events in vivo and in vitro, such as cell signaling pathways, neuromodulation, and tumor treatment without side effects. However, enhancing the performance of photomodulation tools remains challenging due to the limitations of the physicochemical properties of the photoactive molecules. Here, a compact, stable intramolecular π-π stacking conformation forming between the target molecule (naproxen) and the perylene-based photoremovable protecting group is discovered to confine the motion of the photolabile bond and then enhance the photocleavage quantum yield. In conjunction with a red-absorbing photosensitizer, the photocleavage wavelength is extended to the red region via triplet-triplet annihilation. In particular, the triplet lifetime of the prodrug can be extended via the linked steric hindrance to improve the conversion yield via TTA. Using the new photomodulation tool, it is precisely photoreleased cyclooxygenase-2 inhibitors for tumor vascular growth suppression in vivo. In combination with cisplatin, over 90% efficient inhibition of malignant breast tumors is observed via the synergistic tumor treatment strategy. These findings provide a new concept for the rational design of efficient photocleavage and have implications for photomodulating cell signaling pathways in tumor therapy, as well as laying a solid foundation for the development of phototherapeutic approaches.

Synthesis of Caged HMG-CoA Reductase Substrates for the Elucidation of Cellular Pathways

The synthesis of photocaged substrates of the biologically important enzyme HMG-CoA reductase is reported. HMG-CoA bearing a p-hydroxyphenacyl (pHP) photocage moiety was synthesized in an overall yield of 14% over seven steps in addition to caged forms of mevalonate and mevaldehyde. The absorption maximum and quantum yield for the decaging of the photocaged compounds are dependent on pH with a λmax of 330 nm and a ϕ of 5%, respectively, at pH 9.1 but a λmax of 290 nm and a ϕ of 16%, respectively, at pH 6.7.

Photoinitiator-free light-mediated crosslinking of dynamic polymer and pristine protein networks

Light-based patterning of synthetic and biological hydrogels enables precise spatial and temporal control over the formation of chemical bonds. However, photoinitiators are typically used to generate free radicals, which are detrimental to human cells. Here, we report a photoinitiator- and radical-free method based on ortho-nitrobenzyl alcohol (oNBA) photolysis, which gives rise to highly reactive nitroso and benzaldehyde groups. Synthetic hydrogel and pristine protein networks can rapidly form in the presence of these photo-generated reactive species. Thiol -oNBA bonds yield dynamic hydrogel networks (DHNs) via N-semimercaptal linkages that exhibit thixotropy, stress relaxation, and on-demand reversible gel-to-liquid transitions, while amine-oNBA bonds can be exploited to crosslink pristine proteins, such as gelatin and fibrinogen, by targeting their primary amines. Since this approach does not require incorporation of photoreactive moieties along the backbone, the resulting crosslinked proteins are well suited for bioadhesives. Our photoinitiator-free platform provides a versatile approach for rapidly creating synthetic and biological hydrogels for applications ranging from tissue engineering to biomedical devices.

Electron Donor-Acceptor Complex-Assisted Photochemical Conversion of O-2-Nitrobenzyl Protected Hydroxamates to Amides

The hydroxamic acid functionality is present in various medicinal agents and has attracted special interest for synthetic transformations in both organic and medicinal chemistry. The N-O bond cleavage of hydroxamic acid derivatives provides an interesting transformation for the generation of various products. We demonstrate, herein, that O-benzyl-type protected hydroxamic acids may undergo photochemical N-O bond cleavage, in the presence or absence of a catalyst, leading to amides. Although some O-benzyl protected aromatic hydroxamates may be photochemically converted to amides in the presence of a base and anthracene as the catalyst, employing O-2-nitrobenzyl group allowed the smooth conversion of both aliphatic and aromatic hydroxamates to primary or secondary amides in good to excellent yields in the presence of an amine, bypassing the need of a catalyst. DFT and UV-Vis studies supported the effective generation of an electron donor-acceptor (EDA) complex between O-2-nitrobenzyl hydroxamates and amines, which enabled the successful product formation under these photochemical conditions. An extensive substrate scope was demonstrated, showcasing that both aliphatic and aromatic hydroxamates are compatible with this protocol, affording a wide variety of primary and secondary amides.

Adrenochrome formation during photochemical decomposition of "caged" epinephrine derivatives

Control of biological activity with light is a fascinating idea. "Caged" compounds, molecules modified with photolabile protecting group, are one of the instruments for this purpose. Adrenergic receptors are essential regulators of neuronal, endocrine, cardiovascular, vegetative, and metabolic functions. These receptors are largely used as pharmacologic targets. Photolabile "caged" analogs of adrenergic receptor agonists has been reported more than 30 years ago. We report that the photolysis of epinephrine analogs, apart from liberation of the epinephrine, is accompanied by a formation of significant amount of adrenochrome, a compound with neuro- and cardiotoxic effect.

Light-Activable Inhibitor Overcomes Antimicrobial Resistance and Regulates Antibacterial Activity

Overuse of antibiotics and the widespread environmental accumulation of antibiotics drive the evolution and spread of antimicrobial resistance, posing a significant global health threat by reducing the effectiveness of available treatments and increasing the risk of untreatable infections. We designed and synthesized PhoPS, a novel photocaged β-lactamase inhibitor, which incorporates the pharmacophore of sulbactam caged with a photoresponsive moiety of o-nitrobiphenyl derivative. Experimental results demonstrate its rapid photoactivation, good stability in solution, and light-activated β-lactamase inhibition in vitro. PhoPS displays synergy with a cephalosporin antibiotic cefoperazone against both susceptible and resistant strains of Escherichia coli and biofilm formation. Additionally, PhoPS treatment demonstrates the potential to suppress the development of resistance in E. coli. These findings suggest that PhoPS offers a promising approach for restoring the efficacy of existing antibiotics and mitigating the emergence of AMR.

Precision-engineered PROTACs minimize off-tissue effects in cancer therapy

Proteolysis-targeting chimeras (PROTACs) offer a groundbreaking approach to selectively degrade disease-related proteins by utilizing the ubiquitin-proteasome system. While this strategy shows great potential in preclinical and clinical settings, off-tissue effects remain a major challenge, leading to toxicity in healthy tissues. This review explores recent advancements aimed at improving PROTAC specificity, including tumor-specific ligand-directed PROTACs, pro-PROTACs activated in tumor environments, and E3 ligase overexpression strategies. Innovations such as PEGylation and nanotechnology also play a role in optimizing PROTAC efficacy. These developments hold promise for safer, more effective cancer therapies, though challenges remain for clinical translation.

Photo-Claisen Rearrangement in a Coumarin-Caged Peptide Leads to a Surprising Enzyme Hyperactivation

Protein phosphatase-1 (PP1) is a ubiquitous enzyme that counteracts hundreds of kinases in cells. PP1 interacts with regulatory proteins via an RVxF peptide motif that binds to a hydrophobic groove on the enzyme. PP1-disrupting peptides (PDPs) compete with these regulatory proteins, leading to the release of the active PP1 subunit and promoting substrate dephosphorylation. Building on previous strategies employing the ortho-nitrobenzyl (o-Nb) group as a photocage to control PDP activity, we introduced coumarin derivatives into a PDP via an ether bond to explore their effects on PP1 activity. Surprisingly, our study revealed that the coumarin-caged peptides (PDP-DEACM and PDP-CM) underwent a photo-Claisen rearrangement, resulting in an unexpected hyperactivation of PP1. Our findings underscore the importance of linker design in controlling uncaging efficiency of photocages and highlight the need for comprehensive in vitro analysis before cellular experiments.

Photocleavable Protecting Groups Using a Sulfite Self-Immolative Linker for High Uncaging Quantum Yield and Aqueous Solubility

Using light as an external stimulus to control (bio)chemical processes offers many distinct advantages. Most importantly, it allows for spatiotemporal control simply through operating the light source. Photocleavable protecting groups (PPGs) are a cornerstone class of compounds that are used to achieve photocontrol over (bio)chemical processes. PPGs are able to release a payload of interest upon light irradiation. The successful application of PPGs hinges on their efficiency of payload release, captured in the uncaging Quantum Yield (QY). Heterolytic PPGs efficiently release low pKa payloads, but their efficiency drops significantly for payloads with higher pKa values, such as alcohols. For this reason, alcohols are usually attached to PPGs via a carbonate linker. The self-immolative nature of the carbonate linker results in concurrent release of CO2 with the alcohol payload upon irradiation. We introduce herein novel PPGs containing sulfites as self-immolative linkers for photocaged alcohol payloads, for which we discovered that the release of the alcohol proceeds with higher uncaging QY than an identical payload released from a carbonate-linked PPG. Furthermore, we demonstrate that uncaging of the sulfite-linked PPGs results in the release of SO2 and show that the sulfite linker improves water solubility as compared to the carbonate-based systems.

Conditional PROTAC: Recent Strategies for Modulating Targeted Protein Degradation

Proteolysis-targeting chimeras (PROTACs) have emerged as a promising technology for inducing targeted protein degradation by leveraging the intrinsic ubiquitin-proteasome system (UPS). While the potential druggability of PROTACs toward undruggable proteins has accelerated their rapid development and the wide-range of applications across diverse disease contexts, off-tissue effects and side-effects of PROTACs have recently received attentions to improve their efficacy. To address these issues, spatial or temporal target protein degradation by PROTACs has been spotlighted. In this review, we explore chemical strategies for modulating protein degradation in a cell type-specific (spatio-) and time-specific (temporal-) manner, thereby offering insights for expanding PROTAC applications to overcome the current limitations of target protein degradation strategy.

Light-Controlled Anticancer Activity and Cellular Uptake of a Photoswitchable Cisplatin Analogue

A photoactive analogue of cisplatin was synthesized with two arylazopyrazole ligands, able to undergo trans-cis/cis-trans photoisomerizations. The cis photoisomer showed a dark half-life of 9 days. The cytotoxicities of both photoisomers of the complex were determined in several cancer and normal cell lines and compared to that of cisplatin. The trans photoisomer of the complex was much more cytotoxic than both the cis photoisomer and cisplatin, and was more toxic for cancer (4T1) than for normal (NMuMG) murine breast cells. 4T1 cell death occurred through necrosis. Photoisomerization of the trans and cis photoisomers internalized by the 4T1 cells increased and decreased their viability, respectively. The cellular uptake of the trans photoisomer was stronger than that of both the cis photoisomer and cisplatin. Both photoisomers interacted with DNA faster than cisplatin. The trans photoisomer was bound stronger by bovine serum albumin and induced a greater decrease in cellular glutathione levels than the cis photoisomer.

Photocaging of amino acids and short peptides by arylidenethiazoles: mechanism, photochemical characteristics and biological behaviour

A series of fluorophores based on the (5-methyl-4-phenylthiazol-2-yl)-3-phenylacrylonitrile (MPTA) core were designed and synthesised for photocaging of amino acids and peptides. The photophysical characteristics of these compounds and their hybrids with biomolecules were thoroughly investigated through a joint experimental, spectral and computational approach. The photorelease ability of the obtained amino acids-MPTA and peptides-MPTA hybrids was studied under various conditions, including different UV irradiation wavelength and power, and solvents. The main reaction products were identified using high-performance liquid chromatography combined with high-resolution mass spectrometry. Photo uncaging kinetics was quantitatively studied using absorption spectroscopy. The mechanism of photorelease of amino acids and peptides was elucidated through quantum mechanical calculations, which were also used for the exploration of photophysical properties of the excited states, and photodissociation energetics quantification. Relationships between the structure of fluorophores and photodissociation characteristics were estimated, and fluorophores with the good uncaging characteristics (biomolecule photoreleasing yield, uncaging rate, and effectiveness) were identified. Cell viability assays using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide or MTT showed a low cytotoxicity of the hybrids. Confocal microscopy revealed the easy penetration of the hybrids into living cells and their selective accumulation in the endoplasmic reticulum, lipid droplets and mitochondria, depending on their chemical structure.

Nitric Oxide-Photodelivering Materials with Multiple Functionalities: From Rational Design to Therapeutic Applications

The achievement of materials that are able to release therapeutic agents under the control of light stimuli to improve therapeutic efficacy is a significant challenge in health care. Nitric oxide (NO) is one of the most studied molecules in the fascinating realm of biomedical sciences, not only for its crucial role as a gaseous signaling molecule in the human body but also for its great potential as an unconventional therapeutic in a variety of diseases including cancer, bacterial and viral infections, and neurodegeneration. Handling difficulties due to its gaseous nature, reduced region of action due to its short half-life, and strict dependence of the biological effects on its concentration and generation site are critical questions to be solved for appropriate therapeutic uses of NO. Light-activatable NO precursors, namely, NO photodonors (NOPDs), address the above issues since they are stable in the dark and permit in a noninvasive fashion the remote-controlled delivery of NO on demand with great spatiotemporal precision. Engineering biocompatible materials with NOPDs and their combination with additional imaging, therapeutic, and phototherapeutic components leads to intriguing light-responsive multifunctional constructs exhibiting promising potential for biomedical applications. This contribution illustrates the most significant progress made over the last five years in achieving engineered materials including nanoparticles, gels, and thin films, sharing the common feature to deliver NO under the exclusive control of the biocompatible visible/near infrared light inputs. We will highlight the logical design behind the fabrication of these systems, illustrating the potential therapeutic applications with particular emphasis on cancer and bacterial infections.

A supramolecular approach towards the photorelease of encapsulated caged acids in water: 7-diethylaminothio-4-coumarinyl molecules as triggers

Herein, we establish the release of aliphatic acids in water upon excitation of 7-diethylaminothio-4-coumarinyl derivatives encapsulated within the organic host octa acid (OA). The 7-diethylaminothio-4-coumarinyl skeleton, employed here as the trigger, photoreleases caged molecules from the excited triplet state, in contrast to its carbonyl analogue, where the same reaction is known to occur from the excited singlet state. Encapsulation in OA solubilizes molecules in water that are otherwise water-insoluble, and retains the used trigger within itself following the release of the aliphatic acid. Such supramolecular characteristics usher in new features to the photorelease methodology. The thiocarbonyl chromophore extends the absorption of coumarinyl trigger to visible range while enhancing the intersystem crossing (ISC) to the triplet state, making it the reactive state. Despite the non-polar environment within the OA capsules the photocleavage occurs in a heterolytic fashion to release the conjugate base and the used trigger as triplet carbocation in an adiabatic process. Interestingly, the triplet carbocation crosses to the ground singlet surface (closed shell singlet carbocation) with the help of water molecules, possibly aided by C = S chromophore. Utilizing the known excited state dynamics of related thiocoumarinyl and coumarinyl systems, we have identified a few of the important mechanistic features of the photorelease process of 7-diethylaminothio-4-coumarinyl derivatives. Ultrafast excited state dynamic studies and quantum chemical calculations planned should help us better understand the photorelease process so as to effectively exploit the proposed system for potential applications.

BODIPY Compounds Substituted on Boron

BODIPY compounds are important organic dyes with exceptional spectral and photophysical properties and numerous applications in different scientific fields. Their widespread applications have flourished due to their easy structural modifications, which enable the preparation of different molecular structures with tunable spectral and photophysical properties. To date, researchers have mostly devoted their efforts to modifying BODIPY meso-position or pyrrole rings, whereas the substitution of fluorine atoms remains largely unexplored. However, chemistry of the boron atom is possible, and it enables tuning of the photophysical properties of the dyes, without tackling their spectral properties. Furthermore, modifications of boron affect the solubility and aggregation propensity of the molecules. This review article highlights methods for the preparation of 4-substituted compounds and the most important reactions on the boron of the BODIPY dyes. They were divided into reactions promoted by Lewis acid (AlCl3 or BCl3), or bases such as alkoxides and organometallic reagents. By using these two methodologies, it is possible to cleave B-F bonds and substitute them with B-C, B-N, or B-O bonds from different nucleophiles. A special emphasis in this review is given to still underdeveloped photochemical reactions of the boron atom of BODIPY dyes. These reactions have the potential to be used in the development of a new line of BODIPY photo-cleavable protective groups (also known as photocages) with bio-medicinal and photo-pharmacological applications, such as drug delivery.

Time-resolved scattering methods for biological samples at the CoSAXS beamline, MAX IV Laboratory

CoSAXS is a state-of-the-art SAXS/WAXS beamline exploiting the high brilliance of the MAX IV 3 GeV synchrotron. By coupling advances in sample environment control with fast X-ray detectors, millisecond time-resolved scattering methods can follow structural dynamics of proteins in solution. In the present work, four sample environments are discussed. A sample environment for combined SAXS with UV-vis and fluorescence spectroscopy (SUrF) enables a comprehensive understanding of the time evolution of conformation in a model protein upon acid-driven denaturation. The use of microfluidic chips with SAXS allows the mapping of concentration with very small sample volumes. For highly reproducible sequences of mixing of components, it is possible using stopped-flow and SAXS to access the initial effects of mixing at 2 millisecond timescales with good signal to noise to allow structural interpretation. The intermediate structures in a protein are explored under light and temperature perturbations by using lasers to "pump" the protein and SAXS as the "probe". The methods described demonstrate that features at low q, corresponding to cooperative motions of the atoms in a protein, could be extracted at millisecond timescales, which results from CoSAXS being a highly-stable, low background, dedicated SAXS beamline.

Biomimetic light-harvesting antennas via the self-assembly of chemically programmed chlorophylls

The photosynthetic pigment "chlorophyll" possesses attractive photophysical properties, including efficient sunlight absorption, photoexcited energy transfer, and charge separation, which are advantageous for applications for photo- and electro-functional materials such as artificial photosynthesis and solar cells. However, these functions cannot be realized by individual chlorophyll molecules alone; rather, they are achieved by the formation of sophisticated supramolecules through the self-assembly of the pigments. Here, we present strategies for constructing and developing artificial light-harvesting systems by mimicking photosynthetic antenna complexes through the highly ordered supramolecular self-assembly of synthetic dyes, particularly chlorophyll derivatives.

A Photocaged Microtubule-Stabilising Epothilone Allows Spatiotemporal Control of Cytoskeletal Dynamics

The cytoskeleton is essential for spatial and temporal organisation of a wide range of cellular and tissue-level processes, such as proliferation, signalling, cargo transport, migration, morphogenesis, and neuronal development. Cytoskeleton research aims to study these processes by imaging, or by locally manipulating, the dynamics and organisation of cytoskeletal proteins with high spatiotemporal resolution: which matches the capabilities of optical methods. To date, no photoresponsive microtubule-stabilising tool has united all the features needed for a practical high-precision reagent: a low potency and biochemically stable non-illuminated state; then an efficient, rapid, and clean photoresponse that generates a high potency illuminated state; plus good solubility at suitable working concentrations; and efficient synthetic access. We now present CouEpo, a photocaged epothilone microtubule-stabilising reagent that combines these needs. Its potency increases approximately 100-fold upon irradiation by violet/blue light to reach low-nanomolar values, allowing efficient photocontrol of microtubule dynamics in live cells, and even the generation of cellular asymmetries in microtubule architecture and cell dynamics. CouEpo is thus a high-performance tool compound that can support high-precision research into many microtubule-associated processes, from biophysics to transport, cell motility, and neuronal physiology.

Sequential structure probing of cotranscriptional RNA folding intermediates

Cotranscriptional RNA folding pathways typically involve the sequential formation of folding intermediates. Existing methods for cotranscriptional RNA structure probing map the structure of nascent RNA in the context of a terminally arrested transcription elongation complex. Consequently, the rearrangement of RNA structures as nucleotides are added to the transcript can be inferred but is not assessed directly. To address this limitation, we have developed linked-multipoint Transcription Elongation Complex RNA structure probing (TECprobe-LM), which assesses the cotranscriptional rearrangement of RNA structures by sequentially positioning E. coli RNAP at two or more points within a DNA template so that nascent RNA can be chemically probed. We validated TECprobe-LM by measuring known folding events that occur within the E. coli signal recognition particle RNA, Clostridium beijerinckii pfl ZTP riboswitch, and Bacillus cereus crcB fluoride riboswitch folding pathways. Our findings establish TECprobe-LM as a strategy for detecting cotranscriptional RNA folding events directly using chemical probing.

Multifunctional Molecular Hybrids Photoreleasing Nitric Oxide: Advantages, Pitfalls, and Opportunities

The multifaceted role nitric oxide (NO) plays in human physiology and pathophysiology has opened new scenarios in biomedicine by exploiting this free radical as an unconventional therapeutic against important diseases. The difficulties in handling gaseous NO and the strict dependence of the biological effects on its doses and location have made the light-activated NO precursors, namely NO photodonors (NOPDs), very appealing by virtue of their precise spatiotemporal control of NO delivery. The covalent integration of NOPDs and additional functional components within the same molecular skeleton through suitable linkers can lead to an intriguing class of multifunctional photoactivatable molecular hybrids. In this Perspective, we provide an overview of the recent advances in these molecular constructs, emphasizing those merging NO photorelease with targeting, fluorescent reporting, and phototherapeutic functionalities. We will highlight the rational design behind synthesizing these molecular hybrids and critically describe the advantages, drawbacks, and opportunities they offer in biomedical research.

Seeing and Cleaving: Turn-Off Fluorophore Uncaging and Its Application in Hydrogel Photopatterning and Traceable Neurotransmitter Photocages

The advancements in targeted drug release and experimental neuroscience have amplified the scientific interest in photolabile protecting groups (PPGs) and photouncaging. The growing need for the detection of uncaging events has led to the development of reporters with fluorescence turn-on upon uncaging. In contrast, fluorescent tags with turn-off properties have been drastically underexplored, although there are applications where they would be sought after. In this work, a rhodamine-based fluorescent tag is developed with signal turn-off following photouncaging. One-photon photolysis experiments reveal a ready loss of red fluorescence signal upon UV (365 nm) irradiation, while no significant change is observed in control experiments in the absence of PPG or with irradiation around the absorption maximum of the fluorophore (595 nm). The two-photon photolysis of the turn-off fluorescent tag is explored in hydrogel photolithography experiments. The hydrogel-bound tag enables the power-, dwell time-, and wavelength-dependent construction of intricate patterns and gradients. Finally, a prominent caged neurotransmitter (MNI-Glu) is modified with the fluorescent tag, resulting in the glutamate precursor named as GlutaTrace with fluorescence traceability and turn-off upon photouncaging. GlutaTrace is successfully applied for the visualization of glutamate precursor distribution following capillary microinjection and for the selective excitation of neurons in a mouse brain model.

Synthesis of Metal-Organic Cages via Orthogonal Bond Cleavage in 3D Metal-Organic Frameworks

Herein we address the question of whether a supramolecular finite metal-organic structure such as a cage or metal-organic polyhedron (MOP) can be synthesized via controlled cleavage of a three-dimensional (3D) metal-organic structure. To demonstrate this, we report the synthesis of a Cu(II)-based cuboctahedral MOP through orthogonal olefinic bond cleavage of the cavities of a 3D, Cu(II)-based, metal-organic framework (MOF). Additionally, we demonstrate that controlling the ozonolysis conditions used for the cleavage enables Clip-off Chemistry synthesis of two cuboctahedral MOPs that differ by their external functionalization: one in which all 24 external groups represent a mixture of aldehydes, carboxylic acids, acetals and esters, and one in which all are aldehydes.

Design and synthesis of a 2,5-Diarylthiophene chromophore for enhanced near-infrared two-photon uncaging efficiency of calcium ions

The design and synthesis of two-photon-responsive chromophores have recently garnered significant attention owing to their potential applications in materials and life sciences. In this study, a novel π-conjugated system, 2-dimethylaminophenyl-5-nitrophenylthiophene derivatives, featuring a thiophene unit as the π-linker between the donor (NMe2C6H4-) and acceptor (NO2C6H4-) units was designed, synthesized, and applied for the development of two-photon-responsive chromophores as a photoremovable protecting group in the near-infrared region. Notably, the positional effect of the nitro group (NO2), meta versus para position, was observed in the uncaging process of benzoic acid. Additionally, while the para-isomer exhibited a single fluorescence peak, a dual emission was detected for the meta-isomer in polar solvents. The caged calcium ion (Ca2+) incorporating the newly synthesized thiophene unit exhibited a sizable two-photon absorption cross-section value (σ2 = 129 GM at 830 nm). Both one-photon and two-photon photoirradiation of caged calcium ions successfully released calcium ions, indicating the potential utility of 2,5-diarylthiophene derivatives in future biological studies.

Reinvigorating Photo-Activated R-Alkoxysilanes Containing 2-Nitrobenzyl Protecting Groups as Stable Precursors for Photo-Driven Si-O Bond Formation in Polymerization and Surface Modification

This study aimed to revitalize silicon-based sol-gel chemistry methodologies utilizing photoprotected R-alkoxysilanes to control the synthesis of unique silicon-based materials. We have investigated the synthesis, characterization, light-induced deprotection, and subsequent polymerization/surface functionalization through the use of 2-nitrobenzyloxy-based photoremovable protecting groups (PPGs) as alkoxy reactive groups on ethyl and phenyl (R x -(alkoxy) y silanes, with x = 0-3 and y = 1-3). The photochemical dynamics, relative efficiencies, and kinetics of the novel alkoxysilane-based PPGs were thoroughly investigated using UV light irradiation by NMR and UV/vis methods. We then explored the tin-catalyzed coupling of photodeprotected products (R x -silanols) to form polymers/oligomers. We have found that photoenabled removal of PPGs and conversion to silanols from all silane systems studied is achieved. Furthermore, these deprotected species are polymerizable into siloxanes and effectively used as light-controlled surface modifiers with masking techniques of which proof-of-concept examples are given, enabling promising application as photolithographic reagents.