Development of Green/Red-Absorbing Chromophores Based on a Coumarin Scaffold That Are Useful as Caging Groups

Syntheses and Applications of Coumarin-Derived Fluorescent Probes for Real-Time Monitoring of NAD(P)H Dynamics in Living Cells across Diverse Chemical Environments

Fluorescent probes play a crucial role in elucidating cellular processes, with NAD(P)H sensing being pivotal in understanding cellular metabolism and redox biology. Here, the development and characterization of three fluorescent probes, A, B, and C, based on the coumarin platform for monitoring of NAD(P)H levels in living cells are described. Probes A and B incorporate a coumarin-cyanine hybrid structure with vinyl and thiophene connection bridges to 3-quinolinium acceptors, respectively, while probe C introduces a dicyano moiety for replacement of the lactone carbonyl group of probe A which increases the reaction rate of the probe with NAD(P)H. Initially, all probes exhibit subdued fluorescence due to intramolecular charge transfer (ICT) quenching. However, upon hydride transfer by NAD(P)H, fluorescence activation is triggered through enhanced ICT. Theoretical calculations confirm that the electronic absorption changes upon the addition of hydride to originate from the quinoline moiety instead of the coumarin section and end up in the middle section, illustrating how the addition of hydride affects the nature of this absorption. Control and dose-response experiments provide conclusive evidence of probe C's specificity and reliability in identifying intracellular NAD(P)H levels within HeLa cells. Furthermore, colocalization studies indicate probe C's selective targeting of mitochondria. Investigation into metabolic substrates reveals the influence of glucose, maltose, pyruvate, lactate, acesulfame potassium, and aspartame on NAD(P)H levels, shedding light on cellular responses to nutrient availability and artificial sweeteners. Additionally, we explore the consequence of oxaliplatin on cellular NAD(P)H levels, revealing complex interplays between DNA damage repair, metabolic reprogramming, and enzyme activities. In vivo studies utilizing starved fruit fly larvae underscore probe C's efficacy in monitoring NAD(P)H dynamics in response to external compounds. These findings highlight probe C's utility as a versatile tool for investigating NAD(P)H signaling pathways in biomedical research contexts, offering insights into cellular metabolism, stress responses, and disease mechanisms.

The fate of the contact ion pair determines the photochemistry of coumarin-based photocleavable protecting groups

Photocleavable protecting groups (PPGs) enable the precise spatiotemporal control over the release of a payload of interest, in particular a bioactive substance, through light irradiation. A crucial parameter that determines the practical applicability of PPGs is the efficiency of payload release, largely governed by the quantum yield of photolysis (QY). Understanding which parameters determine the QY will prove crucial for engineering improved PPGs and their effective future applications, especially in the emerging field of photopharmacology. The Contact Ion Pair (CIP) has been recognized as an important intermediate in the uncaging process, but the key influence of its fate on the quantum yield has not been explored yet, limiting our ability to design improved PPGs. Here, we demonstrate that the CIP escape mechanism of PPGs is crucial for determining their payload- and solvent-dependent photolysis QY, and illustrate that an intramolecular type of CIP escape is superior over diffusion-dependent CIP escape. Furthermore, we report a strong correlation of the photolysis QY of a range of coumarin PPGs with the DFT-calculated height of all three energy barriers involved in the photolysis reaction, despite the vastly different mechanisms of CIP escape that these PPGs exhibit. Using the insights obtained through our analysis, we were able to predict the photolysis QY of a newly designed PPG with particularly high accuracy. The level of understanding of the factors determining the QY of PPGs presented here will move the ever-expanding field of PPG applications forward and provides a blueprint for the development of PPGs with QYs that are independent of payload-topology and solvent polarity.

Photochemically Controlled Release of the Glucose Transporter 1 Inhibitor for Glucose Deprivation Responses and Cancer Suppression Research

Cancer cells need a greater supply of glucose mainly due to their aerobic glycolysis, known as the Warburg effect. Glucose transport by glucose transporter 1 (GLUT1) is the rate-limiting step for glucose uptake, making it a potential cancer therapeutic target. However, GLUT1 is widely expressed and performs crucial functions in a variety of cells, and its indiscriminate inhibition will cause serious side effects. In this study, we designed and synthesized a photocaged GLUT1 inhibitor WZB117-PPG to suppress the growth of cancer cells in a spatiotemporally controllable manner. WZB117-PPG exhibited remarkable photolysis efficiency and substantial cytotoxicity toward cancer cells under visible light illumination with minimal side effects, ensuring its safety as a potential cancer therapy. Furthermore, our quantitative proteomics data delineated a comprehensive portrait of responses in cancer cells under glucose deprivation, underlining the mechanism of cell death via necrosis rather than apoptosis. We reason that our study provides a potentially reliable cancer treatment strategy and can be used as a spatiotemporally controllable trigger for studying nutrient deprivation-related stress responses.

Cation delocalization and photo-isomerization enhance the uncaging quantum yield of a photocleavable protecting group

Photocleavable protecting groups (PPGs) enable the light-induced, spatiotemporal control over the release of a payload of interest. Two fundamental challenges in the design of new, effective PPGs are increasing the quantum yield (QY) of photolysis and red-shifting the absorption spectrum. Here we describe the combination of two photochemical strategies for PPG optimization in one molecule, resulting in significant improvements in both these crucial parameters. Furthermore, we for the first time identify the process of photo-isomerization to strongly influence the QY of photolysis of a PPG and identify the cis-isomer as the superior PPG.

Comparative study of experimental and DFT calculations for 3-cinnamoyl 4-hydroxycoumarin derivatives

BACKGROUND

Computational research plays an important role in predicting the chemical and physical properties of biologically active compounds important in future structural modifications to improve or modify biological activity.

OBJECTIVE

This research focuses on quantum chemical and spectroscopic investigations properties of synthesized 4-hydroxycoumarin derivatives.

METHODS

Quantum chemical calculations were obtained using B3LYP, HF, and M06-2x level methods with the 6-31++G (d,p) basis set. Afterward, IR, 1H, 13C, UV-Visible experimentally parameters were compared with the results obtained using the B3LYP/6-31+G*(d) basis set of the molecules to be able to characterize the structures.

RESULTS

Based on the quantum chemical calculations compound with acetamido group on the phenyl ring is the most reactive, and compound with nitro substituent is the least reactive and the the strongest electrophile among tested compounds. With the exception of compounds with dimethylamino group, all other compounds have a pronounced tautomer between OH and C = O group. The calculated and experimental values are in agreement with each other.

CONCLUSION

The molecular structure in the ground state of six 3-cinnamoyl 4-hydroxycoumarin derivatives was optimized using density functional theory. The observed and computed values were compared and it can be concluded that the theoretical results were in good linear agreement with the experimental data.

Rational Design of Cost-Effective 4-Styrylcoumarin Fluorescent Derivatives for Biomolecule Labeling

Fluorescent labels are key tools in a wide range of modern scientific applications, such as fluorescence microscopy, flow cytometry, histochemistry, direct and indirect immunochemistry, and fluorescence in situ hybridization (FISH). Small fluorescent labels have important practical advantages as they allow maximizing the fluorescence signal by binding multiple fluorophores to a single biomolecule. At present, the most widely used fluorescent labels available present small Stokes shifts and are too costly to be used in routine applications. In this work we present four new coumarin derivatives, as promising and inexpensive fluorescent labels for biomolecules, obtained through a cost-effective, efficient, and straightforward synthetic strategy. Density functional theory and time-dependent density functional theory calculations of the electronic ground and lowest-lying singlet excited states were carried out in order to gain insights into the observed photophysical properties.

Phototriggered structures: Latest advances in biomedical applications

Non-invasive control of the drug molecules accessibility is a key issue in improving diagnostic and therapeutic procedures. Some studies have explored the spatiotemporal control by light as a peripheral stimulus. Phototriggered drug delivery systems (PTDDSs) have received interest in the past decade among biological researchers due to their capability the control drug release. To this end, a wide range of phototrigger molecular structures participated in the DDSs to serve additional efficiency and a high-conversion release of active fragments under light irradiation. Up to now, several categories of PTDDSs have been extended to upgrade the performance of controlled delivery of therapeutic agents based on well-known phototrigger molecular structures like o-nitrobenzyl, coumarinyl, anthracenyl, quinolinyl, o-hydroxycinnamate and hydroxyphenacyl, where either of one endows an exclusive feature and distinct mechanistic approach. This review conveys the design, photochemical properties and essential mechanism of the most important phototriggered structures for the release of single and dual (similar or different) active molecules that have the ability to quickly reason of the large variety of dynamic biological phenomena for biomedical applications like photo-regulated drug release, synergistic outcomes, real-time monitoring, and biocompatibility potential.

Mitochondria-Targeted COUPY Photocages: Synthesis and Visible-Light Photoactivation in Living Cells

Releasing bioactive molecules in specific subcellular locations from the corresponding caged precursors offers great potential in photopharmacology, especially when using biologically compatible visible light. By taking advantage of the intrinsic preference of COUPY coumarins for mitochondria and their long wavelength absorption in the visible region, we have synthesized and fully characterized a series of COUPY-caged model compounds to investigate how the structure of the coumarin caging group affects the rate and efficiency of the photolysis process. Uncaging studies using yellow (560 nm) and red light (620 nm) in phosphate-buffered saline medium have demonstrated that the incorporation of a methyl group in a position adjacent to the photocleavable bond is particularly important to fine-tune the photochemical properties of the caging group. Additionally, the use of a COUPY-caged version of the protonophore 2,4-dinitrophenol allowed us to confirm by confocal microscopy that photoactivation can occur within mitochondria of living HeLa cells upon irradiation with low doses of yellow light. The new photolabile protecting groups presented here complement the photochemical toolbox in therapeutic applications since they will facilitate the delivery of photocages of biologically active compounds into mitochondria.

Evaluation of on-site testing methods with a novel 3-in-1 miniaturized spectroscopic device for cinnamon screening

"True cinnamon" is often fraudulently replaced by other varieties for economic reasons. In the powdered form, it is not possible to distinguish the varieties visually, but they differ in their sensory profile, in particular in the aromatic compound coumarin content which has also been deemed hepatotoxic in animal models. Molecular and analytical techniques exist which can be used for authentication but are expensive, time-consuming, and destructive. As an alternative, we tested three different miniaturized spectroscopic techniques namely, ultraviolet-visible (UV-Vis), near-infrared (NIR) and fluorescence (FLUO) to authenticate cinnamon samples. Out of the three, UV-Vis and NIR were superior to FLUO. The separation with UV-Vis and FLUO could be visually identified after pre-processing the spectral data and subsequently submitting it to principal component analysis (PCA). When chemometrics were applied a correct classification rate by variety of 89%, 90% and 89% for UV-Vis, NIR, and fluorescence spectroscopy, respectively, was observed. The usage of miniaturized spectrophotometers combined with PCA and classification algorithms was found promising to authenticate cinnamon.

Mono- and Dinuclear Gold(I) Coumarin Complexes: Luminescence Studies and Singlet Oxygen Production

The 4-(thiolmethyl)-7-(diethylamino)-2H-chromen-2-one ligand has been synthesized and used as chromophore in several mono- and dinuclear gold(I) compounds that contain a phosphane at the second coordination position. Four final products were able to obtain in pure form containing one coumarin and one phosphane ligand in the case of PTA (1,3,5-triaza-7-phosphatricyclo[3.3.1.13.7]decane) and PPh₃ (triphenylphosphine); one coumarin and two gold(I)-phosphane groups in the case of phosphane=DAPTA (3,7-diacetyl-1,3,7-triaza-5-phosphabicyclo[3.3.1]nonane) and two coumarin and two gold(I) atoms in the case of phosphane=DPEphos (bis[(2-diphenylphosphino)phenyl]ether), when it was used a diphosphane. Other diphosphane ligands used were not able to give the desired products in pure form. The luminescent properties of the compounds are governed by the fluorescence of the coumarin moiety in all compounds both for measurements carried out in solution and also immobilized in PMMA organic matrix. Phosphorescence emission can be detected in all cases at 77 K both for the uncoordinated coumarin ligand and the gold(I) derivatives, being more favoured in the presence of the gold(I) heavy atom. The compounds have been used as photosensitizers to generate ¹ O₂ with moderate quantum yields values.

Wavelength-Dependent, Orthogonal Photoregulation of DNA Liberation for Logic Operations

In this study, we synthesized two phosphoramidites based on 2,7-bis-{4-nitro-8-[3-(2-propyl)-styryl]}-9,9-bis-[1-(3,6-dioxaheptyl)]-fluorene (BNSF) and 4,4'-bis-{8-[4-nitro-3-(2-propyl)-styryl]}-3,3'-di-methoxybiphenyl (BNSMB) structures as visible light-cleavable linkers for oligonucleotide conjugation. In addition to the commercial ultraviolet (UV) photocleavable (PC) linker, the BNSMB linker was further applied as a building component to construct photoregulated DNA devices as duplex structures, which are functionalized with fluorophores and quenchers. Selective cleavage of PC and BNSMB is achieved in response to ultraviolet (UV) and visible light irradiations as two inputs, respectively. This leads to controllable dissociation of pieces of DNA fragments, which is followed by changes of fluorescence emission as signal outputs of the system. By tuning the number and position of the photocleavable molecules, fluorophores, and quenchers, various DNA devices were developed, which mimic the functions of Boolean logic gates and achieve logic operations in AND, OR, NOR, and NAND gates in response to two different wavelengths of light inputs. By sequence design, the photolysis products can be precisely programmed in DNA devices and triggered to release in a selective and/or sequential manner. Thus, this photoregulated DNA device shows potential as a wavelength-dependent drug delivery system for selective control over the release of multiple individual therapeutic oligonucleotide-based drugs. We believe that our work not only enriches the library of photocleavable phosphoramidites available for bioconjugation but also paves the way for developing spatiotemporal-controlled, orthogonal-regulated DNA-based logic devices for a range of applications in materials science, polymers, chemistry, and biology.

Photodeprotection Reaction Mechanisms of Caged Species Utilizing a Photochromism Function

Acetoxy-1,2,2-tri(aryl)ethanone (1) is a novel and visual release-and-report system that contains the photochromic diarylethylene function attached to the photocage dimethoxybenzoin platform. However, the mechanism of 1 cyclization and a subsequent deprotection remains unclear. Here, we use femtosecond and nanosecond transient absorption spectroscopies in combination with density functional theory computations to study the detailed reaction mechanism. The photodeprotection proceeds with competition between pathways initiated by two different configurations of the singlet excited state of 1 (labeled as ¹1_LE and ¹1_CT); the stepwise elimination after cyclization of ¹1_LE constitutes the predominant pathway, whereas the concerted removal of acetic acid after cyclization of ¹1_CT is the minor pathway. These results contribute to a detailed photodeprotection mechanism of 1 and provide new insights into the effect of geometric configurations of intermediates on the photodeprotection pathways. This new information can help in the further development of this type of the photolabile protecting group (PPG) for the protection of biorelevant molecules and in the design of an improved and versatile release-and-report PPG.

Fluorogenic Cyclopropenones for Multicomponent, Real-Time Imaging

Fluorogenic bioorthogonal reactions enable biomolecule visualization in real time. These reactions comprise reporters that "light up" upon reaction with complementary partners. While the spectrum of fluorogenic chemistries is expanding, few transformations are compatible with live cells due to cross-reactivities or insufficient signal turn-on. To address the need for more suitable chemistries for cellular imaging, we developed a fluorogenic reaction featuring cyclopropenone reporters and phosphines. The transformation involves regioselective activation and cyclization of cyclopropenones to form coumarin products. With optimal probes, the reaction provides >1600-fold signal turn-on, one of the highest fluorescence enhancements reported to date. The bioorthogonal motifs were evaluated in vitro and in cells. The reaction was also found to be compatible with other common fluorogenic transformations, enabling multicomponent, real-time imaging. Collectively, these data suggest that the cyclopropenone-phosphine reaction will bolster efforts to track biomolecule targets in their native settings.

Thiocarbonyl photosensitizer, a feasible way to eliminate the photosensitizer residues in photodynamic therapy

Photodynamic therapy (PDT) has been successfully applied in clinical treatment several years. However, after finished treatment process the residual photosensitizer will spread throughout body, which forces patients stay in the dark room to avoid exposure in sunlight several weeks. Therefore, develop degradable photosensitizer could effectively eliminate this inconvenience. In the past, researchers have developed degradable photosensitizers based on supramolecular structure. In this study, we achieved the same effect in small molecule level. Three thiocarbonyl photosensitizers (PS) have high photogenerated ¹O₂ quantum yield and can be photodegraded by laser irradiation within 15 min. And due to its high phototoxicity and low toxicity, thiocarbonyl PS still maintains its high phototoxicity. Especially, mitochondrial targeting PS 1a has better properties than many BODIPY or cyanine heavy-atom-free photosensitizers. It only needs 1 μM to reduce HeLa cell activity to 30%. Finally the thiocarbonyl PS provided a convenient way to solve the PS residue problem without sacrificing PDT efficiency.

New Red-Shifted 4-Styrylcoumarin Derivatives as Potential Fluorescent Labels for Biomolecules

Important scientific areas, such as cellular biology, medicine, pharmacy, and environmental sciences, are dependent on very sensitive analytical techniques to track and detect biomolecules. In this work, we develop a simple, low-cost and effective synthetic strategy to produce new red-shifted 4-styrylcoumarin derivatives as promising inexpensive fluorescent labels for biomolecules. The extension of the delocalized π-electron system results in bathochromic shifts in these new coumarin derivatives, which also present large Stokes shifts. In addition, density functional theory and time-dependent density functional theory calculations helped to rationalize the photophysical properties observed by the experimental results.

Development of new 2-piperidinium-4-styrylcoumarin derivatives with large Stokes shifts as potential fluorescent labels for biomolecules

New 2-piperidinium-4-styrylcoumarin derivatives, with large Stokes shifts and high fluorescence quantum yields, as potential fluorescent labels for biomolecules.

COUPY Coumarins as Novel Mitochondria-Targeted Photodynamic Therapy Anticancer Agents

Photodynamic therapy (PDT) for cancer treatment has drawn increased attention over the last decades. Herein, we introduce a novel family of low-molecular-weight coumarins as potential PDT anticancer tools. Through a systematic study with a library of 15 compounds, we have established a detailed structure-activity relationship rationale, which allowed the selection of three lead compounds exhibiting effective in vitro anticancer activities upon visible-light irradiation in both normoxia and hypoxia (phototherapeutic indexes up to 71) and minimal toxicity toward normal cells. Acting as excellent theranostic agents targeting mitochondria, the mechanism of action of the photosensitizers has been investigated in detail in HeLa cells. The generation of cytotoxic reactive oxygen species, which has been found to be a major contributor of the coumarins' phototoxicity, and the induction of apoptosis and/or autophagy have been identified as the cell death modes triggered after irradiation with low doses of visible light.

Green Light-Triggered Intraocular Drug Release for Intravenous Chemotherapy of Retinoblastoma

Retinoblastoma is one of the most severe ocular diseases, of which current chemotherapy is limited to the repetitive intravitreal injections of chemotherapeutics. Systemic drug administration is a less invasive route; however, it is also less efficient for ocular drug delivery because of the existence of blood-retinal barrier and systemic side effects. Here, a photoresponsive drug release system is reported, which is self-assembled from photocleavable trigonal small molecules, to achieve light-triggered intraocular drug accumulation. After intravenous injection of drug-loaded nanocarriers, green light can trigger the disassembly of the nanocarriers in retinal blood vessels, which leads to intraocular drug release and accumulation to suppress retinoblastoma growth. This proof-of-concept study would advance the development of light-triggered drug release systems for the intravenous treatment of eye diseases.

Coumarin luciferins and mutant luciferases for robust multi-component bioluminescence imaging

Multi-component bioluminescence imaging requires an expanded collection of luciferase-luciferin pairs that emit far-red or near-infrared light. Toward this end, we prepared a new class of luciferins based on a red-shifted coumarin scaffold. These probes (CouLuc-1s) were accessed in a two-step sequence via direct modification of commercial dyes. The bioluminescent properties of the CouLuc-1 analogs were also characterized, and complementary luciferase enzymes were identified using a two-pronged screening strategy. The optimized enzyme-substrate pairs displayed robust photon outputs and emitted a significant portion of near-infrared light. The CouLuc-1 scaffolds are also structurally distinct from existing probes, enabling rapid multi-component imaging. Collectively, this work provides novel bioluminescent tools along with a blueprint for crafting additional fluorophore-derived probes for multiplexed imaging.

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.

Transformation of COUPY Fluorophores into a Novel Class of Visible-Light-Cleavable Photolabile Protecting Groups

Although photolabile protecting groups (PPGs) have found widespread applications in several fields of chemistry, biology and materials science, there is a growing interest in expanding the photochemical toolbox to overcome some of the limitations of classical caging groups. In this work, the synthesis of a new class of visible-light-sensitive PPGs based on low-molecular weight COUPY fluorophores with several attractive properties, including long-wavelength absorption, is reported. Besides being stable to spontaneous hydrolysis in the dark, COUPY-based PPGs can be efficiently photoactivated with yellow (560 nm) and red light (620 nm) under physiological-like conditions, thereby offering the possibility of unmasking functional groups from COUPY photocages under irradiation conditions in which other PPGs remain stable. Additionally, COUPY photocages exhibit excellent cellular uptake and accumulate selectively in mitochondria, opening the door to the delivery of caged analogues of biologically active compounds into these organelles.

Thiocoumarin Caged Nucleotides: Synthetic Access and Their Photophysical Properties

Photocages have been successfully applied in cellular signaling studies for the controlled release of metabolites with high spatio-temporal resolution. Commonly, coumarin photocages are activated by UV light and the quantum yields of uncaging are relatively low, which can limit their applications in vivo. Here, syntheses, the determination of the photophysical properties, and quantum chemical calculations of 7-diethylamino-4-hydroxymethyl-thiocoumarin (thio-DEACM) and caged adenine nucleotides are reported and compared to the widely used 7-diethylamino-4-hydroxymethyl-coumarin (DEACM) caging group. In this comparison, thio-DEACM stands out as a phosphate cage with improved photophysical properties, such as red-shifted absorption and significantly faster photolysis kinetics.

On-demand and tunable dual wavelength release of antibody using light-responsive hydrogels

There has been an increased interest in the use of protein therapeutics, especially antibodies, for the treatment of a variety of diseases due to their high specificity to tissues and biological pathways of interest. However, the use of antibodies can be hindered by physical aggregation, degradation, and diffusion when injected in vivo leading to the need for antibody-releasing depots for the controlled and localized delivery within tissues of interest. Here, we investigated photolabile hydrogel chemistries for creating on-demand and tunable antibody release profiles. Innovative, scalable synthetic procedures were established and applied for fabricating hydrogels with nitrobenzyl (NB) and coumarin (CMR) photolabile crosslinks that responded to clinically relevant doses of long-wavelength UV and short-wavelength visible light. This synthetic procedure includes a route to make a CMR linker possessing two functional handles at the same ring position with water-stable bonds. The photocleavage properties of NB and CMR crosslinked hydrogels were characterized, as well as their potential for translational studies by degradation through pig skin, a good human skin mimic. The mechanism of hydrogel degradation, bulk versus surface eroding, was determined to be dependent on the wavelength of light utilized and the molar absorptivity of the different photolabile linkers, providing a facile means for altering protein release upon hydrogel degradation. Further, the encapsulation and on-demand release of a model monoclonal antibody was demonstrated, highlighting the ability to control antibody release from these hydrogels through the application of light while retaining its bioactivity. In particular, the newly designed CMR hydrogels undergo surface erosion-based protein release using visible light, which is more commonly used clinically. Overall, this work establishes scalable syntheses and relevant pairings of formulation-irradiation conditions for designing on-demand and light-responsive material systems that provide controlled, tunable release of bioactive proteins toward addressing barriers to preclinical translation of light-based materials and ultimately improving therapeutic regimens.

Coumarin-Containing Light-Responsive Carboxymethyl Chitosan Micelles as Nanocarriers for Controlled Release of Pesticide

Currently, controlled release formulations (CRFs) of pesticides in response to biotic and/or abiotic stimuli have shown great potential for providing “on-demand” smart release of loaded active ingredients. In this study, amphiphilic biopolymers were prepared by introducing hydrophobic (7-diethylaminocoumarin-4-yl)methyl succinate (DEACMS) onto the main chain of hydrophilic carboxymethylchitosan (CMCS) via the formation of amide bonds which were able to self-assemble into spherical micelles in aqueous media and were utilized as light-responsive nanocarriers for the controlled release of pesticides. FTIR and NMR characterizations confirmed the successful synthesis of the CMCS-DEACMS conjugate. The critical micelle concentration (CMC) decreased with the increase in the substitution of DEACMS on CMCS, which ranged from 0.013 to 0.042 mg/mL. Upon irradiation under simulated sunlight, the hydrodynamic diameter, morphology, photophysical properties and photolysis were researched by means of dynamic light scattering (DLS), transmission electron microscopy (TEM), UV-vis absorption spectroscopy and fluorescence spectroscopy. Moreover, 2,4-dichlorophenoxyacetic acid (2,4-D) was used as a model pesticide and encapsulated into the CMCS-DEACMS micelles. In these micelle formulations, the release of 2,4-D was promoted upon simulated sunlight irradiation, during which the coumarin moieties were cleaved from the CMCS backbone, resulting in a shift of the hydrophilic–hydrophobic balance and destabilization of the micelles. Additionally, bioassay studies suggested that this 2,4-D contained which micelles showed good bioactivity on the target plant without harming the nontarget plant. Thereby, the light-responsive CMCS-DEACMS micelles bearing photocleavable coumarin moieties provide a smart delivery platform for agrochemicals.

Core remodeling leads to long wavelength fluoro-coumarins

Low molecular weight, uncharged far-red and NIR dyes would be enabling for a range of imaging applications. Rational redesign of the coumarin scaffold leads to Fluoro-Coumarins (FCs), the lowest molecular weight dyes with emission maxima beyond 700, 800, and 900 nm. FCs display large Stokes shifts and high environmental sensitivity, with a 40-fold increase in emission intensity in hydrophobic solvents. Untargeted variants exhibit selective lipid droplet and nuclear staining in live cells. Furthermore, sulfo-lipid derivatization enables active targeting to the plasma membrane. Overall, these studies report a promising platform for the development of biocompatible, context-responsive imaging agents.

Fluoro-Coumarins are a novel class of far-red and near-infrared solvent sensitive dyes of exceptionally low molecular weight.

An Overview of Coumarin as a Versatile and Readily Accessible Scaffold with Broad-Ranging Biological Activities

Privileged structures have been widely used as an effective template for the research and discovery of high value chemicals. Coumarin is a simple scaffold widespread in Nature and it can be found in a considerable number of plants as well as in some fungi and bacteria. In the last years, these natural compounds have been gaining an increasing attention from the scientific community for their wide range of biological activities, mainly due to their ability to interact with diverse enzymes and receptors in living organisms. In addition, coumarin nucleus has proved to be easily synthetized and decorated, giving the possibility of designing new coumarin-based compounds and investigating their potential in the treatment of various diseases. The versatility of coumarin scaffold finds applications not only in medicinal chemistry but also in the agrochemical field as well as in the cosmetic and fragrances industry. This review is intended to be a critical overview on coumarins, comprehensive of natural sources, metabolites, biological evaluations and synthetic approaches.

A near-infrared light-mediated cleavable linker strategy using the heptamethine cyanine chromophore

Optical methods offer the potential to manipulate living biological systems with exceptional spatial and temporal control. Caging bioactive molecules with photocleavable functional groups is an important strategy that could be applied to a range of problems, including the targeted delivery of otherwise toxic therapeutics. However existing approaches that require UV or blue light are difficult to apply in organismal settings due to issues of tissue penetration and light toxicity. Photocaging groups built on the heptamethine cyanine scaffold enable the targeted delivery of bioactive molecules using near-IR light (up to 780nm) in live animal settings. Here we provide a detailed procedure demonstrating the utility of the heptamethine cyanine caging group to create a light-cleavable linker between an antibody, panitumumab, and a therapeutic small molecule in the duocarmycin class of natural products. Descriptions of the design and synthesis of the small molecule component, assembly of the antibody conjugate, in vitro analysis of uncaging, in vivo imaging, and impact on tumor progression are provided.

Coumarin-Caged Compounds of 1-Naphthaleneacetic Acid as Light-Responsive Controlled-Release Plant Root Stimulators

Six coumarin-caged compounds of 1-naphthaleneacetic acid (NAA) comprising different substituents on the coumarin moiety were synthesized and evaluated for their photophysical and chemical properties as light-responsive controlled-release plant root stimulators. The ¹H NMR and HPLC techniques were used to verify the release of NAA from the caged compounds. After irradiation at 365 nm, the caged compounds exhibited the fastest release rate at t_1/2 of 6.7 days and the slowest release rate at t_1/2 of 73.7 days. Caged compounds at high concentrations (10^-5 and 10^-6 M) significantly stimulate secondary root germination while free NAA at the same level is toxic and leads to inhibition of secondary root germination. The cytotoxicity of the caged compounds against fibroblasts and vero cells were evaluated, and the results suggested that, at 10^-5-10^-6 M, caged compounds exhibited no significant cytotoxicity to the cells. Thus, the caged compounds of NAA in this study could be of great benefit as efficient agrochemicals.

Visible-Light-Activated High-Density Materials for Controlled in Vivo Insulin Release

In this work, we describe the synthesis, characterization, and ultimate in vivo assessment of second-generation insulin photoactivated depot (PAD) materials. These are the first to use visible light to stimulate insulin release and have an in vivo performance that is 28-fold improved relative to first-generation materials. This improvement is due to two major factors linked to the utilized chemistry: (1) we have incorporated the coumarin photocleavable group, which increases the photorelease wavelength into the visible range, enhancing tissue penetration of the light; (2) phototoggling of insulin solubility is produced by linking three insulin molecules to a central bridge via light cleaved groups, and not by bonding to a large polymer. The resulting trimer is, therefore, highly dense (87% insulin dry w/w) but retains the insolubility required of the approach. Only after irradiation with visible light is native, soluble insulin is released from the dermal depot. This high density increases the amount and ease of insulin release, as the density of photolytic groups is 10-20-fold higher than in polymer-based first-generation materials. We have synthesized new azide-terminated coumarin linkers that we react with the amine groups of insulin. Using mass spectrometry methods, we identify the sites of reaction and purify individual isomers, which we demonstrate have in vitro photolysis rates that are within a factor of 2 of each other. We then reacted these terminal azide groups with a tridentate strained alkyne linker. We show that the resulting insulin trimer is highly insoluble, but can be milled into injectable particles that release insulin only in response to light from a 406 nm light source. Finally, we demonstrate that these materials have a significantly improved in vivo performance, releasing 28-fold more insulin on a per energy basis than first-generation materials.

Development of photolabile protecting groups and their application to the optochemical control of cell signaling

Many biological processes are naturally regulated with spatiotemporal control. In order to perturb and investigate them, optochemical tools have been developed that convey similar spatiotemporal precision. Pivotal to optochemical probes are photolabile protecting groups, so called caging groups, and recent developments have enabled new applications to cellular processes, including cell signaling. This review focuses on the advances made in the field of caging groups and their application in cell signaling through caged molecules such as neurotransmitters, lipids, secondary messengers, and proteins.

Combinatorial control of gene function with wavelength-selective caged morpholinos

Caged morpholino oligonucleotides (cMOs) are useful research tools in developmental biology because they allow spatiotemporal control of gene expression in whole organisms. While cMOs are usually triggered by light of a single wavelength, the introduction of spectrally distinct chromophores can enable combinatorial regulation of multiple genes. This chapter describes the general principles and methods of wavelength-selective cMO design and synthesis from commercially available reagents. Synthetic protocols for the linkers and the two-step cMO assembly are described in detail, as well as the microinjection and photoactivation techniques. Following these protocols, spectrally separated cyclic cMOs for multiple genes of interest can be prepared, enabling their inhibition in zebrafish embryos and other animal models.

The synthesis and photophysical properties of tris-coumarins

A structurally unique cyclic tris-coumarin possessing three identical coumarin units bridged by amide linkers as well as two linear analogs has been synthesized. There is a remarkable agreement between crystallographic data, 1H NMR and results of calculations for the cyclic tris-coumarin, showing in all cases a non-symmetric arrangement of identical coumarin moieties. Weak polarization of the coumarin subunits, resulting from the presence of only CONH- groups as electron-donors, results in a hypsochromic shift of both absorption and emission in this dye. We have proven that in non-cyclic, head-to-tail linked tris-coumarins, the photophysics is controlled not only by the substituents but also by the conformation of the molecule, which in turn depends on the nature of the linker's interactions. These can be controlled by the presence/absence of an amide-type hydrogen atom responsible for the formation of intramolecular hydrogen bonds. The presence of a hydrogen bond favors a stretched trans conformation of the dye, while in its absence, folding of the molecule occurs leading to a more compact conformation. Although, the increased number of covalently linked coumarin units does not drastically change the preferred conformation, the fluorescence quantum yields of tris-coumarins are significantly lower than for analogous bis-coumarins composed of the same units.

Dual-wavelength efficient two-photon photorelease of glycine by π-extended dipolar coumarins

Photolabile protecting groups (PPGs) releasing bioactive compounds upon two-photon excitation have emerged as increasingly popular tools to control and study physiological processes. Yet the limited two-photon photosensitivity of many cages is still a critical issue for applications. We herein report the design, synthesis and photophysical study of polarized extended coumarinyl derivatives which show large two-photon sensitivity (up to 440 GM) at two complementary wavelengths in the NIR spectral range. DFT calculations demonstrate that subtle tuning of polarization in the ground-state and confinement of the photo-induced intramolecular charge transfer upon excitation is responsible for enhancing two-photon absorption while maintaining large uncaging efficiency. These findings open a new engineering route towards efficient coumarinyl PPGs.

o-Nitrobenzyl photoremovable groups with fluorescence uncaging reporting properties

o-Nitrobenzyl (o-NB) derivatives are the most widely applied photoremovable groups for the study of dynamic biological processes. By introducing different substituents to the benzylic position we were able to generate a fluorescence signal upon irradiation. This signal originates from the formation of a nitrosoketone by-product able to achieve a keto-enol tautomerism leading to pi-conjugated α-hydroxystilbene derivatives. These o-NB caging groups can be used to directly monitor the uncaging event by the release of a detectable fluorescent side-product.

Solid-Phase Approaches for Labeling Targeting Peptides with Far-Red Emitting Coumarin Fluorophores

Fluorophores based on organic molecules hold great potential for ligand-targeted imaging applications, particularly those operating in the optical window in biological tissues. In this work, we have developed three straightforward solid-phase approaches based on amide-bond formation or a Cu(I)-catalyzed azide-alkyne click (CuAAC) reaction for labeling an octreotide peptide with far-red emitting coumarin-based COUPY dyes. First, the conjugatable versions of COUPY fluorophores incorporating the required functional groups (e.g., carboxylic acid, azide, or alkyne) were synthesized and characterized. All of them were found fully compatible with Fmoc/ tBu solid-phase peptide synthesis, which allowed for the labeling of octreotide either through amide-bond formation or by CuAAC reaction. A near quantitative conversion was obtained after only 1 h of reaction at RT when using CuSO₄ and sodium ascorbate independently of the click chemistry approach used (azido-COUPY/alkynyl-peptide resin or alkynyl-COUPY/azido-peptide resin). COUPY-octreotide conjugates were found stable in cell culture medium as well as noncytotoxic in HeLa cells, and their spectroscopic and photophysical properties were found similar to those of their parent coumarin dyes. Finally, the potential bioimaging applications of COUPY-octreotide conjugates were demonstrated by confocal microscopy through the visualization of living HeLa cells overexpressing the somatostatin subtype-2 receptor.

Solvent-dependent photophysics of a red-shifted, biocompatible coumarin photocage

A novel coumarin photocage with long-wavelength and high photolysis quantum yield shows solvent dependent photolysis.

Photoactivation of MDM2 Inhibitors: Controlling Protein-Protein Interaction with Light

Selectivity remains a major challenge in anticancer therapy, which potentially can be overcome by local activation of a cytotoxic drug. Such triggered activation can be obtained through modification of a drug with a photoremovable protecting group (PPG), and subsequent irradiation in the chosen place and time. Herein, the design, synthesis and biological evaluation is described of a photoactivatable MDM2 inhibitor, PPG-idasanutlin, which exerts no functional effect on cellular outgrowth, but allows for the selective, noninvasive activation of antitumor properties upon irradiation visible light, demonstrating activation with micrometer, single cell precision. The generality of this method has been demonstrated by growth inhibition of multiple cancer cell lines showing p53 stabilization and subsequent growth inhibition effects upon irradiation. Light activation to regulate protein–protein interactions between MDM2 and p53 offers exciting opportunities to control a multitude of biological processes and has the potential to circumvent common selectivity issues in antitumor drug development.

High Photostability in Nonconventional Coumarins with Far-Red/NIR Emission through Azetidinyl Substitution

Replacement of electron-donating N,N-dialkyl groups with three- or four-membered cyclic amines (e.g., aziridine and azetidine, respectively) has been described as a promising approach to improve some of the drawbacks of conventional fluorophores, including low fluorescent quantum yields (Φ_F) in polar solvents. In this work, we have explored the influence of azetidinyl substitution on nonconventional coumarin-based COUPY dyes. Two azetidine-containing scaffolds were first synthesized in four linear synthetic steps and easily transformed into far-red/NIR-emitting fluorophores through N-alkylation of the pyridine moiety. Azetidine introduction in COUPY dyes resulted in enlarged Stokes' shifts with respect to the N,N-dialkylamino-containing parent dyes, but the Φ_F were not significantly modified in aqueous media, which is in contrast with previously reported observations in other fluorophores. However, azetidinyl substitution led to an unprecedented improvement in the photostability of COUPY dyes, and high cell permeability was retained since the fluorophores accumulated selectively in mitochondria and nucleoli of HeLa cells. Overall, our results provide valuable insights for the design and optimization of novel fluorophores operating in the far-red/NIR region, since we have demonstrated that three important parameters (Stokes' shifts, Φ_F, and photostability) cannot be always simultaneously addressed by simply replacing a N,N-dialkylamino group with azetidine, at least in nonconventional coumarin-based fluorophores.

Coumarin-Caged Polyphosphazenes with a Visible-Light Driven On-Demand Degradation

Polymers that, upon photochemical activation with visible light, undergo rapid degradation to small molecules are described. Through functionalization of a polyphosphazene backbone with pendant coumarin groups sensitive to light, polymers which are stable in the dark could be prepared. Upon irradiation, cleavage of the coumarin moieties exposes carboxylic acid moieties along the polymer backbone. The subsequent macromolecular photoacid is found to catalyze the rapid hydrolytic degradation of the polyphosphazene backbone. Water-soluble and non-water-soluble polymers are reported, which due to their sensitivity toward light in the visible region could be significant as photocleavable materials in biological applications.

Redesigning the Coumarin Scaffold into Small Bright Fluorophores with Far-Red to Near-Infrared Emission and Large Stokes Shifts Useful for Cell Imaging

Among the palette of previously described fluorescent organic molecules, coumarins are ideal candidates for developing cellular and molecular imaging tools due to their high cell permeability and minimal perturbation of living systems. However, blue-to-cyan fluorescence emission is usually difficult in in vivo applications due to the inherent toxicity and poor tissue penetration of short visible light wavelengths. Here, we introduce a new family of coumarin-based fluorophores, nicknamed COUPY, with promising photophysical properties, including emission in the far-red/near-infrared (NIR) region, large Stokes shifts, high photostability, and excellent brightness. COUPY fluorophores were efficiently synthesized in only three linear synthetic steps from commercially available precursors, with the N-alkylation of a pyridine moiety being the key step at the end of the synthetic route, as it allows for the tuning of the photophysical properties of the resulting dye. Owing to their low molecular weights, COUPY dyes show excellent cell permeability and accumulate selectively in nucleoli and/or mitochondria of HeLa cells, as their far-red/NIR fluorescence emission is easily detected at a concentration as low as 0.5 μM after an incubation of only 20 min. We anticipate that these coumarin scaffolds will open a way to the development of novel coumarin-based far-red to NIR emitting fluorophores with potential applications for organelle imaging and biomolecule labeling.

Intracellular Uncaging of cGMP with Blue Light

We have made a new caged cGMP that is photolyzed with blue light. Using our recently developed derivative of 7-diethylaminocourmarin (DEAC) called DEAC450, we synthesized coumarin phosphoester derivatives of cGMP with two negative charges appended to the DEAC450 moiety. DEAC450-cGMP is freely soluble in physiological buffer without the need for any organic cosolvents. With a photolysis quantum yield of 0.18 and an extinction coefficient of 43 000 M^-1 cm^-1 at 453 nm, DEAC450-cGMP is the most photosensitive caged cGMP made to date. In patch-clamped neurons in acutely isolated brain slices, blue light effectively uncaged cGMP from DEAC450 and facilitated activation of hyperpolarization and cyclic nucleotide gated cation (HCN) channels in cholinergic interneurons. Thus, DEAC450-cGMP has a unique set of optical and chemical properties that make it a useful addition to the optical arsenal available to neurobiologists.