Photochemical Activation of Tertiary Amines for Applications in Studying Cell Physiology

Rational Design of Hydroxylated Thiazole Orange Photocages for Green Light-Triggered DNA Recombination

The precise control of DNA recombination enables the cell- or time-dependent regulation of gene expression in studies of gene function. Caged estrogen receptor ligands combined with a Cre-ERT2/loxP system are useful tools for light-triggered DNA recombination. However, the photolysis of most caged compounds requires ultraviolet or blue light, which is toxic and displays low tissue penetration. Although a cyanine-based photo-responsive protecting group (PPG) can release estrogen receptor ligands with longer-wavelength light, its low photolytic efficiency requires long illumination times. We developed a caged estrogen receptor ligand with improved green light-responsive PPGs. The rational modification of Hydroxylated Thiazole Orange (HTO) photocages using electron-donating groups (EDGs), such as dimethoxy (DiMeO)-substituted HTO, resulted in high photolytic efficiency (up to ϵΦ ≈320 M-1  cm-1 ). Theoretical calculations demonstrated that the enhanced photolytic efficiencies were derived from the increased intramolecular charge transfer by EDGs upon excitation. The efficient uncaging of estrogen receptor ligands enabled the control of gene recombination in a ligand-dependent Cre-ERT2/loxP system in live cells.

Multi-Photon-Sensitive Chromophore for the Photorelease of Biologically Active Phenols

Phenols confer bioactivity to a plethora of organic compounds. Protecting the phenolic functionality with photoremovable protecting groups (PPGs) sensitive to two-photon excitation (2PE) can block the bioactivity and provide controlled release of these compounds in a spatially and temporally restricted manner by photoactivation with IR light. To develop an efficient 2PE-sensitive PPG for releasing phenols, the (8-cyano-7-hydroxyquinolin-2-yl)methyl (CyHQ) chromophore was functionalized at the C4 position with methyl, morpholine, methoxy, para-tolyl, and 3,4,5-trimethoxyphenyl groups to provide 4-methyl-CyHQ (Me-CyHQ), 4-morpholino-CyHQ (Mor-CyHQ), 4-methoxy-CyHQ (MeO-CyHQ), 4-(p-tolyl)-CyHQ (pTol-CyHQ), and 4-(3,4,5-trimethoxyphenyl)-CyHQ (TMP-CyHQ) PPGs. The probes possess attributes useful for biological use, including high quantum yield (Φu), hydrolytic stability, and good aqueous solubility in physiological conditions. The MeO-CyHQ PPG enhanced the two-photon uncaging action cross section (δu) of dopamine 3.5-fold (0.85 GM) compared to CyHQ (0.24 GM) at 740 nm and 1.49 GM at 720 nm. MeO-CyHQ was used to mediate photoactivation via 2PE of serotonin, rotigotine, N-vanillyl-nonanoylamide (VNA) (a capsaicin analogue), and eugenol. The constructs except rotigotine showed excellent efficiency in 2PE with δu ranging from 0.75 to 1.01 GM at 740 nm and from 1.31 to 1.36 GM at 720 nm high yielding release of the payloads. These probes also performed well by using conventional single photon excitation (1PE). The spatially and temporally controlled release of dopamine from CyHQ-DA and MeO-CyHQ-DA and serotonin (5-HT) from MeO-CyHQ-5HT was quantified in cell culture by using genetically encoded sensors for dopamine and serotonin, respectively. Calcium imaging was employed to quantify the release of VNA and eugenol (EG) from MeO-CyHQ-VNA and MeO-CyHQ-EG, respectively. These tools will enable experiments to understand the intricate mechanisms involved in neurological signaling and the roles played by neurotransmitters, such as dopamine and serotonin, in the activation of their respective receptors.

3-Substituted 2-Aminonaphthalene Photocages for Carboxylic Acids and Alcohols; Decaging Mechanism and Potential Applications in Synthesis

3-Hydroxymethyl-2-aminonaphthalene photocage (photoremovable protecting group) 2 was synthesized and transformed to different ethers and esters to investigate the applicability to decage alcohols and carboxylic acids, respectively. The photoelimination of carboxylic acids takes place relatively efficiently (ΦR = 0.11) upon excitation with near-visible light, contrary to the elimination of alcohols. The scope of the decaging of both alcohols and esters was demonstrated on several examples, including aliphatic and aromatic substrates, carbohydrates, and nonsteroidal anti-inflammatory drugs. The photophysical properties of the photocage and its models, methyl ether 4a and acetyl ester 5a, were investigated. The fluorescence quantum yields (Φf = 0.40-0.002) were found to be reversely proportional to the efficiency of elimination of OH, alcohols, or carboxylic acids. The decaging photochemical reaction mechanism was investigated experimentally by transient absorption techniques with time scales from femtoseconds to seconds and computationally on the TD-DFT level of theory. The photoelimination of carboxylates takes place directly in the singlet excited state by a homolytic cleavage producing a radical pair within 1 ns. The subsequent electron transfer gives rise to aminonaphthalene carbocation and the carboxylate. A wide scope of substrates that can be decaged relatively efficiently with near-visible light and the chromo-orthogonal compatibility of aminonaphthalene and aniline derivatives render these photocages potentially applicable in organic synthesis or biology.

Visible and NIR light photoactivatable o-hydroxycinnamate system for efficient drug release with fluorescence monitoring

Photoactivatable protecting groups (PPGs) have become powerful materials for controlling the activity of biologically important molecules in the biomedical field. However, designing PPGs that can be efficiently activated by biologically benign visible and NIR light with fluorescence monitoring is still a great challenge. Herein, we report o-hydroxycinnamate-based PPGs that can be activated by both visible (one-photon) and NIR (two-photon) light for controlled drug release with real-time monitoring. Thus, a photoremovable 7-diethylamino o-hydroxycinnamate group is covalently attached to an anticancer drug, gemcitabine, to establish a photoactivatable prodrug system. Upon excitation by visible (400-700 nm) or NIR (800 nm) light, the prodrug efficiently releases drug which is quantified by monitoring the formation of a strongly fluorescent coumarin reporter. The prodrug is taken up by the cancer cells and interestingly accumulates within mitochondria as determined by FACS and fluorescence microscopy imaging. Further, the prodrug demonstrates photo-triggered, dose-dependent, and temporally controlled cell death upon irradiation with both visible and NIR light. This photoactivatable system could be useful and adapted in the future for the development of advanced therapies in biomedicine.

Photocatalytic Cleavage of C(sp3 )-N Bond in Trialkylamines to Dialkylamines and Olefins

Development of a new and green strategy for C(sp³ )-N bond cleavage is very interesting. Herein, photocatalytic cleavage of the C(sp³ )-N bond of trialkylamines was achieved, with concurrent formation of dialkylamines and olefins. It was found that a rationally designed 2D-Bi₂ WO₆ @1D-LaPO₄ heterostructure was very efficient for the reaction due to its high light collection efficiency and unique catalytic properties. The strategy could be used for different trialkylamines, including triethylamine, tri-n-propylamine, and ethyl-di-isopropylamine. The mechanistic investigation indicated that the catalyst with heterostructure was not only favorable for charge carrier separation but also rendered excited electrons with high reduction capacity. This work opens a way for C(sp³ )-N bond cleavage of trialkylamines.

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.

Photoactivatable o-Hydroxycinnamic Platforms for Bioimaging and Therapeutic Release

Photoactivatable or photoremovable protecting groups (PPGs) have become a powerful material and gained enormous interest in the field of biomedical applications. PPGs have been utilized for noninvasive, on-demand, spatio-temporal controlled release of biological effectors by irradiation with light to induce biochemical function. Over the past few years, o-hydroxycinnamate (oHC)-based PPGs have received considerable attention for the release of molecules of interest by either UV (one-photon) or near-IR (two-photon) irradiation. In this miniperspective, we have summarized the development of oHC PPGs for bioimaging and the controlled release of therapeutics, bioactive volatiles and other payloads with real-time monitoring. In addition, several future perspectives of oHC systems have been highlighted at the end of this miniperspective.

Quinoline-Based Photolabile Protection Strategy Facilitates Efficient Protein Assembly

Native chemical ligation (NCL) provides a powerful solution to assemble proteins with precise chemical features, which enables a detailed investigation of the protein structure-function relationship. As an extension to NCL, the discovery of desulfurization and expressed protein ligation (EPL) techniques has greatly expanded the efficient access to large or challenging protein sequences via chemical ligations. Despite its superior reliability, the NCL-desulfurization protocol requires orthogonal protection strategies to allow selective desulfurization in the presence of native Cys, which is crucial to its synthetic application. In contrast to traditional thiol protecting groups, photolabile protecting groups (PPGs), which are removed upon irradiation, simplify protein assembly and therefore provide minimal perturbation to the peptide scaffold. However, current PPG strategies are mainly limited to nitro-benzyl derivatives, which are incompatible with NCL-desulfurization. Herein, we present for the first time that quinoline-based PPG for cysteine can facilitate various ligation strategies, including iterative NCL and EPL-desulfurization methods. 7-(Piperazin-1-yl)-2-(methyl)quinolinyl (PPZQ) caging of multiple cysteine residues within the protein sequence can be readily introduced via late-stage modification, while the traceless removal of PPZQ is highly efficient via photolysis in an aqueous buffer. In addition, the PPZQ group is compatible with radical desulfurization. The efficiency of this strategy has been highlighted by the synthesis of γ-synuclein and phosphorylated cystatin-S via one-pot iterative ligation and EPL-desulfurization methods. Besides, successful sextuple protection and deprotection of the expressed Interleukin-34 fragment demonstrate the great potential of this strategy in protein caging/uncaging investigations.

Discovery of AL-GDa62 as a Potential Synthetic Lethal Lead for the Treatment of Gastric Cancer

Diffuse gastric cancer and lobular breast cancer are aggressive malignancies that are frequently associated with inactivating mutations in the tumor suppressor gene CDH1. Synthetic lethal (SL) vulnerabilities arising from CDH1 dysfunction represent attractive targets for drug development. Recently, SLEC-11 (1) emerged as a SL lead in E-cadherin-deficient cells. Here, we describe our efforts to optimize 1. Overall, 63 analogues were synthesized and tested for their SL activity toward isogenic mammary epithelial CDH1-deficient cells (MCF10A-CDH1^-/-). Among the 26 compounds with greater cytotoxicity, AL-GDa62 (3) was four-times more potent and more selective than 1 with an EC₅₀ ratio of 1.6. Furthermore, 3 preferentially induced apoptosis in CDH1^-/- cells, and Cdh1^-/- mammary and gastric organoids were significantly more sensitive to 3 at low micromolar concentrations. Thermal proteome profiling of treated MCF10A-CDH1^-/- cell protein lysates revealed that 3 specifically inhibits TCOF1, ARPC5, and UBC9. In vitro, 3 inhibited SUMOylation at low micromolar concentrations.

Mitochondria-Targeted Photoactivatable Real-Time Monitoring of a Controlled Drug Delivery Platform

The current anticancer therapies are limited by their lack of controlled spatiotemporal release at the target site of action. We report a novel drug delivery platform that provides on-demand, real-time, organelle-specific drug release and monitoring upon photoactivation. The system is comprised of a model anticancer drug doxorubicin, an alkyltriphenylphosphonium moiety to target mitochondria in cancer cells, and a hydroxycinnamate photoactivatable linker that is covalently attached to the drug and mitochondria-targeting moieties such that it can be phototriggered by either UV (one-photon) or NIR (two-photon) light to form a fluorescent coumarin product and facilitate the release of drug payload. The extent of drug release is quantified by the fluorescence intensity of the coumarin formed. Further, the photoactivatable prodrug accumulates in the mitochondria and shows light-triggered temporally controlled cell death. In the future, our platform can be tuned for any biological application of interest, offering immense value in biomedicine.

Direct Detection of the Photorearrangement Reaction of Quinoline-Protected Dialkylanilines†

The photolysis reactions of (8-cyano-7-hydroxyquinolin-2-yl)methyl (CyHQ)-caged amines have been investigated using time-resolved spectroscopy methods. Unexpectedly, an unconventional Hofmann-Martius rearrangement reaction with high yield and regioselectivity occurred during the photolysis of some CyHQ-protected dialkylanilines (such as compounds 1a and 2a). To have more insights into the mechanism of this unexpected photorearrangement reaction, we characterized the reaction intermediates directly using time-resolved spectroscopy. Our new results showed that the anionic form of compound 1a was photoexcited to the singlet excited state, then a heterolytic cleavage of the C-N bond took place to give CyHQ⁺ and the corresponding aniline. Thereafter, the recombined intermediate 6 was found to appear in about 19.7 and 44.3 ps for 1a (A) and 2a (A), respectively, before the generation of an ortho-substituted aniline (1b and 2b) via the excited-state deprotonation of 6. Thus, a logical photodynamic mechanism of this photoinduced Hofmann-Martius rearrangement reaction was deduced. This new insight into the reaction mechanisms may be helpful for the design of novel related photoactivatable aniline molecules and for understanding other similar photorearrangement reaction mechanisms.

Caged Oxime Reactivators Designed for the Light Control of Acetylcholinesterase Reactivation†

Despite its promising role in the active control of biological functions by light, photocaging remains untested in acetylcholinesterase (AChE), a key enzyme in the cholinergic family. Here, we describe synthesis, photochemical properties and biochemical activities of two caged oxime compounds applied in the photocontrolled reactivation of the AChE inactivated by reactive organophosphate. Each of these consists of a photocleavable coumarin cage tethered to a known oxime reactivator for AChE that belongs in an either 2-(hydroxyimino)acetamide or pyridiniumaldoxime class. Of these, the first caged compound was able to successfully go through oxime uncaging upon irradiation at long-wavelength ultraviolet light (365 nm) or visible light (420 nm). It was further evaluated in AChE assays in vitro under variable light conditions to define its activity in the photocontrolled reactivation of paraoxon-inactivated AChE. This assay result showed its lack of activity in the dark but its induction of activity under light conditions only. In summary, this article reports a first class of light-activatable modulators for AChE and it offers assay methods and novel insights that help to achieve an effective design of caged compounds in the enzyme control.

Photoactivatable AMPA for the study of glutamatergic neuronal transmission using two-photon excitation

We report a photoactivatable agonist of the AMPA subtype of ionotropic glutamate receptors, TMP-CyHQ-AMPA, which was designed to study the fast excitatory transmission between neurons. Upon visible light excitation, TMP-CyHQ-AMPA quantitatively released AMPA in high quantum yield on an ultra-short timescale. Intriguingly, the photolyisis can be carried out using 2-photon excitation (2PE) with remarkable efficiency, giving a two-photon uncaging action cross section (δu) value of 1.71 GM. TMP-CyHQ-AMPA is soluble in pysiological buffer and no hydrolysis was detected in the absence of light. Molecular docking experiments indicated that the photocaging strategy abolishes the affinity of AMPA for the GluR2 receptor and no GABAergic effects (as commonly observed in caged glutamates) are expected. TMP-CyHQ-AMPA can be used to study glutamatergic neuronal transmission with exceptional spatial-temporal resolution in complex tissue preparations.

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.

Photoactivatable Odorants for Chemosensory Research

The chemosensory system of any animal relies on a vast array of detectors tuned to distinct chemical cues. Odorant receptors and the ion channels of the TRP family are all uniquely expressed in olfactory tissues in a species-specific manner. Great effort has been made to characterize the molecular and pharmacological properties of these proteins. Nevertheless, most of the natural ligands are highly hydrophobic molecules that are not amenable to controlled delivery. We sought to develop photoreleasable, biologically inactive odorants that could be delivered to the target receptor or ion channel and effectively activated by a short light pulse. Chemically distinct ligands eugenol, benzaldehyde, 2-phenethylamine, ethanethiol, butane-1-thiol, and 2,2-dimethylethane-1-thiol were modified by covalently attaching the photoremovable protecting group (8-cyano-7-hydroxyquinolin-2-yl)methyl (CyHQ). The CyHQ derivatives were shown to release the active odorant upon illumination with 365 and 405 nm light. We characterized their bioactivity by measuring activation of recombinant TRPV1 and TRPA1 ion channels expressed in HEK 293 cells and the electroolfactogram (EOG) response from intact mouse olfactory epithelium (OE). Illumination with 405 nm light was sufficient to robustly activate TRP channels within milliseconds of the light pulse. Photoactivation of channels was superior to activation by conventional bath application of the ligands. Photolysis of the CyHQ-protected odorants efficiently activated an EOG response in a dose-dependent manner with kinetics similar to that evoked by the vaporized odorant amyl acetate (AAc). We conclude that CyHQ-based, photoreleasable odorants can be successfully implemented in chemosensory research.

Photorelease of Pyridines Using a Metal-Free Photoremovable Protecting Group

The photorelease of bioactive molecules has emerged as a valuable tool in biochemistry. Nevertheless, many important bioactive molecules, such as pyridine derivatives, cannot benefit from currently available organic photoremovable protecting groups (PPGs). We found that the inefficient photorelease of pyridines is attributed to intramolecular photoinduced electron transfer (PET) from PPGs to pyridinium ions. To alleviate PET, we rationally designed a strategy to drive the excited state of PPG from S₁ to T₁ with a heavy atom, and synthesized a new PPG by substitution of the H atom at the 3-position of 7-dietheylamino-coumarin-4-methyl (DEACM) with Br or I. This resulted in an improved photolytic efficiency of the pyridinium ion by hundreds-fold in aqueous solution. The PPG can be applied to various pyridine derivatives. The successful photorelease of a microtubule inhibitor, indibulin, in living cells was demonstrated for the potential application of this strategy in biochemical research.

Practical Synthesis of Quinoline-Protected Morpholino Oligomers for Light-Triggered Regulation of Gene Function

Photoactivatable cyclic caged morpholino oligomers (ccMOs) represent a promising tool to selectively regulate gene expression with spatiotemporal control. Nevertheless, some challenges associated with the preparation of these reagents have limited their broader use in biological settings. We describe a novel ccMO design that overcomes many of the challenges and considerably expedites the synthetic preparation. The key factor is the introduction of an ethynyl function on the photocleavable linker to facilitate the use of a Huisgen 1,3-dipolar cycloaddition for the coupling reaction with the oligonucleotide. Compared to previous strategies, this modification reduces the number of synthetic steps and significantly improves the total yield and the stability of the linker. We used the alkynyl-functionalized linker for the preparation of two different ccMOs targeting the mRNA of the glutamic acid decarboxylase genes, gad1 and gad2. HPLC analysis confirms that the caging strategy successfully inhibits the DNA binding ability, and the activity can be restored by brief illumination with 405-nm light. Overall, the straightforward preparation together with the clean and fast photochemistry make these caged antisense reagents excellent tools to modulate gene function in-vivo with spatial and temporal precision.

Photoactivatable Dopamine and Sulpiride to Explore the Function of Dopaminergic Neurons and Circuits

Kinetic analysis of dopamine receptor activation and inactivation and the study of dopamine-dependent signaling requires precise simulation of the presynaptic release of the neurotransmitter dopamine and tight temporal control over the release of dopamine receptor antagonists. The 8-cyano-7-hydroxyquinolinyl (CyHQ) photoremovable protecting group was conjugated to dopamine and the dopamine receptor antagonist sulpiride to generate "caged" versions of these neuromodulators (CyHQ-O-DA and CyHQ-sulpiride, respectively) that could release their payloads with 365 or 405 nm light or through 2-photon excitation (2PE) at 740 nm. These compounds are stable under physiological conditions in the dark, yet photolyze rapidly and cleanly to yield dopamine or sulpiride and the caging remnant CyHQ-OH. CyHQ-O-DA mediated the light activation of dopamine-1 (D1) receptors on the breast cancer cell line MDA-MB-231 in culture. In mouse brain slice from the substantia nigra pars compacta, localized flash photolysis of CyHQ-O-DA accurately mimicked the natural presynaptic release of dopamine and activation of dopamine-2 (D2) receptors, causing a robust, concentration-dependent, and repeatable G protein-coupled inwardly rectifying potassium channel-mediated outward current in whole-cell voltage clamp recordings that was amplified by cocaine and blocked by sulpiride. Photolysis of CyHQ-sulpiride rapidly blocked synaptic activity, enabling measurement of the unbinding rates of dopamine and quinpirole, a D2 receptor agonist. These tools will enable more detailed study of dopamine receptors, their interactions with other GPCRs, and the physiology of dopamine signaling in the brain.

Spacer-Mediated Control of Coumarin Uncaging for Photocaged Thymidine

Despite its importance in the design of photocaged molecules, less attention is focused on linker chemistry than the cage itself. Here, we describe unique uncaging properties displayed by two coumarin-caged thymidine compounds, each conjugated with (2) or without (1) an extended, self-immolative spacer. Photolysis of 1 using long-wavelength UVA (365 nm) or visible (420, 455 nm) light led to the release of free thymidine along with the competitive generation of a thymidine-bearing recombination product. The occurrence of this undesired side reaction, which is previously unreported, was not present with the photolysis of 2, which released thymidine exclusively with higher quantum efficiency. We propose that the spatial separation between the cage and the substrate molecule conferred by the extended linker can play a critical role in circumventing this unproductive reaction. This report reinforces the importance of linker selection in the design of coumarin-caged oligonucleosides and other conjugates.

Competing pathways for photoremovable protecting groups: the effects of solvent, oxygen and encapsulation

Photolysis of p-hydroxyphenacyloxy arenes releases free phenols in good yields governed by their pK_a. At high pK_a, new byproducts (Bvs. A) reveal a change in reaction mechanism.

A two-photon responsive naphthyl tagged p-hydroxyphenacyl based drug delivery system: uncaging of anti-cancer drug in the phototherapeutic window with real-time monitoring

A two-photon responsive drug delivery system having two-photon absorption (TPA) in the phototherapeutic window with a two-photon uncaging cross-section ≥10 GM and exhibiting real-time monitoring of anti-cancer drug release.

Photorearrangement of Quinoline-Protected Dialkylanilines and the Photorelease of Aniline-Containing Biological Effectors

The direct release of dialkylanilines was achieved by controlling the outcome of a photorearrangement reaction promoted by the (8-cyano-7-hydroxyquinolin-2-yl)methyl (CyHQ) photoremovable protecting group. The substrate scope was investigated to obtain structure-activity relationships and to propose a reaction mechanism. Introducing a methyl substituent at the 2-methyl position of the CyHQ core enabled the bypass of the photorearrangement and significantly improved the aniline release efficiency. We successfully applied the strategy to the photoactivation of mifepristone (RU-486), an antiprogestin drug that is also used to induce the LexPR gene expression system in zebrafish and the gene-switch regulatory system based on the pGL-VP chimeric regulator in mammals.

Cyanine Modification Tuned for Amine Photorelease

Cyanines are emerging as useful agents for photoreleasing biological compounds because of their capability of utilizing near-infrared (NIR) light. Another benefit is their ability to self-sensitize to produce singlet oxygen for the release of aryl amines, a process that has not been as feasible in the past. Here, we highlight the paper by Schnermann et al. (https://doi.org/10.1111/php.13090), which reports on a cyanine conjugate for heterolytic photocleavage of aryl amines. This paper is timely-delving into a photorelease mechanism involving a domino rearrangement and β-elimination triggered by NIR light.

Bypassing Glutamic Acid Decarboxylase 1 (Gad1) Induced Craniofacial Defects with a Photoactivatable Translation Blocker Morpholino

γ-Amino butyric acid (GABA) mediated signaling is critical in the central and enteric nervous systems, pancreas, lungs, and other tissues. It is associated with many neurological disorders and craniofacial development. Glutamic acid decarboxylase (GAD) synthesizes GABA from glutamate, and knockdown of the gad1 gene results in craniofacial defects that are lethal in zebrafish. To bypass this and enable observation of the neurological defects resulting from knocking down gad1 expression, a photoactivatable morpholino oligonucleotide (MO) against gad1 was prepared by cyclization with a photocleavable linker rendering the MO inactive. The cyclized MO was stable in the dark and toward degradative enzymes and was completely linearized upon brief exposure to 405 nm light. In the course of investigating the function of the ccMOs in zebrafish, we discovered that zebrafish possess paralogous gad1 genes, gad1a and gad1b. A gad1b MO injected at the 1–4 cell stage caused severe morphological defects in head development, which could be bypassed, enabling the fish to develop normally, if the fish were injected with a photoactivatable, cyclized gad1b MO and grown in the dark. At 1 day post fertilization (dpf), light activation of the gad1b MO followed by observation at 3 and 7 dpf led to increased and abnormal electrophysiological brain activity compared to wild type animals. The photocleavable linker can be used to cyclize and inactivate any MO, and represents a general strategy to parse the function of developmentally important genes in a spatiotemporal manner.

S,S-Chiral Linker Induced U Shape with a Syn-facial Sensitizer and Photocleavable Ethene Group

There is a major need for light-activated materials for the release of sensitizers and drugs. Considering the success of chiral columns for the separation of enantiomer drugs, we synthesized an S,S-chiral linker system covalently attached to silica with a sensitizer ethene near the silica surface. First, the silica surface was modified to be aromatic rich, by replacing 70% of the surface groups with (3-phenoxypropyl)silane. We then synthesized a 3-component conjugate [chlorin sensitizer, S,S-chiral cyclohexane and ethene building blocks] in 5 steps with a 13% yield, and covalently bound the conjugate to the (3-phenoxypropyl)silane-coated silica surface. We hypothesized that the chiral linker would increase exposure of the ethene site for enhanced ¹ O₂ -based sensitizer release. However, the chiral linker caused the sensitizer conjugate to adopt a U shape due to favored 1,2-diaxial substituent orientation; resulting in a reduced efficiency of surface loading. Further accentuating the U shape was π-π stacking between the (3-phenoxypropyl)silane and sensitizer. Semiempirical calculations and singlet oxygen luminescence data provided deeper insight into the sensitizer's orientation and release. This study has lead to insight on modifications of surfaces for drug photorelease and can help lead to the development of miniaturized photodynamic devices.

Direct Observation of an Alkylidenecarbene by Ultrafast Transient Absorption Spectroscopy

Singlet α-methylbenzylidenecarbene has been detected and characterized for the first time by femtosecond transient absorption (fs-TA) spectroscopy. The carbene was generated by photolysis of the hydrocarbon precursor, 1-(1-phenylethylidene)-1a,9b-dihydro-1 H-cyclopropa[ l]-phenanthrene, in acetonitrile. The photolysis initially forms the singlet excited state of the precursor which then extrudes α-methylbenzylidenecarbene in 4.0 ps. The decay of α-methylbenzylidenecarbene, which was previously shown to rearrange into phenylpropyne by a 1,2-phenyl shift, occurred over 13.3 ps. Computed spectra at the TD-B3LYP/6-311+G** level of theory are consistent with the experimental observations. CASSCF(10,10)/6-311+G** calculations, using one of the carbene conformers as a model, indicated that the reference wave function is dominated by a closed-shell description.

Photoactivatable drugs for nicotinic optopharmacology

Photoactivatable pharmacological agents have revolutionized neuroscience, but the palette of available compounds is limited. We describe a general method for caging tertiary amines by using a stable quaternary ammonium linkage that elicits a red shift in the activation wavelength. We prepared a photoactivatable nicotine (PA-Nic), uncageable via one- or two-photon excitation, that is useful to study nicotinic acetylcholine receptors (nAChRs) in different experimental preparations and spatiotemporal scales.

Control of Protein Activity and Gene Expression by Cyclofen-OH Uncaging

The use of light to control the expression of genes and the activity of proteins is a rapidly expanding field. Whereas many of these approaches use fusion between a light-activable protein and the protein of interest to control the activity of the latter, it is also possible to control the activity of a protein by uncaging a specific ligand. In that context, controlling the activation of a protein fused to the modified estrogen receptor (ERT) by uncaging its ligand cyclofen-OH has emerged as a generic and versatile method to control the activation of proteins quantitatively, quickly, and locally in a live organism. We present that approach and its uses in a variety of physiological contexts.