Model dyads for 2PA uncaging of a protecting group via photoinduced electron transfer

Theoretical Design, Synthesis, and In Vitro Neurobiological Applications of a Highly Efficient Two-Photon Caged GABA Validated on an Epileptic Case

In this paper, we present an additional, new cage-GABA compound, called 4-amino-1-(4'-dimethylaminoisopropoxy-5',7'-dinitro-2',3'-dihydro-indol-1-yl)-1-oxobutane-γ-aminobutyric acid (iDMPO-DNI-GABA), and currently, this compound is the only photoreagent, which can be applied for GABA uncaging without experimental compromises. By a systematic theoretical design and successful synthesis of several compounds, the best reagent exhibits a high two-photon efficiency within the 700-760 nm range with excellent pharmacological behavior, which proved to be suitable for a complex epileptic study. Quantum chemical design showed that the optimal length of the cationic side chain enhances the two-photon absorption by 1 order of magnitude due to the cooperating internal hydrogen bonding to the extra nitro group on the core. This feature increased solubility while suppressing membrane permeability. The efficiency was demonstrated in a systematic, wide range of in vitro single-cell neurophysiological experiments by electrophysiological as well as calcium imaging techniques. Scalable inhibitory ion currents were elicited by iDMPO-DNI-GABA with appropriate spatial-temporal precision, blocking both spontaneous and evoked cell activity with excellent efficiency. Additionally, to demonstrate its applicability in a real neurobiological study, we could smoothly and selectively modulate neuronal activities during artificial epileptic rhythms first time in a neural network of GCaMP6f transgenic mouse brain slices.

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.

High efficiency two-photon uncaging coupled by the correction of spontaneous hydrolysis

Two-photon (TP) uncaging of neurotransmitter molecules is the method of choice to mimic and study the subtleties of neuronal communication either in the intact brain or in slice preparations. However, the currently available caged materials are just at the limit of their usability and have several drawbacks. The local and focal nature of their use may for example be jeopardized by a high spontaneous hydrolysis rate of the commercially available compounds with increased photochemical release rate. Here, using quantum chemical modelling we show the mechanisms of hydrolysis and two-photon activation, and synthesized more effective caged compounds. Furthermore, we have developed a new enzymatic elimination method removing neurotransmitters inadvertently escaping from their compound during experiment. This method, usable both in one and two-photon experiments, allows for the use of materials with an increased rate of photochemical release. The efficiency of the new compound and the enzymatic method and of the new compound are demonstrated in neurophysiological experiments.

Photoinduced Electron Transfer (PET)-Mediated Fragmentation of Picolinium-Derived Redox Probes

The photolysis of covalently linked N-alkyl picolinium phenylacetate-carbazole dyads was analyzed experimentally and by using density functional theory (DFT) and time dependent-DFT (TD-DFT) calculations. In contrast to earlier observations efficient one and two-photon fragmentations conditions were found for 15 c (δ_u =0.16 GM at 730 nm) opening the way for the design of a novel class of "caged" compounds.

Cloaked Caged Compounds: Chemical Probes for Two-Photon Optoneurobiology

Caged neurotransmitters, in combination with focused light beams, enable precise interrogation of neuronal function, even at the level of single synapses. However, most caged transmitters are, surprisingly, severe antagonists of ionotropic gamma-aminobutyric acid (GABA) receptors. By conjugation of a large, neutral dendrimer to a caged GABA probe we introduce a "cloaking" technology that effectively reduces such antagonism to very low levels. Such cloaked caged compounds will enable the study of the signaling of the inhibitory neurotransmitter GABA in its natural state using two-photon uncaging microscopy for the first time.

Design and Synthesis of a 4-Nitrobromobenzene Derivative Bearing an Ethylene Glycol Tetraacetic Acid Unit for a New Generation of Caged Calcium Compounds with Two-Photon Absorption Properties in the Near-IR Region and Their Application in Vivo

Among biologically active compounds, calcium ions (Ca2+) are one of the most important species in cell physiological functions. Development of new calcium chelators with two-photon absorption (TPA) properties is a state-of-the-art challenge for chemists. In this study, we report the first and efficient synthesis of 5-bromo-2-nitrobenzyl-substituted ethylene glycol tetraacetic acid (EGTA) as a platform for a new generation of calcium chelators with TPA properties in the near-infrared region. New calcium chelators with high TPA properties, that is, a two-photon (TP) fragmentation efficiency of δu = 20.7 GM at 740 nm for 2-(4-nitrophenyl)benzofuran (NPBF)-substituted EGTA (NPBF-EGTA, K d = 272 nM) and δu = 7.8 GM at 800 nm for 4-amino-4'-nitro-1,1'-biphenyl (BP)-substituted EGTA (BP-EGTA, K d = 440 nM) derivatives, were synthesized using Suzuki-Miyaura coupling reactions of the bromide with benzofuran-2-boronic acid and 4-(dimethylamino)phenyl boronic acid, respectively. The corresponding acetoxymethyl (AM) esters were prepared and successfully applied to the Ca2+-uncaging reaction triggered by TP photolysis in vivo.

Development of Anionically Decorated Caged Neurotransmitters: In Vitro Comparison of 7-Nitroindolinyl- and 2-(p-Phenyl-o-nitrophenyl)propyl-Based Photochemical Probes

Neurotransmitter uncaging, especially that of glutamate, has been used to study synaptic function for over 30 years. One limitation of caged glutamate probes is the blockade of γ-aminobutyric acid (GABA)-A receptor function. This problem comes to the fore when the probes are applied at the high concentrations required for effective two-photon photolysis. To mitigate such problems one could improve the photochemical properties of caging chromophores and/or remove receptor blockade. We show that addition of a dicarboxylate unit to the widely used 4-methoxy-7-nitroindolinyl-Glu (MNI-Glu) system reduced the off-target effects by about 50-70 %. When the same strategy was applied to an electron-rich 2-(p-Phenyl-o-nitrophenyl)propyl (PNPP) caging group, the pharmacological improvements were not as significant as in the MNI case. Finally, we used very extensive biological testing of the PNPP-caged Glu (more than 250 uncaging currents at single dendritic spines) to show that nitro-biphenyl caging chromophores have two-photon uncaging efficacies similar to that of MNI-Glu.

Two-photon sensitive protecting groups operating via intramolecular electron transfer: uncaging of GABA and tryptophan

Improved photo-labile protecting groups, with high sensitivity to two-photon excitation, are needed for the controlled release of drugs, as tools in neuroscience and physiology. Here we present a new modular approach to the design of caging groups based on photoinduced electron transfer from an electron-rich two-photon dye to an electron acceptor, followed by scission of an ester to release a carboxylic acid. Three different electron acceptors were tested: nitrobenzyl, phenacyl and pyridinium. The nitrobenzyl system was ineffective, giving only photochemical decomposition and no release of the carboxylic acid. The phenacyl system also performed poorly, liberating the carboxylic acid in 20% chemical yield and 0.2% photochemical yield. The pyridinium system was most successful, and was tested for the release of two carboxylic acids: γ-amino butyric acid (GABA) and tryptophan. The caged GABA undergoes photochemical cleavage with a chemical yield of >95% and a photochemical yield of 1%; it exhibits a two-photon absorption cross section of 1100 GM at 700 nm, corresponding to a two-photon uncaging cross section of 10 ± 3 GM.