Caged Zn2+ Photolysis in Zebrafish Whole Brains Reveals Subsecond Modulation of Dopamine Uptake.

Free, ionic zinc (Zn2+) modulates neurotransmitter dynamics in the brain. However, the sub-s effects of transient concentration changes of Zn2+ on neurotransmitter release and uptake are not well understood. To address this lack of knowledge, we have combined the photolysis of the novel caged Zn2+ compound [Zn(DPAdeCageOMe)]+ with fast scan cyclic voltammetry (FSCV) at carbon fiber microelectrodes in live, whole brain preparations from zebrafish (Danio rerio). After treating the brain with [Zn(DPAdeCageOMe)]+, Zn2+ was released by application of light that was gated through a computer-controlled shutter synchronized with the FSCV measurements and delivered through a 1 mm fiber optic cable. We systematically optimized the photocage concentration and light application parameters, including the total duration and light-to-electrical stimulation delay time. While sub-s Zn2+ application with this method inhibited DA reuptake, assessed by the first-order rate constant (k) and half-life (t1/2), it had no effect on the electrically stimulated DA overflow ([DA]STIM). Increasing the photocage concentration and light duration progressively inhibited uptake, with maximal effects occurring at 100 µM and 800 ms, respectively. Furthermore, uptake was inhibited 200 ms after Zn2+ photorelease, but no measurable effect occurred after 800 ms. We expect that application of this method to the zebrafish whole brain and other preparations will help expand the current knowledge of how Zn2+ affects neurotransmitter release/uptake in select neurological disease states.

Probing the Ni2+ -selective Response of Fluorescent Probe NiSensor-1 with the NiCast Photocaged Complex.

CTEA (N,N-bis[2-(carboxylmethyl)thioethyl]amine) is a mixed donor ligand that has been incorporated into multiple fluorescent sensors such as NiSensor-1 that was reported to be selective for Ni2+ . Other metal ions such as Zn2+ do not produce an emission response in aqueous solution. To investigate the coordination chemistry and selectivity of this receptor, we prepared NiCast, a photocage containing the CTEA receptor. Cast photocages undergo a photoreaction that decreases electron density on a metal-bound aniline nitrogen atom, which shifts the binding equilibrium toward unbound metal ion. The unique selectivity of CTEA was examined by measuring the binding affinity of NiCast and the CTEA receptor for Ni2+ , Zn2+ , Cd2+ and Cu2+ under different conditions. In aqueous solution, Ni2+ binds more strongly to the aniline nitrogen atom than Cd2+ ; however, in CH3 CN, the change in affinity virtually disappears. The crystal structure of [Cu(CTEA)], which exhibits a Jahn-Teller-distorted square pyramidal structure, was also analyzed to gain more insight into the underlying coordination chemistry. These studies suggest that the fluorescence selectivity of NiSensor-1 in aqueous solution is due to a stronger interaction between the aniline nitrogen atom and Ni2+ compared to other divalent metal ions except Cu2+ .

Detection and Quantification of Tightly Bound Zn2+ in Blood Serum Using a Photocaged Chelator and a DNAzyme Fluorescent Sensor.

DNAzymes have emerged as a powerful class of sensors for metal ions due to their high selectivity over a wide range of metal ions, allowing for on-site and real-time detection. Despite much progress made in this area, detecting and quantifying tightly bound metal ions, such as those in the blood serum, remain a challenge because the DNAzyme sensors reported so far can detect only mobile metal ions that are accessible to bind the DNAzymes. To overcome this major limitation, we report the use of a photocaged chelator, XDPAdeCage to extract the Zn2+ from the blood serum and then release the chelated Zn2+ into a buffer using 365 nm light for quantification by an 8-17 DNAzyme sensor. Protocols to chelate, uncage, extract, and detect metal ions in the serum have been developed and optimized. Because DNAzyme sensors for other metal ions have already been reported and more DNAzyme sensors can be obtained using in vitro selection, the method reported in this work will significantly expand the applications of the DNAzyme sensors from sensing metal ions that are not only free but also bound to other biomolecules in biological and environmental samples.

Coordination Chemistry of a Controlled Burst of Zn2+ in Bulk Aqueous and Nanosized Water Droplets with a Zincon Chelator.

The light-induced photolysis of [Zn(NTAdeCage)]- generates a temporally controlled burst of Zn2+, which is rapidly chelated in situ by the free ligand Zincon2-. The [Zn(Zincon)]2- coordination progress is monitored using absorption spectroscopy in bulk aqueous buffer and reverse micelle environments. The [Zn(NTAdeCage)]- photocage and free ligand Zincon2- have different reverse micelle locations that affect the [Zn(Zincon)]2- formation at the nanoscale compared to the bulk aqueous buffer. The formation of [Zn(Zincon)]2- in a bulk aqueous buffer is more efficient despite the released Zn2+ and Zincon2- being physically closer within reverse micelles. The observed reduction of complex formation is attributed to the interfacial partitioning of Zincon2-, distinct from the Zn2+ photocage in the water pool, requiring diffusion for the species to meet to form [Zn(Zincon)]2-. This work introduces a proof-of-concept methodology to experimentally measure fast chelation reactions in confined spaces and thus provides an approach to exploring cellular responses.

On-demand guest release from MOF-5 sealed with nitrophenylacetic acid photocapping groups.

Recently, we demonstrated that triphenylacetic acid could be used to seal dye molecules within MOF-5, but guest release required the digestion of the framework by treatment with acid. We prepared the sterically bulky photocapping group [bis-(3-nitro-benzyl)-amino]-(3-nitro-phenyl)-acetic acid (PC1) that can prevent crystal violet dye diffusion from inside MOF-5 until removed by photolysis.

Zinc Photocages with Improved Photophysical Properties and Cell Permeability Imparted by Ternary Complex Formation.

Photocaged complexes can control the availability of metal ions to interrogate cellular signaling pathways. We describe a new photocage, {bis[(2-pyridyl)methyl]amino}(9-oxo-2-xanthenyl)acetic acid (XDPAdeCage, 1), which utilizes a 2-xanthone acetic acid group to mediate a photodecarboxylation reaction. XDPAdeCage photolyzes with a quantum yield of 27%, and binds Zn2+ with 4.6 pM affinity, which decreases by over 4 orders of magnitude after photolysis. For comparison to our previous approach to Zn2+ release via photodecarboxylation, the analogous photocage {bis[(2-pyridyl)methyl]amino}(m-nitrophenyl)acetic acid (DPAdeCage, 2), which uses a m-nitrobenzyl chromophore, was also prepared and characterized. The advantages of the 2-xanthone acetic acid chromophore include red-shifted excitation and a higher extinction coefficient at the preferred uncaging wavelength. The neutral ternary complex of [Zn(XDPAdeCage)]+ with the anionic ligand pyrithione is membrane permeable, which circumvents the need to utilize invasive techniques to introduce intracellular Zn2+ fluctuations. Using fluorescent imaging, we have confirmed transport of Zn2+ across membranes; in addition, RT-PCR experiments demonstrate changes in expression of Zn2+-responsive proteins after photolysis.

MOF Decomposition and Introduction of Repairable Defects Using a Photodegradable Strut.

Photoswitchable components can modulate the properties of metal organic frameworks (MOFs); however, photolabile building blocks remain underexplored. A new strut NPDAC (2-nitro-1,4-phenylenediacetic acid) that undergoes photodecarboxylation has been prepared and incorporated into a MOF, using post-synthetic linker exchange (PSLE) from the structural analogue containing PDAC (p-phenylenediacetic acid). Irradiation of NPDAC-MOF leads to MOF decomposition and concomitant formation of amorphous material. In addition to complete linker exchange, MOFs containing a mixture of PDAC and NPDAC can be obtained through partial linker exchange. In NPDAC30-MOF, which contains approximately 30 % NPDAC, the MOF retains crystallinity after irradiation, but the MOF contains defect sites consistent with loss of decarboxylated NPDAC linkers. The defect sites can be repaired by exposure to additional PDAC or NPDAC linkers at a much faster rate than the initial exchange process. The photoremoval and replacement process may lead to a more general approach to customizable MOF structures.

Detection of adsorbates on emissive MOF surfaces with X-ray photoelectron spectroscopy.

Luminescent metal-organic frameworks (MOFs) have been explored extensively as potential probes for nitroaromatic molecules, which are common constituents of explosive devices. Guest encapsulation within MOF pores is often cited as the prerequisite for emission changes, but the evidence for this signal transduction mechanism is often inadequate. Using the unique bipyridyl ligand AzoAEpP (2,2'-bis[N,N'-(4-pyridyl)ethyl]diaminoazobenzene), we constructed two luminescent pillared paddle-wheel Zn2+ MOFs using aryl dicarboxylate ligands 1,4-naphthalenedicarboxylic acid (ABMOF-1) and benzene 1,4-dicarboxylic acid (ABMOF-2). While both MOFs exhibit luminescence, 2,4-dinitrophenol only extinguishes ABMOF-1 emission. Since the size of the pores in ABMOF-1 precludes guest inclusion, we used X-ray photoelectron spectroscopy (XPS) to confirm the surface interaction and obtain insight into the nature of the quenching process. XPS experiments utilized a fluorinated nitroaromatic molecule, 4-trifluoromethyl-2,6-dinitrophenol, that extinguishes ABMOF-1 emission, and verified surface adsorption through a series of angle-resolved (ARXPS) and argon-ion sputter depth profile experiments. By further developing these techniques, we hope to develop a general approach for distinguishing between the various intermolecular interactions between MOFs and analytes that lead to changes in luminescence.

Publisher Correction: Neutron stardust and the elements of Earth.

In the version of this Comment originally published, the image was incorrectly credited to Chelsea Anne Bar; it should have been to Brett F. Thornton. This has now been corrected.

Emissive Azobenzenes Delivered on a Silver Coordination Polymer.

Azobenzene has become a ubiquitous component of functional molecules and polymeric materials because of the light-induced trans → cis isomerization of the diazene group. In contrast, there are very few applications utilizing azobenzene luminescence, since the excitation energy typically dissipates via nonradiative pathways. Inspired by our earlier studies with 2,2'-bis[ N,N'-(2-pyridyl)methyl]diaminoazobenzene (AzoAM oP) and related compounds, we investigated a series of five aminoazobenzene derivatives and their corresponding silver complexes. Four of the aminoazobenzene ligands, which exhibit no emission under ambient conditions, form silver coordination polymers that are luminescent at room temperature. AzoAE pP (2,2'-bis[ N,N'-(4-pyridyl)ethyl]diaminoazobenzene) assembles into a three-dimensional coordination polymer (AgAAE pP) that undergoes a reversible loss of emission upon the addition of metal-coordinating analytes such as pyridine. The switching behavior is consistent with the disassembly and reassembly of the coordination polymer driven by displacement of the aminoazobenzene ligands by coordinating analytes.

The zinc paradigm for metalloneurochemistry.

Neurotransmission and sensory perception are shaped through metal ion-protein interactions in various brain regions. The term "metalloneurochemistry" defines the unique field of bioinorganic chemistry focusing on these processes, and zinc has been the leading target of metalloneurochemists in the almost 15 years since the definition was introduced. Zinc in the hippocampus interacts with receptors that dictate ion flow and neurotransmitter release. Understanding the intricacies of these interactions is crucial to uncovering the role that zinc plays in learning and memory. Based on receptor similarities and zinc-enriched neurons (ZENs) in areas of the brain responsible for sensory perception, such as the olfactory bulb (OB), and dorsal cochlear nucleus (DCN), zinc participates in odor and sound perception. Development and improvement of methods which allow for precise detection and immediate manipulation of zinc ions in neuronal cells and in brain slices will be critical in uncovering the synaptic action of zinc and, more broadly, the bioinorganic chemistry of cognition.

Zn(2+) at a cellular crossroads.

Zinc is an essential micronutrient for cellular homeostasis. Initially proposed to only contribute to cellular viability through structural roles and non-redox catalysis, advances in quantifying changes in nM and pM quantities of Zn(2+) have elucidated increasing functions as an important signaling molecule. This includes Zn(2+)-mediated regulation of transcription factors and subsequent protein expression, storage and release of intracellular compartments of zinc quanta into the extracellular space which modulates plasma membrane protein function, as well as intracellular signaling pathways which contribute to the immune response. This review highlights some recent advances in our understanding of zinc signaling.