Probing Metal Ion Complexation of Ligands with Multiple Metal Binding Sites: The Case of Spiropyrans

Among known molecular switches, spiropyrans attract considerable interest because of their reversible responsiveness to external stimuli and the deep conformational and electronic changes that characterize the switching process between the two isomeric forms [spiropyran (SP) and merocyanine (MC)]. Metal coordination is one of the most interesting aspects of spiropyrans for its potential in sensing, catalysis, and medicinal chemistry, but little is known about the details surrounding spiropyran–metal ion binding. We investigated the interplay between an N‐modified 8‐methoxy‐6‐nitrospiropyran (SP‐E), designed to provide appropriate molecular flexibility and a range of competing/collaborative metal binding sites, with Mg²⁺, Cu²⁺ and Zn²⁺, which were chosen for their similar coordination geometry preferences while differing in their hard/soft character. The formed molecular complexes were studied by means of UV/Vis, fluorescence, and NMR spectroscopies and mass spectrometry, and the crystal structure of the SP‐E–Cu complex was also obtained. The results indicate that the Mg²⁺, Zn²⁺ and Cu²⁺ complexes have identical coordination stoichiometry. Furthermore, the Mg²⁺ and Zn²⁺ complexes display fluorescence properties in solution and visible‐light responsiveness. These results provide important spectroscopic and structural information that can serve as a foundation for rational design of spiropyran‐based smart materials for metal sensing and scavenging applications.     

A FRET fluorescent chemosensor SPAQ for Zn based on a dyad bearing spiropyran and 8-aminoquinoline unit

A novel FRET fluorescent sensor SPAQ containing 8-aminoquinoline (donor) and spiropyran derivative (acceptor) was designed and synthesized for detecting Zn²⁺. The probe successfully exhibited a fluorescence turn on and ratiometric response for Zn²⁺ in ethanol solution with high selectivity. Upon excitation at 370 nm, the modulation of the emission intensity of SPAQ at 645 and 470 nm was achieved in the presence of Zn²⁺ by fluorescent resonance energy transfer (FRET) and chelation-enhanced fluorescence (CHEF) effects.     

Nonionic Dendritic and Carbohydrate Based Amphiphiles: Self‐Assembly and Transport Behavior

Herein, a new series of non‐ionic dendritic and carbohydrate based amphiphiles is synthesized employing biocompatible starting materials and studied for supramolecular aggregate formation in aqueous solution. The dendritic amphiphiles 12 and 13 possessing poly(glycerol) [G2.0] as hydrophilic unit and C‐10 and C‐18 hydrophobic alkyl chains, respectively, exhibit low critical aggregation concentration (CAC) in the order of 10⁻⁵m and hydrodynamic diameters in the 8–10 nm range and supplemented by cryogenic transmission electron microscopy. Ultraviolet‐visible (UV‐Vis) and fluorescence spectroscopy suggests the effective solubilization of hydrophobic guests by the self‐assembled architectures, with the nanotransporters 12 and 13 possessing the highest encapsulation efficiency of 80.74 and 98.03% for curcumin. Efficient uptake of encapsulated curcumin in adenocarcinomic human alveolar basal epithelial (A549) cells is observed by confocal laser scanning microscopy. Amphiphiles 12 and 13 are non‐cytotoxic at the concentrations studied, however, curcumin encapsulated samples efficiently reduce the viability of A549 cells in vitro. Experimental studies indicate the ability of amphiphile 13 to encapsulate 1‐anilinonaphthalene‐8‐sulfonic acid (ANS) and curcumin with binding constant of 1.16 × 105⁵m ⁻¹ and 1.43 × 10⁶m ⁻¹, respectively. Overall, our findings demonstrate the potential of these dendritic amphiphiles for the development of prospective nanocarriers for the solubilization of hydrophobic drugs.     

Micelles and reverse micelles with a photo and thermo double‐responsive block copolymer

A novel photo and thermo double‐responsive block copolymer was developed to fabricate micelles and reverse micelles in aqueous solution. The block copolymer was synthesized by ATRP block copolymerization of a spiropyran‐ containing methacrylate (SPMA) with di(ethylene glycol) methyl ether methacrylate (DEGMMA). By facile control of the photo irradiation and solution temperature, PSPMA‐core and PDEGMMA‐core micelles can be obtained, respectively. The thermo‐ and photo‐responsive micelles were used as smart polymeric nanocarriers for controlled encapsulation, triggered release, and re‐encapsulation of model drug coumarin 102. The double‐responsive self‐assembly and disassembly were tracked by dynamic light scattering, transmission electron microscopy, and fluorescence spectroscopy. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2855–2861, 2010     

Aqueous-phase synthesis of upconversion metal-organic frameworks for ATP-responsive in situ imaging and targeted combinational cancer therapy

Herein, the nanoscaled ATP-responsive upconversion metal-organic frameworks（UCMOFs） are aqueousphase synthesized for co-delivery of therapeutic protein cytochrome c（Cyt c） and chemodrugs doxorubicin（DOX）, achieving targeted combinational therapy of human cervical cancer. The UCMOFs are rationally fabricated by growing ZIF-90 on mesoporous silica-coated upconversion nanoparticles（UCNPs）,in which the ZIF-90 layer attenuates the upconversion luminescence（UCL） and the rigid frameworks increase the s...     

Dual Enzyme‐Responsive Capsules of Hyaluronic Acid‐block‐Poly(Lactic Acid) for Sensing Bacterial Enzymes

The synthesis of novel amphiphilic hyaluronic acid (HYA) and poly(lactic acid) (PLA) block copolymers is reported as the key element of a strategy to detect the presence of pathogenic bacterial enzymes. In addition to the formation of defined HYA‐block‐PLA assemblies, the encapsulation of fluorescent reporter dyes and the selective enzymatic degradation of the capsules by hyaluronidase and proteinase K are studied. The synthesis of the dual enzyme‐responsive HYA‐b‐PLA is carried out by copper–catalyzed Huisgen 1,3‐dipolar cycloaddition. The resulting copolymers are assembled in water to form vesicular structures, which are characterized by scanning electron microscopy, transmission electron microscopy, dynamic light scattering (DLS), and fluorescence lifetime imaging microscopy (FLIM). DLS measurements show that both enzymes cause a rapid decrease in the hydrodynamic diameter of the nanocapsules. Fluorescence spectroscopy data confirm the liberation of encapsulated dye, which indicates the disintegration of the capsules and validates the concept of enzymatically triggered payload release. Finally, cytotoxicity assays confirm that the HYA‐b‐PLA nanocapsules are biocompatible with primary human dermal microvascular endothelial cells.     

Reversibly light‐responsive biodegradable poly(carbonate) micelles constructed via CuAAC reaction

Light‐responsive poly(carbonate)s PEG₁₁₃‐b‐PMPC_n‐SP were synthesized via copper catalyzed azide‐alkyne cycloaddition reaction between azide‐modified spiropyran (SP‐N₃) and amphiphilic copolymer PEG₁₁₃‐b‐PMPC_n. PEG₁₁₃‐b‐PMPC₂₅‐SP can self‐assemble to biocompatible micelles with an average diameter of ～96 nm and a critical aggregation concentration of 0.0148 mg mL^?1. Under 365 nm UV light irradiation, the characteristic absorption intensity of merocyanine (MC) progressively increased and most of the micellar aggregations were disrupted within 10 min, suggesting the completion of the transformation of hydrophobic SP to hydrophilic MC. Subsequent exposuring the micelles to 620 nm visible light, spherical micelles aggregated again. The light‐controlled release and re‐encapsulation behaviors of coumarin 102‐loaded micelles were further investigated by fluorescence spectroscopy. This study provides a convenient way to construct smart poly(carbonate)s nanocarriers for controlled release and re‐encapsulation of hydrophobic drugs. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 750–760     

Carbohydrate-Derived Metal-Chelator-Triggered Lipids for Liposomal Drug Delivery

Liposomes are versatile three-dimensional, biomaterial-based frameworks that can spatially enclose a variety of organic and inorganic biomaterials for advanced targeted-delivery applications. Implementation of external-stimuli-controlled release of their cargo will significantly augment their wide application for liposomal drug delivery. This paper presents the synthesis of a carbohydrate-derived lipid, capable of changing its conformation depending on the presence of Zn²⁺: an active state in the presence of Zn²⁺ ions and back to an inactive state in the absence of Zn²⁺ or when exposed to Na₂EDTA, a metal chelator with high affinity for Zn²⁺ ions. This is the first report of a lipid triggered by the presence of a metal chelator. Total internal reflection fluorescence microscopy and a single-liposome study showed that it indeed was possible for the lipid to be incorporated into the bilayer of stable liposomes that remained leakage-free for the fluorescent cargo of the liposomes. On addition of EDTA to the liposomes, their fluorescent cargo could be released as a result of the membrane-incorporated lipids undergoing a conformational change.     

Effects of the Molecular Weight of PLGA on Degradation and Drug Release In vitro from an mPEG-PLGA Nanocarrier

We successfully synthesized four kinds of copolymers with varying molecular weights of poly(lactide-co-glycolide)(PLGA) to yield methoxy-poly(ethylene glycol)-block-poly(lactide-co-glycolide)(mPEG-PLGA) nanocarriers:mPEG-PLGA(3k), mPEG-PLGA(9k), mPEG-PLGA(11k) and mPEG-PLGA(16k). An antitumor drug, 10-hydroxycamptothecin(HCPT), was encapsulated into the mPEG-PLGA nanocarrier cores by self-assembly in dialysis. The lower molecular weight nanocarriers degraded more quickly, resulting in mass loss, pH decline, and a rapid HCPT release rate in vitro. The degradation and drug release of the nanocarriers were dependent on the PLGA molecular weight. However, the larger molecular weight nanocarriers could not increase the loading content and encapsulation efficiency. Considering the antitumor effect of these nanocarriers, the mPEG-PLGA(9k) nanocarrier, which had the highest drug loading content[(7.72±0.57)%] and a relatively high encapsulation efficiency[(22.71±5.53)%], is an optimum agent for drug delivery.     

Efficient complexation of pyrrole-bridged dizinc(II) bisporphyrin with fluorescent probe pyrene: synthesis, structure, and photoinduced singlet-singlet energy transfer

A diethylpyrrole‐bridged dizinc(II) bisporphyrin (Zn₂DEP) is reported that encapsulates fluorescent probe pyrene molecules through strong π–π interactions, which can relay information about the chemical environment in the interior of the host–guest supramolecular assembly. X‐ray structures of both Zn₂DEP and the encapsulated pyrene complex are reported, which provides a rare opportunity to investigate the structural changes upon guest binding. A comparative structural analysis demonstrated the exceptional ability of this bisporphyrin platform to open its binding pocket for pyrene encapsulation by a vertical displacement of more than 2.45 Å, although both Zn₂DEP and the pyrene complex have nearly parallel porphyrin ring orientations. The ¹H NMR spectrum of the encapsulated pyrene complex in solution shows the upfield shifts of the pyrene protons due to a strong ring current effect, which demonstrates the retention of the solid‐state structure in solution. To further assess the extent to which pyrene guests remain encapsulated in solution, a known fluorescence quencher, dimethylaniline, was added to the host–guest assembly, which shows no exciplex formation for days in nonpolar solvents. Thus, the assembly also retained the structural integrity in solution for a long time. The association constant (K_asso) for such a complexation process in solution was observed to be 1.78×10⁵ M ^?2 for 1:2 binding. Steady‐state fluorescence and lifetime studies indicate significant photoinduced singlet–singlet energy transformation from the excited state of pyrene to zinc bisporphyrin.     

Sugar‐Decorated Dendritic Nanocarriers: Encapsulation and Release of the Octahedral Rhenium Cluster Complex [Re6S8(OH)6]4−

The encapsulation of a nanometer‐sized octahedral anionic rhenium cluster complex with six terminal hydroxo ligands [Re₆S₈(OH)₆]^4? in maltose‐decorated poly(propylene amine) dendrimers (POPAM, generation 4 and 5) has been investigated. Ultrafiltration experiments showed that maximal loading capacity of the dendrimers with the cluster complex is achieved after about ten hours in aqueous solution. To study the inclusion phenomena, three different methods have been applied: UV/Vis, time‐resolved laser‐induced fluorescence spectroscopy (TRLFS), and laser‐induced liquid bead ion desorption mass spectrometry (LILBID‐MS). From the results obtained, it could be concluded that: a) the hydrolytic stability of the rhenium cluster complex is significantly enhanced in the presence of dendritic hosts; b) the cluster anions are preferentially bound inside the dendrimers; c) the number of cluster complexes encapsulated in the dendrimers increases with rising dendrimer generation. On average, four to five cluster anions can preferentially be captured in the interior of sugar‐coated dendritic carriers. An asymptotic progression of the release of cluster complexes from the loaded dendrimers was observed under physiologically relevant conditions (isotonic sodium chloride solution: approximately 93 % within 4 days for loaded POPAM‐G4‐maltose; approximately 86 % within 4 days for loaded POPAM‐G5‐maltose). These encapsulation and release properties of maltose‐decorated nanocarriers imply the possibility for the development of the next generation of dendritic nanocarriers with specific targeting of destined tissue for therapeutic treatments.     

A lysosomal targeting fluorescent probe and its zinc imaging in SH-SY5Y human neuroblastoma cells

A fluorescent probe based PET mechanism was designed, and the probe could image endogenous release of Zn²⁺ upon H₂O₂ stimulation in SH-SY5Y cells.     

Excited State Proton Transfers in Hybrid Compound Based on Indoline Spiropyran of the Coumarin Type and Azomethinocoumarin in the Presence of Metal Ions

Spectral-luminescence properties of a hybrid compound containing a coumarin-type spiropyran and an azomethinocoumarin fragment in toluene-acetonitrile solution in the presence of Li⁺, Ca²⁺, Zn²⁺ and Mg²⁺ ions are reported. Two excited state proton transfers can occur in the hybrid compound—the transfer of a proton from the OH group of the 7-hydroxy coumarin tautomer to the N atom of the C=N bond of the azomethine fragment leading to green ESIPT fluorescence with a maximum at 540 nm and from the OH group of the 7-hydroxy coumarin tautomer to the carbonyl group of the pyrone chromophore, which leads to the formation of the 2-hydroxyl-tautomer T of coumarin with blue fluorescence with a maximum at 475 nm. Dependence of these excited state proton transfers on the metal nature and irradiation with an external UV source is discussed.     

The Bioinorganic Chemistry of Apoptosis: Potential Inhibitory Zinc Binding Sites in Caspase‐3

Zn²⁺ inhibits the action of several of the caspases and thus may act as a regulator of apoptosis. Reversal of this inhibition is one possible approach for the development of apoptosis‐based therapies. Few studies describe the molecular details of the Zn²⁺–caspase interaction, the understanding of which is essential for the success of any therapeutic strategies. Enzyme kinetics and biophysical studies have shown that the inhibition is of mixed type with prominent (ca. 60 % of inhibition) uncompetitive characteristics and an IC₅₀ of 0.8 μM under the conditions used. Fluorescence‐based techniques confirmed that, during inhibition in the sub‐micromolar range, substrate binding remains unaffected. A new zinc binding site composed of the catalytic histidine and a nearby methionine residue, rather than the catalytic histidine and cysteine dyad, is proposed based on the experimental observations. DFT models were used to demonstrate that the proposed site could be the preferred inhibitory zinc binding site.     

Selective detection of Zn2+ and Cd2+ ions in water using a host-guest complex between chromone and Q[7]

In this paper, the host-guest interaction of cucurbit[7]uril (Q[7]) and chromone (CMO) has been developed as a fluorescent probe for the highly selective detection of Zn²⁺ and Cd²⁺ in water based on a chelation-enhanced fluorescence (CHEF) mechanism. There was a good linear relationship between the fluorescence intensity of the CMO@Q[7] probe and the concentration of Zn²⁺ or Cd²⁺ in the range of 0–3.0 × 10^–5 mol/L and the detection limit for Zn²⁺ and Cd²⁺ was found to be 2.03 × 10^–6 mol/L and 1.89 × 10^–6 mol/L, respectively. The X-ray crystal structure indicated that different coordination fashions were triggered by Zn²⁺ and Cd²⁺ in the CMO@Q[7] complexes, respectively. However, both metal ions coordinated with the carbonyl oxygen of CMO, which was encapsulated in the cavity of Q[7], thus leading to the enhancement of recognition fluorescence emission of CMO.     

A FRET-based fluorescent Zn2+ sensor: 3D ratiometric imaging,flow cytometric tracking and cisplatin-induced Zn2+ fluctuation monitoring

Monitoring labile Zn²⁺ homeostasis is of great importance for the study of physiological functions of Zn²⁺ in biological systems. Here we report a novel ratiometric fluorescent Zn²⁺ sensor, CPBT, which was constructed based on chelation-induced alteration of FRET efficiency. CPBT was readily cell membrane permeable and showed a slight preferential localization in the endoplasmic reticulum. With this sensor, 3D ratiometric Zn²⁺ imaging was first realized in the head of zebra fish larvae via Z-stack mode. CPBT could track labile Zn²⁺ in a large number of cells through ratiometric flow cytometric assay. More interestingly, both ratiometric fluorescence imaging and flow cytometric assay demonstrated that the labile Zn²⁺ level in MCF-7 cells (cisplatin-sensitive) decreased while that in SKOV3 cells (cisplatin-insensitive) increased after cisplatin treatment, indicating that Zn²⁺ may play an important role in cisplatin induced signaling pathways in these cancer cells.

A Zn²⁺ sensor exhibiting 3D ratiometric imaging and flow cytometric ability was constructed based on the FRET mechanism, and cisplatin-induced endogenous labile Zn²⁺ fluctuations were monitored in real time.     

Aggregation-Induced Emission-Based Sensing Platform for Selective Detection of Zn2+: Experimental and Theoretical Investigations

Fluorescent chemosensors with aggregation induced emission enhancement (AIEE) emerge as promising tools in the field of sensing materials. Herein, we report the design, synthesis and applicability of a Schiff base chemosensor 1-(benzo[1,3]dioxol-4-ylmethylene-hydrazonomethyl)-naphthalen-2-ol ( Hbdhn ) of AIE characteristics that exhibits highly effective and selective response towards Zn²⁺. The sensing effect of Hbdhn was evaluated by means of absorption/emission spectra and corresponding underlying photophysical mechanisms were proposed based on extensive quantum-chemical (TD)DFT calculations. The aggregated states in different DMSO/H₂O ratios and in a presence of Zn²⁺ were examined by fluorescence lifetime measurements, dynamic light scattering and scanning electron microscopy studies. The bioimaging abilities of Hbdhn were evaluated for Zn²⁺ in HepG2 cancer cells. The results demonstrate instant, stable in time and reproducible, colorimetric turn-on response with superb selectivity and sensitivity of Hbdhn towards Zn²⁺, based on chelation enhanced fluorescence mechanism. AIEE improves further Hbdhn properties, leading to strong, long-lived fluorescence, with appearance of rod-like particles, in 90 % of water in DMSO and only 10 % of water in DMSO in the presence of Zn²⁺. All these features combined with successful biomaging studies make Hbdhn one of the most promising candidate for practical applications among recently proposed related systems.     

A Zinc(II) Photocage Based on a Decarboxylation Metal Ion Release Mechanism for Investigating Homeostasis and Biological Signaling

Metal ion signaling in biology has been studied extensively with ortho‐nitrobenzyl photocages; however, the low quantum yields and other optical properties are not ideal for these applications. We describe the synthesis and characterization of NTAdeCage, the first member in a new class of Zn²⁺ photocages that utilizes a light‐driven decarboxylation reaction in the metal ion release mechanism. NTAdeCage binds Zn²⁺ with sub‐pM affinity using a modified nitrilotriacetate chelator and exhibits an almost 6 order of magnitude decrease in metal binding affinity upon uncaging. In contrast to other metal ion photocages, NTAdeCage and the corresponding Zn²⁺ complex undergo efficient photolysis with quantum yields approaching 30 %. The ability of NTAdeCage to mediate the uptake of ⁶⁵Zn²⁺ by Xenopus laevis oocytes expressing hZIP4 demonstrates the viability of this photocaging strategy to execute biological assays.     

A far-red emitting probe for unambiguous detection of mobile zinc in acidic vesicles and deep tissue

Imaging mobile zinc in acidic environments remains challenging because most small-molecule optical probes display pH-dependent fluorescence. Here we report a reaction-based sensor that detects mobile zinc unambiguously at low pH. The sensor responds reversibly and with a large dynamic range to exogenously applied Zn²⁺ in lysosomes of HeLa cells, endogenous Zn²⁺ in insulin granules of MIN6 cells, and zinc-rich mossy fiber boutons in hippocampal tissue from mice. This long-wavelength probe is compatible with the green-fluorescent protein, enabling multicolor imaging, and facilitates visualization of mossy fiber boutons at depths of >100 μm, as demonstrated by studies in live tissue employing two-photon microscopy.     

A quinoline-based selective ‘turn on’ chemosensor for zinc(II) via quad-core complex,and its application in live cell imaging

An efficient quinoline-based fluorescent chemosensor (QLNPY) was successfully developed for the detection of zinc ions (Zn²⁺). This novel chemosensor displayed higher sensitivity and selectivity toward Zn²⁺ over other competitive metal ions accompanying with obvious fluorescence enhancement. The QLNPY-Zn²⁺ complex can be further used as a new fluorescent “turn-off” sensor for pyrophosphate (PPi) and sulfur ion (S^2?) via a Zn²⁺ displacement approach. The limits of detection were calculated to be 3.8 × 10^?8 M for Zn²⁺, 3.7 × 10^?7 M for PPi and 4.9 × 10^?7 M for S^2?. The binding mechanism of QLNPY and Zn²⁺ was investigated through NMR, HR-MS analysis and further studied by crystallographic analysis. Additionally, further application of QLNPY for sequential bioimaging of Zn²⁺ and PPi was studied in HepG2 cells, suggesting that the quinoline-based chemosensor possesses great potential applications for the detection of intracellular Zn²⁺ and PPi in vivo.