Metal complexation chemistry used for phosphate and nucleotide determination: an investigation of the Yb3+–pyrocatechol violet sensor

Metal complexation reactions can be used effectively as sensors to measure concentrations of phosphate and phosphate analogs. Recently, a method was described for the detection of phosphate or ATP in aqueous solution based on the displacement by these ligands of pyrocatechol violet (PV) from a putative 2:1 (Yb³⁺)₂PV complex. We have not been able to reproduce this stoichiometry and report this work in order to correct the coordination chemistry upon which sensor applications are based. In our work, colorimetric and spectrophotometric detection of phosphate was confirmed qualitatively (blue PV + Yb³⁺; yellow + P_i); however, the sequence of visual changes on the titration of PV with 2 equiv. of Yb³⁺ and back titration with ATP as described previously could not be reproduced. In contrast to the linear response to P_i that was reported previously, the absorbance response at 443 or 623 nm was found to be sigmoidal using the recommended 2:1 Yb³⁺:PV solution (100 μM:50 μM, pH 7, HEPES). Furthermore, both continuous variation titration and molar ratio analysis (Job plot) experiments are consistent with 1:1, not 2:1, YbPV complex stoichiometry at pH 7 in HEPES buffer, indicating that the deviation from linearity is produced by excess Yb³⁺. Indeed, using a 1:1 Yb³⁺:PV ratio produces a linear response in ΔAbs at 443 or 623 nm on back titration with analyte (phosphate or ATP). In addition, speciation analysis of the Yb–ATP system demonstrates that a 1:1 complex containing Yb³⁺ and ATP predominates in solution at μM metal ion and ATP concentrations. Paramagnetic ¹H NMR spectroscopy directly establishes the formation of Yb³⁺–solute complexes in dilute aqueous solution. The 1:1 YbPV complex can be used for the colorimetric measurement of phosphate and ATP concentrations from ~2 μM. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.

Stable lysozyme-cello-oligosaccharide complexes

Stable complexes that formed when [¹⁴C]cello-oligosaccharides and lysozyme were incubated under various conditions were isolated chromatographically and characterized. Complexation occurred over the pH range of 3–9 but was favored at high pH; the extent of complexation was inversely proportional to temperature. The stoichiometry of the complex was 1:1 sugar-protein, but no radiolabeled peptides could be isolated from a tryptic digest. The minimum requirements for association were native enzyme and a substrate composed of three or more (1→4)-β-D-glucopyranosyl residues. Acceptors and competitive inhibitors of lysozyme inhibited complexation and stimulated dissociation of the complex. Lysozyme did not hydrolyze the cello-oligosaccharides nor did it use them as D-glucosyl donors. Ultracentrifugation, molecular-sieve chromatography, and light-scattering studies indicated that the precipitated complexes were large, heterogeneous aggregates of protein and oligosaccharide which conform to the following equilibrium: n(protein+oligosaccharide)?(protein-oligosaccharide)_n. Polymerization is a cooperative phenomenon.     

Facile synthesis,cytotoxicity and bioimaging of Fe3+ selective fluorescent chemosensor

The designing and development of fluorescent chemosensors have recently been intensively explored for sensitive and specific detection of environmentally and biologically relevant metal ions in aqueous solution and living cells. Herein, we report the photophysical results of alanine substituted rhodamine B derivative 3 having specific binding affinity toward Fe³⁺ with micro molar concentration level. Through fluorescence titration at 599 nm, we were confirmed that ligand 3 exhibited ratiometric fluorescence response with remarkable enhancement in emission intensity by complexation between 3 and Fe³⁺ while it appeared no emission in case of the competitive ions (Sc³⁺, Yb³⁺, In³⁺, Ce³⁺, Sm³⁺, Cr³⁺, Sn²⁺, Pb²⁺, Ni²⁺, Co²⁺, Cu²⁺, Ba²⁺, Ca²⁺, Mg²⁺, Ag⁺, Cs⁺, Cu⁺, K⁺) in aqueous/methanol (60:40, v/v) at neutral pH. However, the fluorescence as well as colorimetric response of ligand–iron complex solution was quenched by addition of KCN which snatches the Fe³⁺ from complex and turn off the sensor confirming the recognition process was reversible. Furthermore, bioimaging studies against L-929 cells (mouse fibroblast cells) and BHK-21 (hamster kidney fibroblast), through confocal fluorescence microscopic experiment indicated that ligand showed good permeability and minimum toxicity against the tested cell lines.     

Zn-polyphenol chelation: complexes with quercetin, (+)-catechin, and derivatives: I optical and NMR studies

The complexation of Zn²⁺ by quercetin, (+)-catechin and several derivatives has been investigated in the aim to determine the stoichiometry and the stereospecificity and its influence on their antioxidant properties. These studies have been conducted under anaerobic conditions in hydro-organic solvents buffered at pH 7 using UV-Vis and ¹H NMR spectroscopies. From the evolution of the spectra of the flavonoids with the concentration of ZnAc their stoichiometry and the coordination sites were determined.     

A Schiff‐based sensor with turn‐on fluorescence for selective detection of Hg2+

A simple Schiff‐base colorimetric receptor 1 was prepared. It exhibits an ‘off–on‐type’ mode with high sensitivity in the presence of Hg²⁺. The change in color is very easily observed by the naked eye in the presence of Hg²⁺, whereas other metal cations do not induce such a change. A Job plot indicated a 1 : 1 complexation stoichiometry between receptor 1 and Hg²⁺. The association constant for 1–Hg²⁺ in Tetrahydrofuran (THF) was determined to be 1.3 × 10⁹ M^‐1 using a Hill plot. Copyright © 2013 John Wiley & Sons, Ltd.     

Effect of Bi3+ addition on the upconversion luminescence properties of NaYbF4:Er

In this study, Bi³⁺ incorporation in NaYbF₄:Er lattice and its influence on upconversion luminescence properties have been investigated in detail using techniques such as temperature‐dependent luminescence, Fourier transform infrared spectroscopy and X‐ray diffraction (XRD). The study was carried out to develop phosphors with improved upconversion luminescence. From photoluminescence and lifetime measurements it is inferred that luminescence intensity from NaYbF₄:Er increases with Bi³⁺ addition. The sample containing 50 at.% Bi³⁺ ions exhibited optimum upconversion luminescence. Increased distance between Yb³⁺–Yb³⁺ and Er³⁺–Er³⁺ due to Bi³⁺ incorporation into the lattice and associated decrease in the extent of dipolar interaction/self‐quenching are responsible for increase in lifetime values and luminescence intensities from Er³⁺ ions. Incorporation of Bi³⁺ into NaYbF₄:Er lattice reduced self‐quenching among Yb³⁺–Yb³⁺ions and this facilitated energy transfer from Yb³⁺ to Er³⁺. This situation also explains decrease in the extent of temperature‐assisted quenching of emission from thermally coupled ²H_11/2 and ⁴S_3/2 levels of Er³⁺. Based on Rietveld refinement of XRD patterns it was confirmed that a maximum of 10 at.% of Bi³⁺added was incorporated into the NaYbF₄:Er lattice and the remaining complex co‐exists as a BiOF phase. These results are of significant interest in the area of development of phosphors based on Yb³⁺–Er³⁺ upconversion luminescence.     

Photoluminescence properties of rare‐earth‐doped (Er3+,Yb3+) Y2O3 nanophosphors by a combustion synthesis method

In this work, we report the synthesis of Y₂O₃:Er³⁺, Y₂O₃:Yb³⁺ and Y₂O₃:Er³⁺,Yb³⁺ nanophosphors by the combustion synthesis method using urea as fuel. The doping agents were incorporated in the form of erbium nitrate and ytterbium nitrate. X‐Ray diffraction (XRD) patterns revealed that the synthesized particles have a body‐centered cubic structure with space group Ia‐3. The photoluminescence (PL) properties were investigated after UV and infrared irradiation at room temperature. A strong characteristic emission of Er³⁺ and Yb³⁺ ions was identified, and the influence of doping concentration on the PL properties was systematically studied. Energy transfer from Yb³⁺ to Er³⁺ ions was observed in Y₂O₃ nanophosphors. The obtained result may be useful in potential applications such as bioimaging. Copyright © 2015 John Wiley & Sons, Ltd.     

Formation of ion-translocating oligomers by nigericin

Summary At pH 4.0, >10^–7 m nigericin was found capable of conducting net charge transfer across bimolecular lecithin membranes, with a stoichiometry of three uncharged ionophore moieties per cation. At neutral or alkaline pH, nigericin catalyzed the transfer of net charge through dimer forms. In agreement with these results, quantitative analysis of nigericin-potassium complexes formed at pH 4.0 showed a 31 ratio, and a 21 ratio at neutral or alkaline pH. A 11 stoichiometry was observed when the ionophore complex was not transferred from methanol-water to chloroform. Moreover,¹H-NMR spectra of nigericin-cation complexes formed at pH 4.0, displayed clear-cut chemical shift variations different to those observed at neutral or alkaline pH. Thus, it is apparent that acid pH causes a transition from dimeric to trimeric forms of nigericin-cation complexes. The membrane conductance increased up to ten times when negatively charged phosphatidyl glycerol was used, while the conductance decreased in positively charged cetylpyridinium containing membranes at pH 4.0. These results suggest that the nigericin-K⁺ oligomeric complex is positively charged. In this respect, pK _a values around 8.0 were obtained for the nigericin carboxylate group in media of different dielectric constant, indicating that this chemical group is undissociated under these conditions. Moreover, the values for the complex formation constants as well as the G values calculated for the dimers and trimers indicated that such ionophore cation oligomeric complexes are thermodynamically stable.     

A Study and Optimization of Complexation of Cobalt Ions with Dihydroquercetin in Aqueous Solutions

This work continues a series of studies devoted to complex formation of ions of biogenic metals with the flavonoid dihydroquercetin (DHQ). The interaction of Со²⁺ ions with DHQ in aqueous solutions has been investigated. It has been found that, at different pH of a solution, complex compounds (CC) with different stoichiometry are formed; a variation of the pH value of a solution from 6.0 to 7.0 results in the formation of compounds (1)–(3) with the metal : flavonoid ligand ratio (Met : L) from 1 : 2 at рН 6.0 (1), through 2 : 3 at pH 6.4–6.7 (2), to 1 : 1 at рН 6.8–7.0 (3). By using the thermogravimetric method and the data of the elemental analysis, the most probable composition of the compounds with the determination of the amount of bound water has been proposed: [CoL₂(H₂O)₄] for (1), [Co₂L₃(ОН)(H₂O)₄] for (2), and [CoL(ОН)(H₂O)₂] for (3). Conditions for the optimization of product yield in the complexation reaction of Со²⁺ ions with DHQ in an aqueous solution have been determined for compound (2): the рН value of solution 6.7; the reaction time 15 min; the temperature of the reaction solution 90°С; the molar ratio of the initial reagents DHQ : Со²⁺ 1 : 1.5; the initial concentration of DHQ 0.020 mol/L and that of Со²⁺ 0.030 mol/L; and the use of CoSO₄ ? 7H₂O as a source of cobalt ions. The yield of the product is 81.8%.

     

Preparation and up‐conversion luminescence properties of LaOBr:Yb3+/Er3+ nanofibers via electrospinning

LaOBr:Yb³⁺/Er³⁺ nanofibers were synthesized for the first time by calcinating electrospun PVP/[La(NO₃)₃ + Er(NO₃)₃ + Yb(NO₃)₃ + NH₄Br] composites. The morphology and properties of the final products were investigated in detail using scanning electron microscopy (SEM), energy dispersion spectroscopy (EDS), X‐ray diffractometry (XRD) and fluorescence spectroscopy. The results indicate that LaOBr:Yb³⁺/Er³⁺ nanofibers are tetragonal in structure with a space group of P4/nmm. The diameter of LaOBr:Yb³⁺/Er³⁺ nanofibers is ~ 147 nm. Under the excitation of a 980‐nm diode laser, LaOBr:Yb³⁺/Er³⁺ nanofibers emit strong green and red up‐conversion emission centering at 519, 541 and 667 nm, ascribed to the ²H_11/2, ⁴S_3/2 → ⁴I_15/2 and ⁴ F_9/2 → ⁴I_15/2 energy‐level transitions of Er³⁺ ions, respectively. The up‐conversion luminescent mechanism of LaOBr:Yb³⁺/Er³⁺ nanofibers is advanced. Moreover, near‐infrared emission of LaOBr:Yb³⁺/Er³⁺ nanofibers is obtained under the excitation of a 532‐nm laser. The formation mechanism of LaOBr:Yb³⁺/Er³⁺ nanofibers is proposed. LaOBr:Yb³⁺/Er³⁺ nanofibers could be important up‐conversion luminescent materials. Copyright © 2014 John Wiley & Sons, Ltd.     

Albumin-binding PARACEST agents

Lanthanide complexes (Eu³⁺, Gd³⁺ and Yb³⁺) of two different 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid tetraamide derivatives containing two (2) and four (3) O-benzyl-l-serine amide substituents were synthesized and their chemical exchange saturation transfer (CEST) and relaxometric properties were examined in the presence and absence of human serum albumin (HSA). Both Eu2 and Eu3 display a significant CEST effect from a single slowly exchanging Eu³⁺-bound water molecule, making these PARACEST complexes potentially useful as vascular MRI agents. Yb2 also showed a detectable CEST effect from both the Yb³⁺-bound water protons and the exchangeable NH amide protons, making it potentially useful as a vascular pH sensor. Fluorescence displacement studies using reporter molecules indicate that both Gd2 and Gd3 displace dansylsarcosine from site II of HSA with inhibition constants of 32 and 96 μM, respectively, but neither complex significantly displaces warfarin from site I. Water proton relaxation enhancements of 135 and 171% were observed upon binding of Gd2 and Gd3 to HSA, respectively, at 298 K and pH 7.4.     

Structural, spectroscopic and semiempirical characterisation of the calcium cation complexes with 14-membered macrocyclic ligand of cyclic oxaalkyl diamide of o-phthalic acid

Cyclic oxaalkyl diamide of o-phthalic acid (CPhDA) has been obtained and its ability to form complexes with calcium cation has been studied by X-ray, ESI MS, ¹H and ¹³C NMR, FT-IR and PM5 semiempirical methods. The ESI MS measurements have proved that in gas phase the 3:1, 2:1 and 1:1 CPhDA-Ca²⁺ as well as 3:1 and 2:1 CPhDA-Ca(ClO₄)⁺ complexes are formed. In the solid state a 3:1 complex between CPhDA and calcium perchlorate of the CPhDA-Ca(ClO₄)₂-H₂O (3:1:0.5) stoichiometry crystallises as hemihydrate in centrosymmetric space group (R-3) of the rhombohedral system. In crystal, the central Ca²⁺ cation is coordinated by the three CPhDA ligands via the carbonyl oxygen atoms in a distorted trigonal antiprism. The cationic [Ca(CPhDA)₃]²⁺ complex exhibits a threefold symmetry. Two [Ca(CPhDA)₃]²⁺ cations related by an inversion centre interact with oxygen atom of water molecule that statistically occupies two positions around the inversion centre along the Ca···Ca axis. The FT-IR spectra show the characteristic changes in the frequencies of the amide I and amide II bands upon complexation. The structures of the CPhDA and its complexes with calcium cation are visualised using DFT and PM5 methods and discussed in detail.     

Composition and metal ion complexation behavour of humic fractions derived from corn tissue

Decomposition of fresh plant residues produces humic fractions with different molecular size and composition. It was hypothesized that the functional group-type and content of humic fractions depended on molecular size, which was expected to influence heavy-metal complexation behavior. In this study, corn (Zea maysL.) stalks and leaves were collected from the field and decomposed for an 8-month period to produce humic substances which were separated into three water soluble fractions, HF1, HF2 and HF3, from highest to lowest relative molecular size. Functional group determination showed that total, carboxylic and phenolic OH acidity increased as relative molecular size of humic fractions decreased. Furthermore, C/O ratios decreased, whereas N/C and H/C ratios remained relatively unaffected as relative molecular size of humic fractions decreased. Formation of Ca²⁺, Cd²⁺ and Cu²⁺ -humic fraction complexes and how these complexes were affected by pH and relative (humic fraction) molecular size were studied using potentiometric titration. Metal-humic complexes exhibited at least two types of sites with respect to Ca²⁺, Cd²⁺ and Cu²⁺ complexation. Relative molecular size had a large significant influence on total metal-ion complexation, but it had a relatively small influence on complex stability at low levels of metal-ion complexation. Strength of metal-ion humic complexes followed the order Cu²⁺ > Cd²⁺ > Ca²⁺ and was affected by pH, especially for low affinity sites. Carboxylic and phenolic OH groups were most likely involved in complex formation. Magnitude of the metal-humic formation constants at the lowest equilibrium metal-ion concentration, under the various pH values tested, varied from 5.39 to 5.90 for Ca²⁺, from 5.36 to 6.01 for Cd²⁺ and from 6.93 to 7.71 for Cu²⁺. Furthermore, the formation constants appeared to be positively influenced by decreasing molecular size of water-soluble humic fraction, and increasing pH. These results inferred that soil management practices causing build-up of humic substances would affect mobility and bioavailability of metal-ions.     

Role of Mg2+ in Chromomycin A3 – DNA Interaction: A Molecular Modeling Study

Chromomycin A₃ (CHR) is an antitumor antibiotic that inhibits macromolecular biosynthesis by reversibly binding to double stranded DNA via the minor groove, with GC-base specificity. At and above physiological pH when CHR is anionic, interaction of CHR with DNA requires the presence of divalent metal ions like Mg²⁺. However, at acidic pHthe molecule is neutral and it binds DNA even in absence of Mg²⁺. Molecular dynamics simulation studies at 300K of neutral CHR and 1:1 CHR:Mg²⁺ complexes formed at pH 5.2 and 8.0 show that hydrophobicity of CHR:Mg²⁺ complex formed with the neutral drug is greater than that of the two other species. Interactions of CHR with DNA in presence and absence of Mg²⁺ have been studied by simulated annealing to understand the role of Mg²⁺ in the DNA binding potential of CHR. This shows that the antibiotic has the structural potential to bind to DNA even in the absence of metal ion. Evaluation of the direct interaction energy between the ligand and DNA does not explain the observed GC-base specificity of the antibiotic. When energy contributions from structural alteration of the interacting ligand and DNA as a sequel to complex formation are taken into account, atrue picture of the theoretical binding propensity emerges. This implies that DNA and/or the ligand undergo significant structural alterations during the process of association, particularly in presence of Mg²⁺. Accessible surface area calculations give idea about the entropy contribution to the binding free energy which is found to be different depending upon the presence and absence of Mg²⁺.     

Halogenated nucleic acids: effects of 5-fluorouracil on the conformation and properties of a polyribonucleotide and its constituents

The conformational properties of 5-fluorouracil derivatives are compared to uracil derivatives. FUrd, 5′-FUMP, and poly(FU) are studied as a function of pH and temperature by ¹⁹F- and ¹H-nmr spectroscopy, and the corresponding uracil derivatives by ¹H-nmr spectroscopy. FUrd exhibits no significant conformational changes with solution pH (5–10). In contrast, at low pH (6–7) 5′-FUMP and 5′-UMP show similar conformational features, while at high pH (9) 5′-FUMP shows significant conformational alterations. Also, poly(U) and poly(FU) are conformationally similar at low pH, but increasing pH induces changes in poly(FU). These changes are observed in the backbone [γ(C4′-C5′)], furanose, and furanose-base conformations. The apparent pK_a of N3-H ionization of the FUra base is determined by ¹H- and ¹⁹F-nmr to range from 7.5 to 8.2 [FUrd < 5′-FUMP < 5′-FUDP < poly(FU)]. These observations are interpreted as a result of electrostatic interactions generated between the ionized phosphate group and the negatively charged base moiety as the pH is raised. The interaction properties of poly(FU) with ApA are studied by ¹H- and ¹⁹F-nmr spectroscopy, and these properties compared to those published for poly(U). Poly(FU) forms a complex with ApA inducing upfield ¹H-shifts in both components, and downfield ¹⁹F- shifts in poly(FU). The base stoichiometry of the complex for poly(U)·ApA is 2U:1A at various U/A ratios. In contrast, the base stoichiometry of the poly(FU)·ApA complex appears to be dependent on the FU/A ratio. At high FU/A ratio, the complex is 2FU:1A, and as the FU/A ratio approaches unity the complex becomes 1FU:1A.     

The effect of elevated temperature on the complexation of am3+ with chloride

The temperature dependence of the formation of the 1:1 Am³⁺-Cl^- solution complex is reported. Stability constants were determined as a function of temperature using a solvent extraction technique. The stability constant for the formation of the AmCl²⁺ complex is measurably higher at 50‡C compared to values determined under ambient conditions. From these studies, we have estimated the enthalpy of complexation in this temperature range. A small, positive enthalpy was observed. The results are discussed in the context of previous studies.     

Importance of hydrogen ions and aluminium in regulating the structure and function of stream ecosystems: an experimental test

1. Experiments simulating spring acidic snowmelt episodes were conducted to determine the effects of short-term inputs of H⁺ and Al on the chemistry and biology of a poorly buffered mountain stream. HC1 and A1C1₃ were added in separate experiments to first- to third-order reaches of a New Hampshire stream. 2. Cation exchange and Alⁿ⁺ dissolution reactions neutralized experimentally added H ⁺, whereas groundwater dilution was insignificant. Mobilized Ca⁺⁺, Mg⁺⁺ and Alⁿ⁺ concentrations progressively increased from third- to first-order reaches during HC1 additions. Mobilization of Ca⁺⁺ and Mg⁺⁺ was greater during A1C1₃ than HC1 addition. 3. Total phosphorus was mobilized from stream sediments during both HCI and A1C1₃ addition. Dissolved organic carbon (DOC) decreased during A1C1₃ addition in the second-order but not in the third-order reach. DOC concentration decreased during HCI addition only when Al^{n +} mobilized from the stream bottom was >0.28 mg AI 1¹. 4. Production of foam at the water surface during AlCl₃ addition to a second-order and HCl addition to a first-order reach indicated a reduction in surface tension of the streamwater and may be related to complexation reactions between Al and DOC at low pH (4–5). 5. Mayfly nymphs and blackfly and chironomid larvae drifted at greater rates from HCl- and AlCl₃treated sections of first- and second-order streams than from corresponding reference areas. 6. When stream pH was lowered to 5.25–5.5 by HCI alone (15 μg monomeric inorganic Al l^?1), the behaviour of aquatic invertebrates did not change, but pH reduced to the same range during Al additions (280μg Al 1^?1) did affect it. Therefore, fluctuating aluminium concentrations in low-order streams at a pH range of 4.5–5.5 may alter the biology and geochemistry of poorly buffered waters.     

A colorimetric sensor for the selective detection of fluoride ions

A colorimetric receptor L was prepared. Receptor L can selectively sense F^? based on distinct color changes among a series of ions. It can selectively sense F^? through an intramolecular hydrogen bond interaction. A Job plot indicated a 1:1 complexation stoichiometry between receptor L and F^?. The association constant for L –F^? in CH₃CN was determined as 9.70 × 10⁴ M^?1 using a Stern–Volmer plot.     

Formation of transition metal-doxorubicin complexes inside liposomes

Doxorubicin complexation with the transition metal manganese (Mn²⁺) has been characterized, differentiating between the formation of a doxorubicin-metal complex and doxorubicin fibrous-bundle aggregates typically generated following ion gradient-based loading procedures that rely on liposome encapsulated citrate or sulfate salts. The physical and chemical characteristics of the encapsulated drug were assessed using cryo-electron microscopy, circular dichroism (CD) and absorbance spectrophotometric analysis. In addition, in vitro and in vivo drug loading and release characteristics of the liposomal formulations were investigated. Finally, the internal pH after drug loading was measured with the aim of linking formation of the Mn²⁺ complex to the presence or absence of a transmembrane pH gradient. Doxorubicin was encapsulated into either 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC)/cholesterol (Chol) or 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC)/Chol liposomes, where the entrapped salts were citrate, MnSO₄ or MnCl₂. In response to a pH gradient or a Mn²⁺ ion gradient, doxorubicin accumulated inside to achieve a drug-to-lipid ratio of approximately 0.2:1 (wt/wt). Absorbance and CD spectra of doxorubicin in the presence of Mn²⁺ suggested that there are two distinct structures captured within the liposomes. In the absence of added ionophore A23187, drug loading is initiated on the basis of an established pH gradient; however, efficient drug uptake is not dependent on maintenance of the pH gradient. Drug release from DMPC/Chol is comparable regardless of whether doxorubicin is entrapped as a citrate-based aggregate or a Mn²⁺ complex. However, in vivo drug release from DSPC/Chol liposomes indicate less than 5% or greater than 50% drug loss over a 24-h time course when the drug was encapsulated as an aggregate or a Mn²⁺ complex, respectively. These studies define a method for entrapping drugs possessing coordination sites capable of complexing transition metals and suggest that drug release is dependent on lipid composition, internal pH, as well as the nature of the crystalline precipitate, which forms following encapsulation.     

A zinc (II) complex comprising an aminoethyl-nitropyridine-derived N,N,O-donor Schiff base ligand serves as an efficient ON–OFF probe for Cu(II)

A new fluorescent zinc (II) complex-based probe 1 encompassing a Schiff's base (E)-2-methoxy-6-((2-[5-nitropyridin-2-ylamino]ethylimino)methyl)phenol ( HL ) was designed, synthesized, and used for the highly selective detection of Cu²⁺. Ligand HL and complex 1 were characterized using various spectroscopic techniques such as ¹H, ¹³C-NMR, and FTIR spectroscopy, high-resolution mass spectronomy (HRMS), UV/visible light spectroscopy, and fluorescence studies. Ligand HL did not exhibit any considerable change in fluorescence in the presence of various cations. Notably, its Zn(II) complex 1 exhibited highly selective ‘TURN-OFF’ fluorescence signalling towards Cu²⁺ that remained uninterrupted with competing analytes. Probe 1 interacted with Cu²⁺ in 1:2 (1:Cu²⁺) stoichiometry as estimated through a Job's plot. Moreover, the selectivity of 1 was further confirmed through the interaction of the 1 + Cu²⁺ complex with some possible interfering metal ions inducing an insignificant response. Additionally, the association and quenching constant were determined to be 3.30 × 10⁴ M⁻¹ and 0.21 × 10⁵ M⁻¹ through the Benesi–Hildebrand method and Stern–Volmer plot, respectively.