Synthesis and luminescence properties of lanthanide complexes with a new tripodal ligand featuring N‐thenylsalicylamide arms

To explore the relationship between the structure of the ligands and the luminescent properties of the lanthanide complexes, luminescent lanthanide complexes of a new tripodal ligand, featuring N‐thenylsalicylamide arms, were synthesized and characterized by elemental analysis, IR and TGA measurements. Photophysical properties of the complexes were studied by means of UV ? visible absorption and steady‐state luminescence spectroscopy. The results of UV ? vis spectra indicate that metal binding does not disturb the electronic structure of the ligand. Excited‐state luminescence lifetimes and quantum yields of the complexes were determined. The photoluminescence analysis suggested that there is an efficient ligand ? Ln(III) energy transfer for the Tb(III) complex, and the ligand is an efficient 'antenna' for Tb(III). From a more general perspective, the results demonstrated the potential application of the lanthanide complex as luminescent materials in material chemistry. Copyright © 2012 John Wiley & Sons, Ltd.     

Lanthanide chelate complexes of salicyloyl hydrazide-salicylaldehyde schiff base (SHSASB) and anthranilic acid-salicylaldehyde schiff base (AASASB)

New lanthanide complexes of salicylaldehyde- Schiff bases with salicyloyl hydrazide and anthranilic acid, were synthesized by a novel method consisting of refluxing the mixtures of Schiff base ligands and lanthanide trichloroacetate in acetone. Solid complexes of formulae Ln(SHSASB)₃·2H₂O and Ln₂(AASASB)₃·2H₂O where Ln = La---Yb and Y, were isolated. Pronto NMR and IR spectra for the complexes reveal the bidentate binding of both the Schiff base ligands to the lanthanide ion. Electronic spectra along with the conductance data for the complexes indicate a coordination number of six for the lanthanide ion in the complexes of both the Schiff bases.     

Increasing the luminescence of lanthanide complexes.

This review compares the chemical and physical properties of lanthanide ion complexes and of other narrow-emitting species that can be used as labels for cytometry. A series of luminescent lanthanide ion macrocyclic complexes, Quantum Dyes, which do not release or exchange their central lanthanide ion, do accept energy transfer from ligands, and are capable of covalent binding to macromolecules, including proteins and nucleic acids, is described and their properties are discussed.Two methods are described for increasing the luminescence intensity of lanthanide ion complexes, which intrinsically is not as high as that of standard fluorophores or quantum dots. One method consists of adding a complex of a second lanthanide ion in a micellar solution (columinescence); the other method produces dry preparations by evaporation of a homogeneous solution containing an added complex of a second lanthanide ion or an excess of an unbound antenna ligand. Both methods involve the Resonance Energy Transfer Enhanced Luminescence, RETEL, effect as the mechanism for the luminescence enhancement.     

EPR and O2.- scavenger activity: Cu(II)-peptide complexes as superoxide dismutase models.

Several copper(II) complexes with aminoacids and peptides are known to show superoxide dismutase (SOD)-like activity. EPR spectroscopy has proved to be a useful tool for studying the complex equilibria of the copper(II) ion and various ligands of biological importance in solution. In the present work, a variety of copper(II) complexes with di-, tri- and tetra-peptides containing only glycine residues (GG, GGG and GGGG) and others containing a histidyl residue in different positions (HGG, GHG, GGH and GGHG) have been investigated. EPR parameters obtained by extensive use of computer simulation of spectra lead to reliable spin Hamiltonian EPR parameters at both room temperature and in frozen solution. The molecular orbital coefficients computed from the anisotropic EPR data and the d-d electronic energies are used to characterize different arrangements of the complexes. Estimation of the scavenger activity of the complexes due to the particular environment created by the ligands around copper is discussed in the frame of the structure-activity relationship.     

Near-infrared Yb(3+) vibronic sideband spectroscopy: application to Ca(2+)-binding proteins.

We have used near-infrared (NIR) vibronic fluorescence spectroscopy to study the vibrational structure of ligands associated with model complexes of the lanthanide Yb(3+). This technique exploits the similar binding properties of the lanthanide Yb(3+) to probe Ca(2+)-binding sites in proteins. The (NIR) fluorescence of complexed Yb(3+) exhibits, in addition to main 0-0 (2F5/2----2F7/2) electronic transition of Yb(3+), weak vibronic sidebands which provide infrared-like, local vibrational spectra of the chelates (inner sphere ligands) of Yb(3+). A similar approach has been used for the lanthanide Gd(3+) (MacGregor, R.B., Jr (1989) Arch. Biochem. Biophys. 274, 312-316) which fluoresces in the UV and which is usually complicated by amino-acid residues fluorescing in the same spectral region. In this same spectral region, other complications in studying photosynthetic membranes occur in the form of the excitation wavelength being actinic, promoting photodegradation of the membranes, as well as the reabsorption of Gd(3+) fluorescence. NIR excitation and fluorescence detection of Yb(3+) avoid these problems when studying photosynthetic membranes. A preliminary study has been conducted here on rat muscle parvalbumin.     

Synthesis,characterization, and binding assessment with human serum albumin of three bipyridine lanthanide(III) complexes

In this work, the terbium(III), dysprosium(III), and ytterbium(III) complexes containing 2, 2′-bipyridine (bpy) ligand have been synthesized and characterized using CHN elemental analysis, FT-IR, UV–Vis and ¹H-NMR techniques and their binding behavior with human serum albumin (HSA) was studied by UV–Vis, fluorescence and molecular docking examinations. The experimental data indicated that all three lanthanide complexes have high binding affinity to HSA with effective quenching of HSA fluorescence via static mechanism. The binding parameters, the type of interaction, the value of resonance energy transfer, and the binding distance between complexes and HSA were estimated from the analysis of fluorescence measurements and Förster theory. The thermodynamic parameters suggested that van der Waals interactions and hydrogen bonds play an important role in the binding mechanism. While, the energy transfer from HSA molecules to all these complexes occurs with high probability, the order of binding constants (BpyTb > BpyDy > BpyYb) represents the importance of radius of Ln³⁺ ion in the complex-HSA interaction. The results of molecular docking calculation and competitive experiments assessed site 3 of HSA, located in subdomain IB, as the most probable binding site for these ligands and also indicated the microenvironment residues around the bound mentioned complexes. The computational results kept in good agreement with experimental data.     

Self-consistent reaction field computation of the reactive characteristics of DNA bases in water

The effect of the solvent in the molecular properties of DNA bases has been explored by using a self-consistent reaction field (SCRF) method based on the AM1 (Austin Model 1) Hamiltonian and a modified version of the high level Miertus–Scrocco–Tomasi (MST) algorithm. MST/AM1 estimates of free energies of hydration compare qualitatively well with the available experimental data, as well as with the results obtained from molecular dynamic simulations. Furthermore, the changes in the dipole predicted by the MST/AM1 method are in good agreement with Monte Carlo/quantum mechanical data, as well as with AM1-SM2 (Soluation Model 2) estimates. AM1/MST calculations of Mulliken, and electrostatic charges, dipoles, molecular electrostatic potentials and molecular interaction potentials in both vacuum and solution allowed us to quantify the effect of the water on the reactive characteristics of the DNA bases. This effect is large and complex, and cannot be neglected in theoretical calculations where an accurate representation of the DNA bases is needed. The possibility of including the polarization effect of the water into force-field simulations of DNA structures is discussed. © 1993 John Wiley & Sons, Inc.     

Lanthanide octaethylprophyrins: preparation, association, and interaction with axial ligands

The synthesis of virtually all the lanthanide octaethylporphyrin complexes have been achieved by heating appropriate anhydrous lanthanide halide and octaethylporphyrin in imidazole melt at 210 degrees C for two hours. The lighter lanthanide porphyrin complexes are very susceptible to hydrolysis, the middle lanthanide porphyrin complexes are moderately stable, and the heavier lanthanide porphyrin complexes are relatively more stable to hydrolysis. Two out of four lanthanide porphyrin complexes studied in detail, namely ytterbium and lutetium octaethylporphyrins, aggregate in benzene and the Soret bands in their absorption spectra are about 6 nm shifted to higher energies upon a hundred-fold increase in their concentrations. The aggregations of these lanthanide porphyrin complexes in non-coordinating solvents have been further verified by 1H NMR spectral studies. This spectral behavior can be interpreted qualitatively in terms of the model of the molecular exciton interactions with stacking of at least two prophyrins. A dimeric structure of these lanthanide porphyrin complexes has been proposed on the basis of geometrical considerations. On the contrary, the europium and gadolinium octaethylporphyrins associate very weakly in benzene in the concentration range studied. All four lanthanide porphyrin complexes interact with pyridine and piperidine, and the Soret bands in their absorption spectra are about 8 nm shifted to low energies as compared with their values in pure benzene.     

Synthesis and luminescence properties of pyrazolone derivatives and their terbium complexes

Seven novel pyrazolone derivatives were synthesized and characterized by ¹H NMR and ¹³C NMR spectra, mass spectra, infrared spectra and elemental analysis. Their terbium complexes were prepared and characterized by elemental analysis, EDTA titrimetric analysis, UV/vis spectra, infrared spectra and molar conductivity, as well as thermal analysis. The fluorescence properties and fluorescence quantum yields of the complexes were investigated at room temperature. The results indicated that pyrazolone derivatives had good energy‐transfer efficiency for the terbium ion. All the terbium complexes emitted green fluorescence characteristic of terbium ions, possessed strong fluorescence intensity, and showed relatively high fluorescence quantum yields. Cyclic voltammograms of the terbium complexes were studied and the highest occupied molecular orbital (HOMO) and lowest occupied molecular orbital (LUMO) energy levels of these complexes were estimated. Copyright © 2014 John Wiley & Sons, Ltd.     

1H NMR and UV-Vis spectroscopic characterization of sulfonamide complexes of nickel(II)-carbonic anhydrase. Resonance assignments based on NOE effects.

The binding of acetazolamide, p-fluorobenzensulfonamide, p-toluenesulfonamide, and sulfanilamide to nickel(II)-substituted carbonic anhydrase II has been studied by 1H NMR and electronic absorption spectroscopies. These inhibitors bind to the metal ion forming 1:1 complexes and their affinity constants were determined. The 1H NMR spectra of the formed complexes show a number of isotropically shifted signals corresponding to the histidine ligands. The complexes with benzene-sulfonamides gave rise to very similar 1H NMR spectra. The NMR data suggest that these aromatic sulfonamides bind to the metal ion altering its coordination sphere. In addition, from the temperature dependence of 1H NMR spectra of the p-fluorobenzenesulfonamide adduct, a conformational change is suggested. The T1 values of the meta-like protons of the coordinated histidines have been measured and resonance assignments based on NOE experiments were performed.     

Chemiluminescence and energy transfer mechanism of lanthanide ions in different media based on peroxomonosulfate system

The chemiluminescence (CL) phenomena of lanthanide (Ln) ions and their coordinate complexes in peroxomonosulfate system and the energy transfer mechanism during the process were investigated in this work. A strong and sharp CL signal was yielded when the Eu(III) or Tb(III) solution was added to the peroxymonosulfate solution. The CL intensity was greatly enhanced by 2,6‐pyridinedicarboxylic acid (DPA) ligand [maximum enhancement reached when Ln(III):DPA was 1:1] and hexadecyltrimethylammonium chloride micelles. The degree of enhancement of DPA and micelles on Ln(III) CL was related to the fluorescence lifetimes of Ln(III) in different media. According to the ESR spin‐trapping experiments of 2,2,6,6‐tetramethyl‐4‐piperidone and the specific quenching experiments of 1,4‐diazabicyclo[2.2.2]octane and sodium azide, singlet oxygen was generated though the Ln(III) ion‐catalyzed decomposition of peroxymonosulfate. From the comparisons of the fluorescence and CL spectra, lanthanide ions were the luminescence emitter and the ligand DPA absorbed the energy from singlet oxygen and transferred it to Ln(III) ions in the coordinate complexes. Micelles can enhance the CL intensity by improving intermolecular energy transfer efficiencies, removing the quenching effect of water and prolonging the lifetime of singlet oxygen. Copyright © 2009 John Wiley & Sons, Ltd.     

Oxovanadium(IV) and dioxouranium(VI) complexes with thiocarbohydrazones

Oxovanadium(IV) and dioxouranium(VI) complexes with thiocarbohydrazones have been prepared in an ethanolic medium and characterised by elemental analysis and molecular weight determination. They have 1:1 stoichiometry. The IR observations suggest that the ligands have coordinated through azomethine nitrogen atoms and reacted through hydroxy groups. The v(MN) and v(MO) vibrations have been assigned. The PMR spectral information supports the IR inference. The oxovanadium(IV) complexes show magnetic moments in the range of 1.74–1.94 B.M. The electronic spectra have been interpreted in the light of the BG model. Various NSH parameters have been calculated. The ESR spectra also render support for the spectral information. On the basis of this information it is suggested that oxovanadium(IV) complexes exhibit coordination number five and dioxouranium coordination number six.     

Adducts of europium β-diketonates with nitrogen p,p′-disubstituted bipyridine and phenanthroline ligands: Synthesis, structural characterization, and luminescence studies

Europium complexes featuring fluorinated β-diketonate ligands [thenoyltrifluoroacetone (tta), 4,4,4-trifluoro-1-phenyl-1,3-butanedione (btfac), and 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate (hfac)] and nitrogen p,p′-disubstituted bipyridine and phenanthroline ligands [4,4′-dimethoxy-2,2′-bipyridine (dmbipy) and 4,7-dimethyl-1,10-phenanthroline (dmphen)] were synthesized. Their structures were determined by single crystal X-ray diffraction. Octacoordinate complexes were obtained using trifluorinated tta and btfac, while nonacoordinated complexes were produced using hexafluorinated hfac. The differences in coordination number and bond lengths of these complexes are rationalized in terms of the electronic and steric features of the ligands. UV excitation of the complexes led to red luminescence characteristic of trivalent europium ion. The high overall quantum yields observed for the europium complexes bearing hfac and dmbipy or dmphen ligands are rationalized in terms of the relatively high ligand-to-metal energy transfer efficiencies.     

Structure and NIR luminescence of lanthanide(III) complexes with an organic tridentate ligand

Some lanthanide (Ln) complexes (Ln = Er, Nd, Yb) with an organic ligand, 6-diphenylamine carbonyl 2-pyridine carboxylic acid (HDPAP), have been synthesized. The crystal structure and near infrared luminescence of these complexes (Er-DPAP, Nd-DPAP and Yb-DPAP) have been investigated. The results showed that the lanthanide complexes have electroneutral structures and the near infrared (NIR) emission exhibits characteristic narrow emission of the lanthanide ions. The energy transfer mechanisms in the lanthanide complexes were discussed.     

Electronic excited states and excitation transfer kinetics in the Fenna-Matthews-Olson protein of the photosynthetic bacterium Prosthecochloris aestuarii at low temperatures

The molecular structure-function relationship of the Fenna-Matthews-Olson light-harvesting complex of the photosynthetic green bacterium Prosthecochloris aestuarii has been investigated. It has been assumed that the electronic excited states responsible for the function (transfer of electronic excitation energy) result from the dipole-dipole interactions between the bacteriochlorophyll molecules bound to the polypeptide chain of the complex at a specific three-dimensional geometry. The molecular structure-electronic excited states relationship has been addressed on the basis of simultaneous simulations of several spectroscopic observations. Current electronic excited state models for the Fenna-Matthews-Olson complex have generally been based on obtaining an optimal match between the information contents of the optical steady-state spectra and the bacteriochlorophyll organization. Recent kinetic and spectral information gathered from ultrafast time-resolved measurements have not yet been used effectively for further refinement of the excited state models and for quantification of the relation between the excited states and the energy transfer processes. In this study, we have searched for a model that not only can explain the key features of several steady-state spectra but also the temporal and spectral evolution observed in a recent absorption difference experiment and we have discussed the implications of this model for equilibration of the electronic excitation energy in systems at low temperatures. Received: 12 June 1998 / Revised version: 19 October 1998 / Accepted: 30 November 1998     

Polynuclear complexes of lanthanides with nickel and copper schiff bases as ligands

Polynuclear complexes of lanthanide(III) nitrates, thiocyanates, chlorides and perchlorates with N,N′- 1,3-propylen-bis(salicylideniminato)nickel(II) and N,N'?1,3-propylen-bis(salicylideniminato)copper(II) have been prepared and characterized by vibrational and electronic spectra, magnetic susceptibility, molar conductivity and thermal analysis measurements. Reliable information relating to the lanthanide- ligand bondings and geometries of the complexes have been inferred from infrared and electronic spectral data.     

Lanthanide ion catalysis of the trans-cis isomerization of trans-bis(oxalato)-diaquochromate(III) and trans-bis(malonato)diaquochromate(III) [1]

The lanthanide ion catalyzed trans-cis isomerizations of trans-bis(oxalato)diaquochromate(II) and trans-bis(malonato)diaquochromate(III) have been studied. A linear free energy relationship was found correlating the catalytic rate constants for the oxalate reaction with the corresponding formation constants of complexes formed between simple monocarboxylic acids and the light (LaGd) members of the lanthanide series. The results indicates that for this portion of the series, the reaction mechanism is related to the formation of monocarboxylate complex intermediates. When the ionic radius of the lanthanide ion decreases below a particular value (as in the latter half of the series), the metal ion remains coordinated to both carboxylates of the oxalate ion rather than simply binding to only one carboxylate. In either situation, isomerization to the cis product eventually occurs, and the lanthanide ion is released.The reaction rates associated with the trans-bis(malonato)diaquochromate(III) reaction were found to be significantly slower than those of the corresponding oxalate system. However, in the malonate system, no linear free energy relationship was found relating the catalytic rate constants with the corresponding formation constants of monocarboxylic acids. One does find a linear relationship between the catalytic rate constants for the malonate reaction and the log K₁ values for the corresponding lanthanide/malonate complexes. During the course of the trans-cis isomerization, the lanthanide ion chelates the dissociated malonate group of a pentavalent Cr(III) intermediate. In the mechanism the lanthanide ion does not aid in ring opening, and neither does it singly bond to the intermediate     

A molecular model for singlet/singlet energy transfer of monovalent ligand/receptor interactions

A stochastic model is described that predicts the degree of singlet/singlet energy transfer in complexes formed between monovalent ligands and monovalent receptors. The modeling approach is intended to serve as an analytical tool for approximating the level of fluorescence quenching that can be expected to occur in fluorescently labeled monovalent ligands and receptors that are bound together in complexes. This approach has utility in areas such as modeling protein/protein interactions and designing fluorescence energy transfer assays.Using the crystallographic data for papain (monovalent ligand ) and concanavalin A (monovalent receptor ) along with a molecular graphics computational package the ligand and receptor were docked together to form a ligand/receptor complex. The intermolecular distances between the lysine resides of the ligand and receptor were then estimated, receptor complex was calculated assuming a value for the characteristic length R(0) of the donor/acceptor pair. Results from the stochastic model were used to calculate the level of fluorescence quenching one would expect for a resonance energy transfer competition assay based on the monovalent ligand/pair.Three key assumptions were made during the model development. First, all lysine resides for the ligand and receptor were equally reactive with the dye molecules so the stoichiometry of the donor and acceptor chromophores was governed by a binomial distribution. Second, the dye molecules were located at the alpha-carbon position for each reactive lysine residue. Finally, in the energy transfer competition assay, it was assumed that equilibrium existed between the ligand, receptor, and competing hapten at all times. Based on these assumptions, results are presented that indicate the maximum energy transfer for the monovalent papain/concanavalin. A complex is strongly dependent on the number of acceptor chromophores and on the value of R(0). Results are also presented on the approximate level of fluorescence quenching that may occur in a competition assay based on the papin/pConA complex. Lastly, a strategy is discussed for maximizing the dynamic range and linearity of energy transfer assays by optimizing several key design variables.     

Hydration of CO2 by carbonic anhydrase: intramolecular proton transfer between Zn2+-bound H2O and histidine 64 in human carbonic anhydrase II

The energy barrier for the intramolecular proton transfer between zinc-bound water and His 64 in the active site of human carbonic anhydrase II (HCA II) has been studied at the partial retention of diatomic differential overlap (PRDDO) level. The most important stabilizing factor for the intramolecular proton transfer is the zinc ion, which lowers the pKa of zinc-bound water and electrostatically repels the proton. The energy barrier of 127.5 kcal/mol for proton transfer between a water dimer is completely removed in the presence of the zinc ion. The zinc ligands, which donate electrons to the zinc ion, raise the barrier slightly to 34 kcal/mol for a 4-coordinated zinc complex including three imidazole ligands from His 94, His 96, and His 119 and to 54 kcal/mol for the 5-coordinated zinc complex including the fifth water ligand. A few model calculations indicate that these energy barriers are expected to be reduced to within experimental range (approximately 10 kcal/mol) when large basis set, correlation energies, and molecular dynamics are considered. The proton-transfer group, which functions as proton receiver in the intramolecular proton transfer, helps to attract the proton; and the partially ordered active site water molecules are important for proton relay function.