Lena J. Daumann

New Insights into Structure and Luminescence of Eu III and Sm III Complexes of the 3,4,3-LI(1,2-HOPO) Ligand

We report the preparation and new insight into photophysical properties of luminescent hydroxypyridonate complexes [MIIIL]− (M = Eu or Sm) of the versatile 3,4,3-LI(1,2-HOPO) ligand (L). We report the crystal structure of this ligand with EuIII as well as insights into the coordination behavior and geometry in solution by using magnetic circular dichroism. In addition TD-DFT calculations were used to examine the excited states of the two different chromophores present in the 3,4,3-LI(1,2-HOPO) ligand. We find that the EuIII and SmIII complexes of this ligand undergo a transformation after in situ preparation to yield complexes with higher quantum yield (QY) over time. It is proposed that the lower QY in the in situ complexes is not only due to water quenching but could also be due to a lower degree of f-orbital overlap (in a kinetic isomer) as indicated by magnetic circular dichroism measurements.

Promiscuity comes at a price: Catalytic versatility vs efficiency in different metal ion derivatives of the potential bioremediator GpdQ☆

The glycerophosphodiesterase from Enterobacter aerogenes (GpdQ) is a highly promiscuous dinuclear metallohydrolase with respect to both substrate specificity and metal ion composition. While this promiscuity may adversely affect the enzymes catalytic efficiency its ability to hydrolyse some organophosphates (OPs) and by-products of OP degradation have turned GpdQ into a promising candidate for bioremedial applications. Here, we investigated both metal ion binding and the effect of the metal ion composition on catalysis. The prevalent in vivo metal ion composition for GpdQ is proposed to be of the type Fe(II)Zn(II), a reflection of natural abundance rather than catalytic optimisation. The Fe(II) appears to have lower binding affinity than other divalent metal ions, and the catalytic efficiency of this mixed metal center is considerably smaller than that of Mn(II), Co(II) or Cd(II)-containing derivatives of GpdQ. Interestingly, metal ion replacements do not only affect catalytic efficiency but also the optimal pH range for the reaction, suggesting that different metal ion combinations may employ different mechanistic strategies. These metal ion-triggered modulations are likely to be mediated via an extensive hydrogen bond network that links the two metal ion binding sites via residues in the substrate binding pocket. The observed functional diversity may be the cause for the modest catalytic efficiency of wild-type GpdQ but may also be essential to enable the enzyme to evolve rapidly to alter substrate specificity and enhance k(cat) values, as has recently been demonstrated in a directed evolution experiment. This article is part of a Special Issue entitled: Chemistry and mechanism of phosphatases, diesterases and triesterases.

Immobilization of the enzyme GpdQ on magnetite nanoparticles for organophosphate pesticide bioremediation

Annually thousands of people die or suffer from organophosphate (pesticide) poisoning. In order to remove these toxic compounds from the environment, the use of enzymes as bioremediators has been proposed. We report here a Ser127Ala mutant based on the enzyme glycerophosphodiesterase (GpdQ) from Enterobacter aerogenes. The mutant, with improved metal binding abilities, has been immobilized using glutaraldehyde on PAMAM dendrimer-modified magnetite nanoparticles. The immobilized system was characterized using elemental analysis as well as infrared, transmission electron and X-ray photoelectron spectroscopies. The amount of GpdQ that was immobilized with the optimized procedure was 1.488 nmol per g MNP. A kinetic assay has been designed to evaluate the activity of the system towards organophosphoester substrates. The specific activity towards BPNPP directly after immobilization was 3.55 μmol mg(-1)min(-1), after one week 3.39 μmol mg(-1)min(-1) and after 120 days 3.36 μmol mg(-1)min(-1), demonstrating that the immobilized enzyme was active for multiple cycles and could be stored on the nanoparticles for a prolonged period.

Cadmium(II) complexes: mimics of organophosphate pesticide degrading enzymes and metallo-β-lactamases.

Cadmium(II) complexes of ethyl 4-hydroxy-3,5-bis(((2-hydroxyethyl)(pyridin-2-ylmethyl)amino)methyl)benzoate (CO(2)EtH(3)L1) and ethyl 4-hydroxy-3,5-bis(((2-methoxyethyl)(pyridin-2-ylmethyl)amino)methyl)benzoate (CO(2)EtHL2) are described. The two ligands possess an ethyl ester (CO(2)Et-) at the position para to the phenolic -OH; CO(2)EtHL2, with methyl ether donors in contrast to potentially nucleophilic alkoxide donors in CO(2)EtH(3)L1, offers a direct comparison of potential ligand-centered nucleophiles. The complex with CO(2)EtH(3)L1 was characterized using (1)H and (13)C NMR spectroscopy, mass spectrometry and microanalysis; X-ray crystallography defined a tetranuclear structure [Cd(4)(CO(2)EtH(2)L1)(2)(CH(3)COO)(3.75)Cl(0.25)(H(2)O)(2)](PF(6))(2). Functional studies of the cadmium(II) complexes were undertaken with the substrates bis(2,4-dinitrophenyl)phosphate (BDNPP), and nitrocefin to assess their phosphatase and β-lactamase activities, respectively. The complexes with CO(2)EtH(3)L1 and CO(2)EtHL2 are competent phosphoesterase mimics with K(M) = 9.4 ± 2.1 mM and 10.1 ± 3.4 mM, k(cat) = 9.4 ± 0.2 × 10(-3) s(-1) and 9.7 ± 2.7 × 10(-3) s(-1), respectively. Use of a solvent mixture containing H(2)(18)O/H(2)(16)O in the reaction with BDNPP showed that for the complex with CO(2)EtH(3)L1 the (18)O label was incorporated in the reaction product suggesting that the nucleophile involved is a Cd-OH moiety and not a metal bound alkoxide; for CO(2)EtHL2 the presence of the methyl-ether dictates that the active nucleophile must also be a hydroxide. The cadmium(II) complex with CO(2)EtH(3)L1 was furthermore found to be a competent β-lactamase mimic with k(cat) = 1.39 × 10(-2) ± 3 × 10(-3) s(-1), K(M) = 0.11 ± 0.03 mM, and pK(a) = 7.9 ± 0.1. Mass spectral evidence suggested that the active nucleophile in this reaction is the alkoxide; lack of β-lactamase activity of the complex with CO(2)EtHL2 supports this assignment. Similar to enzyme-catalyzed reactions, a blue reaction intermediate in the β-lactamase reaction of the CO(2)EtH(3)L1 complex was also identified. It is proposed that the Cd(II) complexes of CO(2)EtH(3)L1 and CO(2)EtHL2 react identically as phosphatases, with a terminal hydroxide as the nucleophile; the former exhibits β-lactamase activity with the alkoxide as a nucleophile, while the latter, without a potentially nucleophilic alkoxide, is inactive.

Asymmetric zinc(II) complexes as functional and structural models for phosphoesterases.

We report two asymmetric ligands for the generation of structural and functional dinuclear metal complexes as phosphoesterase mimics. Two zinc(II) complexes, [Zn2(CH3L4)(CH3CO2)2](+) (CH3HL4 = 2-(((2-methoxyethyl)(pyridin-2-ylmethyl)amino)methyl)-4-methyl-6-(((pyridin-2-ylmethyl)amino)methyl)phenol) and [Zn2(CH3L5)(CH3CO2)2](+) (CH3HL5 = 2-(((2-methoxyethyl)(pyridine-2-ylmethyl)amino)methyl)-4-methyl-6-(((pyridin-2-ylmethyl)(4-vinylbenzyl)amino)methyl)phenol) were synthesized and characterized by X-ray crystallography. The structures showed that the ligands enforce a mixed 6,5-coordinate environment in the solid state. (1)H-, (13)C- and (31)P-NMR, mass spectrometry and infrared spectroscopy were used to further characterize the compounds in the solid state and in solution. The zinc(II) complexes hydrolyzed the organophosphate substrate bis-(2,4-dinitrophenol)phosphate (BDNPP), the nucleophile proposed to be a terminal water molecule (pK(a) 7.2). The ligand CH3HL4 was immobilised on Merrifield resin and its zinc(II) complex generated. Infrared spectroscopy, microanalysis and XPS measurements confirmed successful immobilisation, with a catalyst loading of ~1.45 mmol g(-1) resin. The resin bound complex was active towards BDNPP and displayed similar pH dependence to the complex in solution.

Mono- and Dinuclear Copper(II) and Iron(III) Complexes of a Tetradentate Bispidine-diacetate Ligand

The synthesis of a new tetradentate bispidine ligand (LH2 = 2,2 � -(1,5-dimethyl-3,7-diazabicyclo[3.3.1]nonan-3,7diyl)diacetic acid), containing two tertiary amine and two carboxylic groups, is reported along with the preparation and characterization of the corresponding Cu(ii) and Fe(iii) complexes. The mononuclear [LCu(OH2)]·4H2 O( 1) complex contains a five-coordinate Cu(ii) centre, which adopt a square pyramidal geometry with the four donor atoms of the ligand (N2O2) occupying the equatorial plane and a water molecule occupying the axial position. An axial electron paramagnetic resonance (EPR) signature is observed for 1 (gx = 2.054, gy = 2.050, gz = 2.234; Ax = 18 × 10 −4 cm −1 , Ay = 20 × 10 −4 cm −1 , Az = 188 × 10 −4 cm −1 ) in frozen methanolic solution (0.1 mM). Dimerization of 1 in concentrated solution (10 mM) was observed by EPR spectroscopy (g� = 2.24, g⊥ = 2.07, A� = 195 × 10 −4 cm −1 , and A⊥ = 12 × 10 −4 cm −1 for each Cu centre). The structure of the dimeric species [LCu(OH2)]2 (1b) was determined by a combination of molecular mechanics with the simulation of the EPR spectrum (MM-EPR). The dimer has each Cu(ii) centre coordinated by the two amines and one carboxylate of one ligand (L), while the other carboxylate bridges to the second Cu(ii) centre; each coordination sphere is completed by an axial water ligand, with the Cu ··· Cu distance 5.5 A (relative orientation from EPR simulation: α = 60 ◦ , β = 0 ◦ , γ = 25 ◦ ). The aqueous reaction between the tetradentate ligand (L) and Fe(ii) leads to the formation of an oxo-bridged diiron(iii) complex, [LFe-(µ-O)-FeL] (2), with a Fe–O–Fe angle of 180 ◦ (dFe···Fe = 3.516 A), as revealed by X-ray crystallography. The Mossbauer spectrum of 2 consists of one quadrupole doublet with an isomer shift (δ) of 0.37 mm s −1 and a quadrupole splitting (� EQ) of 0.73 mm s −1 , which is consistent with S = 5/2 Fe(iii) centres. Variable-temperature magnetic susceptibility measurements show the presence of intramolecular antiferromagnetic interactions between the two Fe(iii) centres, with an exchange coupling constant J of −91(3) cm −1

Effects of Ligand Geometry on the Photophysical Properties of Photoluminescent Eu(III) and Sm(III) 1-Hydroxypyridin-2-one Complexes in Aqueous Solution

A series of 10 tetradentate 1-hydroxy-pyridin-2-one (1,2-HOPO) ligands and corresponding eight-coordinated photoluminescent Eu(III) and Sm(III) complexes were prepared. Generally, the ligands differ by the linear (nLI) aliphatic linker length, from 2 to 8 methylene units between the bidentate 1,2-HOPO chelator units. The photoluminescent quantum yields (Φtot) were found to vary with the linker length, and the same trend was observed for the Eu(III) and Sm(III) complexes. The 2LI and 5LI bridged complexes are the brightest (Φtotxε). The change in ligand wrapping pattern between 2LI and 5LI complexes observed by X-ray diffraction (XRD) is further supported by density functional theory (DFT) calculations. The bimodal Φtot trends of the Eu(III) and Sm(III) complexes are rationalized by the change in ligand wrapping pattern as the bridge (nLI) is increased in length.

Siderophore inspired tetra- and octadentate antenna ligands for luminescent Eu(III) and Tb(III) complexes.

Following the success of the siderophore-inspired 1,2-hydroxypyridonate (HOPO) and 2-hydroxisophthalamide (IAM) chromophores in Eu(III) and Tb(III) luminescence, we designed three new ligands bearing both chromophores. Syntheses of the octadentate ligands 3,4,3-LI-IAM-1,2-HOPO and 3,4,3-LI-1,2-HOPO-IAM, where the chromophores are attached to different positions in the (LI=linear) spermine backbone, are reported in addition to a tetradentate ligand based on 1,5-diaminopentane. The Gd(III) complexes were prepared and revealed localized triplet states typical for the IAM and HOPO chromophores. Photophysical characterization of the Eu(III) and Tb(III) complexes revealed that the chromophores need to reside at a primary amine of the spermine backbone to be efficient in lanthanide excitation. These systems help us to understand the antenna effect in siderophore inspired chromophores and could be potential targets for sensing and biological imaging applications.

Optimization of the Sensitization Process and Stability of Octadentate Eu(III) 1,2-HOPO Complexes.

The synthesis of a series of octadentate ligands containing the 1-hydroxypyridin-2-one (1,2-HOPO) group in complex with europium(III) is reported. Within this series, the central bridge connecting two diethylenetriamine units linked to two 1,2-HOPO chromophores at the extremities (5-LIN-1,2-HOPO) is varied from a short ethylene chain (H(2,2)-1,2-HOPO) to a long pentaethylene oxide chain (H(17O5,2)-1,2-HOPO). The thermodynamic stability of the europium complexes has been studied and reveals these complexes may be effective for biological measurements. Extension of the central bridge results in exclusion of the inner-sphere water molecule observed for [Eu(H(2,2)-1,2-HOPO)]− going from a nonacoordinated to an octacoordinated Eu(III) ion. With the longer chain length ligands, the complexes display increased luminescence properties in aqueous medium with an optimum of 20% luminescence quantum yield for the [Eu(H(17O5,2)-1,2-HOPO)]− complex. The luminescence properties for [Eu(H(14O4,2)-1,2-HOPO)]− and [Eu(H(17O5,2)-1,2-HOPO)]− are better than that of the model bis-tetradentate [Eu(5LINMe-1,2-HOPO)2]− complex, suggesting a different geometry around the metal center despite the geometric freedom allowed by the longer central chain in the H(mOn,2) scaffold. These differences are also evidenced by examining the luminescence spectra at room temperature and at 77 K and by calculating the luminescence kinetic parameters of the europium complexes.

Structural and Mechanistic Studies of Zn(II) Complexes as Phosphoesterase Models

Zn(II) is a common metal in all forms of life; it is not only the second most abundant metal in biological systems after iron, but also occurs in the active site of over 200, mostly hydrolytic, enzymes.