Lanthanide coordination polymers with tetrafluoroterephthalate as a bridging ligand: thermal and optical properties

By slow diffusion of triethylamine into a solution of 2,3,5,6-tetrafluoroterephthalic acid (H2tfBDC) and the respective lanthanide salt in EtOH/DMF single crystals of seven nonporous coordination polymers, (∞)(2)[Ln(tfBDC)(NO(3))(DMF)(2)]·DMF (Ln(3+) = Ce, Pr, Nd, Sm, Dy, Er, Yb; C2/c, Z = 8) have been obtained. In the crystal structures, two-dimensional square grids are found, which are composed of binuclear lanthanide nodes connected by tfBDC(2-) as a linking ligand. The coordination sphere of each lanthanide cation is completed by a nitrate anion and two DMF molecules (CN = 9). This crystal structure is unprecedented in the crystal chemistry of coordination polymers based on nonfluorinated terephthalate (BDC(2-)) as a bridging ligand; as for tfBDC(2-), a nonplanar conformation of the ligand is energetically more favorable, whereas for BDC(2-), a planar conformation is preferred. Differential thermal analysis/thermogravimetric analysis (DTA/TGA) investigations reveal that the noncoordinating DMF molecule is released first at temperatures of 100-200 °C. Subsequent endothermal weight losses correspond to the release of the coordinating DMF molecules. Between 350 and 400 °C, a strong exothermal weight loss is found, which is probably due to a decomposition of the tfBDC(2-) ligand. The residues could not be identified. The emission spectra of the (∞)(2)[Ln(tfBDC)(NO(3))(DMF)(2)]·DMF compounds reveal intense emission in the visible region of light for Pr, Sm, and Dy with colors from orange, orange-red, to warm white.     

Promotion of the oxidation of carbon monoxide at stepped platinum single-crystal electrodes in alkaline media by lithium and beryllium cations

The role of alkali cations (Li(+), Na(+), K(+), Cs(+), and Be(2+)) on the blank voltammetric response and the oxidative stripping of carbon monoxide from stepped Pt single-crystal electrodes in alkaline media has been investigated by cyclic voltammetry. A strong influence of the nature of the cation on both the blank voltammetric profile and the CO oxidation is observed and related to the influence of the cation on the specific adsorption of OH on the platinum surface. Especially Li(+) and Be(2+) cations markedly affect the adsorption of OH and thereby have a significant promoting effect on CO(ads) oxidation. The voltammetric experiments suggest that, on Pt(111), the influence of Li(+) (and Be(2+)) is primarily through a weakening of the repulsive interactions between the OH in the OH adlayer, whereas in the presence of steps also, the onset of OH adsorption is at a lower potential, both on steps and on terraces.     

Reversible low-light induced photoswitching of crowned spiropyran-DO3A complexed with gadolinium(III) ions

Photoswitchable spiropyran has been conjugated to the crowned ring system DO3A, which improves its solubility in dipolar and polar media and stabilizes the merocyanine isomer. Adding the lanthanide ion gadolinium(III) to the macrocyclic ring system leads to a photoresponsive magnetic resonance imaging contrast agent that displays an increased spin-lattice relaxation time (T?) upon visible light stimulation. In this work, the photoresponse of this photochromic molecule to weak light illumination using blue and green light emitting diodes was investigated, simulating the emission spectra from bioluminescent enzymes. Photon emission rate of the light emitting diodes was changed, from 1.75 × 101? photons·s?1 to 2.37 × 1012 photons·s?1. We observed a consistent visible light-induced isomerization of the merocyanine to the spiropyran form with photon fluxes as low as 2.37 × 1012 photons·s?1 resulting in a relaxivity change of the compound. This demonstrates the potential for use of the described imaging probes in low light level applications such as sensing bioluminescence enzyme activity. The isomerization behavior of gadolinium(III)-ion complexed and non-complexed spiropyran-DO3A was analyzed in water and ethanol solution in response to low light illumination and compared to the emitted photon emission rate from over-expressed Gaussia princeps luciferase.     

Orbital signatures of methyl in L-alanine

Molecular orbital signatures of the methyl substituent in L-alanine have been identified with respect to those of glycine from information obtained in coordinate and momentum space, using dual space analysis. Electronic structural information in coordinate space is obtained using ab initio (MP2/TZVP) and density functional theory (B3LYP/TZVP) methods, from which the Dyson orbitals are simulated based on the plane wave impulse approximation into momentum space. In comparison to glycine, relaxation in geometry and valence orbitals in L-alanine is found as a result of the attachment of the methyl group. Five orbitals rather than four orbitals are identified as methyl signatures. That is, orbital 6a in the core shell, orbitals 11a and 12a in the inner valence shell, and orbitals 19a and 20a in the outer valence shell. In the inner valence shell, the attachment of methyl to glycine causes a splitting of its orbital 10a' into orbitals 11a and 12a of L-alanine, whereas in the outer valence shell the methyl group results in an insertion of an additional orbital pair of 19a and 20a. The frontier molecular orbitals, 24a and 23a, are found without any significant role in the methylation of glycine.     

Iron(II) carboxylate complexes based on a tetraimidazole ligand as models of the photosynthetic non-heme ferrous sites: synthesis, crystal structure, and Mössbauer and magnetic studies

The preparations, X-ray structures, and detailed physical characterization are presented for new complexes involving an iron(II) center, a tetraimidazole ligand (TIM), and different carboxylates. [Fe(TIM)(C(6)H(5)CH(2)CO(2))](ClO(4)) (1) crystallizes in the Pbca space group with a = 10.8947(13), b = 20.343(2), and c = 22.833(3) A, Z = 8, and V = 5060.6(11) A(3). [Fe(TIM)(CH(3)CO(2))](ClO(4)) (2) crystallizes in the Ia space group with a = 17.117(2), b = 10.3358(12), and c = 25.658(3) A, beta = 90.301(13) degrees, Z = 8, and V = 4539.5(9) A(3). In both structures, the iron(II) is hexacoordinated to the four N(imidazole) donors of the TIM ligand and the two O donors of a bidentate carboxylate. The flexibility of the carboxylate bidentate coordination, symmetrical or more or less asymmetrical, associated with the steric demand of the TIM ligand results in a remarkable versatility of the Fe(II)N(4)O(2) coordination geometry. The diversity in carboxylate bidentate coordination modes has allowed us to clearly show the importance of the structural and electronic effects, through IR and M?ssbauer spectroscopy, of this apparently tenuous carboxylate shift. Comparison of the structural and M?ssbauer properties of these complexes with the non-heme ferrous site of photosynthetic systems (i) shows that the metric parameters of site 2b, including the symmetrically chelated bidentate carboxylate, are closer to those of the non-heme ferrous site in the bacterial reaction centers of Rhodopseudomonas viridis and R. sphaeroides and (ii) suggests that the ligand environment of the non-heme ferrous center of PS 2 is close to the axially distorted octahedral symmetry resulting from an asymmetrical bidentate coordination of the -CO(2) motif, as in complex 1.     

Prediction of high-valent iron K-edge absorption spectra by time-dependent density functional theory

In recent years, a number of high-valent iron intermediates have been identified as reactive species in iron-containing metalloproteins. Inspired by the interest in these highly reactive species, chemists have synthesized Fe(IV) and Fe(V) model complexes with terminal oxo or nitrido groups, as well as a rare example of an Fe(VI)-nitrido species. In all these cases, X-ray absorption spectroscopy has played a key role in the identification and characterization of these species, with both the energy and intensity of the pre-edge features providing spectroscopic signatures for both the oxidation state and the local site geometry. Here we build on a time-dependent DFT methodology for the prediction of Fe K- pre-edge features, previously applied to ferrous and ferric complexes, and extend it to a range of Fe(IV), Fe(V) and Fe(VI) complexes. The contributions of oxidation state, coordination environment and spin state to the spectral features are discussed. These methods are then extended to calculate the spectra of the heme active site of P450 Compound II and the non-heme active site of TauD. The potential for using these methods in a predictive manner is highlighted.     

Synthesis and characterization of a novel nonlinear optical hyperbranched polymer containing a highly performing chromophore

We report herein the peripheral functionalization of a high glass transition temperature hyperbranched polyimide with a new and highly performing electro‐optic chromophore for the elaboration of a second‐order nonlinear optical material. In this study, the CPO1 chromophore was selected for its very high quadratic hyperpolarizability coefficient (μβ = 31,000 ? 10^?48 esu at 1990 nm) and its ease of synthesis in multigram scale. As a result, the new electro‐optic polymer was characterized by an r₃₃ coefficient around 40 pm/V at 1.5 µm, although the poling conditions were not optimized. For sake of comparison, the electro‐optic r₃₃ coefficient of our previously reported similar polymer functionalized with the well‐known Disperse Red One chromophore was also measured using the technique and gave a much lower r₃₃ coefficient. This study underscores that hyperbranched polymers are particularly promising matrices to host highly efficient chromophore to achieve more efficient and more stable electro‐optic devices than classical linear polymers. Copyright © 2013 John Wiley & Sons, Ltd.     

Synthesis of Potential Cytotoxic α-Methylene γ-Lactams

α-Methylene γ-lactones are known to have physiological activity, especially as cytotoxic agents, and are widely found in nature (1). However they are often toxic which makes them inappropriate for cancer treatment.     

An analytical workflow for the molecular dissection of irreversibly modified fluorescent proteins

Owing to their ability to be genetically expressed in live cells, fluorescent proteins have become indispensable markers in cellular and biochemical studies. These proteins can undergo a number of covalent chemical modifications that may affect their photophysical properties. Among other mechanisms, such covalent modifications may be induced by reactive oxygen species (ROS), as generated along a variety of biological pathways or through the action of ionizing radiations. In a previous report [1], we showed that the exposure of cyan fluorescent protein (ECFP) to amounts of ^?OH that mimic the conditions of intracellular oxidative bursts (associated with intense ROS production) leads to observable changes in its photophysical properties in the absence of any direct oxidation of the ECFP chromophore. In the present work, we analyzed the associated structural modifications of the protein in depth. Following the quantified production of ^?OH, we devised a complete analytical workflow based on chromatography and mass spectrometry that allowed us to fully characterize the oxidation events. While methionine, tyrosine, and phenylalanine were the only amino acids that were found to be oxidized, semi-quantitative assessment of their oxidation levels showed that the protein is preferentially oxidized at eight residue positions. To account for the preferred oxidation of a few, poorly accessible methionine residues, we propose a multi-step reaction pathway supported by data from pulsed radiolysis experiments. The described experimental workflow is widely generalizable to other fluorescent proteins, and opens the door to the identification of crucial covalent modifications that affect their photophysics.