Carboxylate ligands drastically enhance the rates of oxo exchange and hydrogen peroxide disproportionation by oxo manganese compounds of potential biological significance

To mimic the carboxylate-rich active site of the manganese catalases more closely we introduced carboxylate groups into dimanganese complexes in place of nitrogen ligands. The series of dimanganese(III,IV) complexes of tripodal ligands [Mn(2)(L)(2)(O)(2)](3+/+/-/3-) was extended from those of tpa (1) and H(bpg) (2) to those of H(2)(pda) (3) and H(3)(nta) (4) (tpa=tris-picolylamine, H(bpg)=bis-picolylglycylamine, H(2)(pda)=picolyldiglycylamine, H(3)(nta)=nitrilotriacetic acid). While 3 [Mn(2)(pda)(2)(O)(2)][Na(H(2)O)(3)] could be synthesized at -20 degrees C and characterized in the solid state, 4 [Mn(2)(nta)(2)(O)(2)](3-) could be obtained and studied only in solution at -60 degrees C. A new synthetic procedure for the dimanganese(III,III) complexes was devised, using stoichiometric reduction of the dimanganese(III,IV) precursor by the benzil radical with EPR monitoring. This enabled the preparation of the parent dimanganese(III,III) complex 5 [Mn(2)(tpa)(2)(O)(2)](ClO(4))(2), which was structurally characterized. The UV/visible, IR, EPR, magnetic, and electrochemical properties of complexes 1-3 and 5 were analyzed to assess the electronic changes brought about by the carboxylate replacement of pyridine ligands. The kinetics of the oxo ligand exchanges with labeled water was examined in acetonitrile solution. A dramatic effect of the number of carboxylates was evidenced. Interestingly, the influence of the second carboxylate substitution differs from that of the first one probably because this substitution occurs on an out-of-plane coordination while the former occurs in the plane of the [Mn(2)O(2)] core. Indeed, on going from 1 to 3 the exchange rate was increased by a factor of 50. Addition of triethylamine caused a rate increase for 1, but not for 3. The abilities of 1-3 to disproportionate H(2)O(2) were assessed volumetrically. The disproportionation exhibited a sensitivity corresponding to the carboxylate substitution. These observations strongly suggest that the carboxylate ligands in 2 and 3 act as internal bases.     

Definition of magneto-structural correlations for the MnII ion

The electronic properties of the high spin mononuclear MnII complexes [Mn(tpa)(NCS)2] (1) (tpa=tris-2-picolylamine), [Mn(tBu3-terpy)2](PF6)2 (2) (tBu3-terpy=4,4',4'-tri-tert-butyl-2,2':6',2'-terpyridine) and [Mn(terpy)2](I)2 (3) (terpy=2,2':6',2'-terpyridine) with an N6 coordination sphere have been determined by multifrequency EPR spectroscopy. The X-ray structures of 1.CH3CN and 2.C4H10 O.0.5 C2H5OH.0.5 CH3OH reveal that the MnII ion lies at the center of a distorted octahedron. The D-values of 1-3 all fall in the narrow range of 0.041 to 0.105 cm(-1). The comparison of the results reported here and those found in the literature is consistent with the following observation: the D value is sensitive to the coordination number (6 or 5) of the MnII ion as long as the coordination sphere involves only nitrogen and/or oxygen based ligands. This magneto-structural correlation has been analyzed in this work though DFT model calculations. The zero-field splitting (zfs) parameters of 1-3 have been calculated and are in reasonable agreement with the experimental values. Hypothetical simplified models [Mn(NH3)x(OH2)y]2+ (x+y=5 or 6 and [Mn(NH3)5X]+ (X=OH, Cl)) have been constructed to investigate the origin of the zfs. This investigation reveals i) that D is sensitive to the coordination number (5 or 6) of the MnII ion, ii) for the five coordinate systems the major contribution to D is the spin-orbit coupling part, iii) for the six coordinate systems the major contribution to D is the spin-spin interaction and iv) the deprotonation of a water ligand leads to an increase of D, consistent with the relative ligand fields of OH(-) versus H2O.     

Facile synthesis of a "ready to use" precursor of porphobilinogen and its amino Acid derivatives

A practical synthesis of porphobilinogen based on the biosynthetic mechanism is described. The crossed Mukayiama aldol reaction is the key step creating the central carbon-carbon bond between the two protected forms of 5-aminolevulinic acids. The optimized sequence gives a crystalline, storable precursor, which can be transformed in high yield into porphobilinogen and bioconjugates thereof. The enzymatic hydrolysis of the precursor produces porphobilinogen in quantitative yield.     

Genotoxicity of visible light (400-800 nm) and photoprotection assessment of ectoin, L-ergothioneine and mannitol and four sunscreens

This study was designed to determine the genotoxic effects of visible (400-800nm) and ultraviolet A (UVA)/visible (315-800nm) lights on human keratinocytes and CHO cells. The alkaline comet assay was used to quantify DNA-damage. In addition, photo-dependent cytogenetic lesions were assessed in CHO cells by the micronucleus test. Three protective compounds [ectoin, l-ergothioneine (ERT) and mannitol] were tested with the comet assay for their effectiveness to reduce DNA single-strand breaks (SSB). Finally, the genomic photoprotections of two broad-band sunscreens and their tinted analogues were assessed by the comet assay. The WST-1 cytotoxicity assay revealed a decrease of the keratinocyte viability of 30% and 13% for the highest UVA/visible and visible irradiations (15 and 13.8J/cm(2), respectively). Visible as well as UVA/visible lights induced DNA SSB and micronuclei, in a dose-dependent manner. The level of DNA breakage induced by visible light was 50% of the one generated by UVA/visible irradiation. However, UVA radiations were 10 times more effective than visible radiations to produce SSB. The DNA lesions induced by visible and UVA/visible lights were reduced after a 1-h preincubation period with the three tested compounds. The maximal protective effects were 92.7%, 97.9% and 52.0% for ectoin (0.1mM), ERT (0.5mM) and mannitol (1.5mM), respectively, against visible light and 68.9%, 59.8% and 62.7% for ectoin (0.1mM), ERT (0.5mM) and mannitol (1.5mM), respectively, against UVA/visible light. Thus, visible light was genotoxic on human keratinocytes and CHO cells through oxidative stress mechanisms similar to the ones induced by UVA radiations. The four tested sunscreens efficiently prevented DNA lesions that were induced by both visible and UVA/visible irradiations. The tinted sunscreens were slightly more effective that their colorless analogues. There is a need to complement sunscreen formulations with additional molecules to obtain a complete internal and external photoprotection against both UVA and visible lights.     

Efficient Recycling of Gold and Copper from Electronic Waste by Selective Precipitation

The recycling of metals from electronic waste (e-waste) using efficient, selective, and sustainable processes is integral to circular economy and net-zero aspirations. Herein, we report a new method for the selective precipitation of metals such as gold and copper that offsets the use of organic solvents that are traditionally employed in solvent extraction processes. We show that gold can be selectively precipitated from a mixture of metals in hydrochloric acid solution using triphenylphosphine oxide (TPPO), as the complex [(TPPO)₄(H₅O₂)][AuCl₄]. By tuning the acid concentration, controlled precipitation of gold, zinc and iron can be achieved. We also show that copper can be selectively precipitated using 2,3-pyrazinedicarboxylic acid (2,3-PDCA), as the complex [Cu(2,3-PDCA-H)₂]_n ⋅ 2n(H₂O). The combination of these two precipitation methods resulted in the recovery of 99.5 % of the Au and 98.5 % of the Cu present in the connector pins of an end-of-life computer processing unit. The selectivity of these precipitation processes, combined with their straightforward operation and the ability to recycle and reuse the precipitants, suggests potential industrial uses in the purification of gold and copper from e-waste.     

Experimental sensitivity analysis applied to sample preparation uncertainties: are ruggedness tests enough for measurement uncertainty estimates?

It has been suggested that typical ruggedness tests might lead directly to uncertainty estimates. This assertion is tested using simple experimental studies of uncertainties associated with sample grinding and oven-drying operations. The results are used to predict the outcome of typical ruggedness tests on the same systems. It is concluded that uncertainty estimation from ruggedness tests is appropriate only where a strong effect can be observed. Since current practice in ruggedness testing is predisposed to confirming insignificance, typical ruggedness tests are not likely to lead to reliable uncertainty estimates; instead, lack of statistical significance in ruggedness tests is better interpreted as reason to leave an effect out of the uncertainty budget. Only where the ruggedness study is modified in order to achieve statistically significant change is it useful for uncertainty estimation. Received: 27 November 2000 Accepted: 13 February 2001     

Commutative reduction of aromatic ketones to arylmethylenes/alcohols by hypophosphites catalyzed by Pd/C under biphasic conditions

An efficient method is reported to reduce aromatic ketones selectively into arylmethylenes or alcohols with hypophosphites and Pd/C, depending on the selected conditions. This study could represent a promising alternative to the classical uses of standard hydrides or molecular hydrogen involved in reduction and deoxygenation procedures.     

A Luminescent and Sublimable DyIII-Based Single-Molecule Magnet

The reaction of [Ln(hfac)₃] ⋅ 2 H₂O and pyridine-N-oxide (PyNO) leads to isostructural dimers of the formula [Ln(hfac)₃(PyNO)]₂ (Ln=Eu, Gd, Tb, Dy). The Dy derivative shows a remarkable single-molecule magnet behavior with complex hysteresis at 1.4 K. The dynamics of the magnetization features are two relaxation regimes: a thermally activated one at high temperature (τ₀=(5.62±0.4)×10⁻¹¹ s and Δ=(167±1) K) and a quantum tunneling regime at low temperature with a tunneling frequency of 0.42 Hz. The analysis of the Gd derivative evidences intradimer antiferromagnetic interactions (J=(−0.034±0.001) cm⁻¹). Moreover, the Eu, Tb, and Dy derivatives are luminescent with quantum yield of 51, 53, and 0.1 %, respectively. The thermal investigation of [Dy(hfac)₃(PyNO)]₂ shows that the dimers can be sublimated intact, suggesting their possible exploit as active materials for surface-confined nanostructures to be investigated by fluorimetry methods.