Controlling the Flexibility of MIL-88A(Sc) Through Synthetic Optimisation and Postsynthetic Halogenation

Breathing behaviour in metal-organic frameworks (MOFs), the distinctive transformation between a porous phase and a less (or non) porous phase, often controls the uptake of guest molecules, endowing flexible MOFs with highly selective gas adsorptive properties. In highly flexible topologies, breathing can be tuned by linker modification, which is typically achieved pre-synthetically using functionalised linkers. Herein, it was shown that MIL-88A(Sc) exhibits the characteristic flexibility of its topology, which can be tuned by 1) modifying synthetic conditions to yield a formate-buttressed analogue that is rigid and porous; and 2) postsynthetic bromination across the alkene functionality of the fumarate ligand, generating a product that is rigid but non-porous. In addition to providing different methodologies for tuning the flexibility and breathing behaviour of this archetypal MOF, it was shown that bromination of the formate-bridged analogue results in an identical material, representing a rare example of two different MOFs being postsynthetically converted to the same end product.     

Highly non-planar dendritic porphyrin for pH sensing: observation of porphyrin monocation

Metal-free porphyrin-dendrimers provide a convenient platform for the construction of membrane-impermeable ratiometric probes for pH measurements in compartmentalized biological systems. In all previously reported molecules, electrostatic stabilization (shielding) of the core porphyrin by peripheral negative charges (carboxylates) was required to shift the intrinsically low porphyrin protonation pK(a)'s into the physiological pH range (pH 6-8). However, binding of metal cations (e.g., K(+), Na(+), Ca(2+), Mg(2+)) by the carboxylate groups on the dendrimer could affect the protonation behavior of such probes in biological environments. Here we present a dendritic pH nanoprobe based on a highly non-planar tetraaryltetracyclohexenoporphyrin (Ar(4)TCHP), whose intrinsic protonation pK(a)'s are significantly higher than those of regular tetraarylporphyrins, thereby eliminating the need for electrostatic core shielding. The porphyrin was modified with eight Newkome-type dendrons and PEGylated at the periphery, rendering a neutral water-soluble probe (TCHpH), suitable for measurements in the physiological pH range. The protonation of TCHpH could be followed by absorption (e.g., ε(Soret)(dication)～270,000 M(-1) cm(-1)) or by fluorescence. Unlike most tetraarylporphyrins, TCHpH is protonated in two distinct steps (pK(a)'s 7.8 and 6.0). In the region between the pK(a)'s, an intermediate species with a well-defined spectroscopic signature, presumably a TCHpH monocation, could be observed in the mixture. The performance of TCHpH was evaluated by pH gradient measurements in large unilamellar vesicles. The probe was retained inside the vesicles and did not pass through and/or interact with vesicle membranes, proving useful for quantification of proton transport across phospholipid bilayers. To interpret the protonation behavior of TCHpH we developed a model relating structural changes on the porphyrin macrocycle upon protonation to its basicity. The model was validated by density functional theory (DFT) calculations performed on a planar and non-planar porphyrin, making it possible to rationalize higher protonation pK(a)'s of non-planar porphyrins as well as the easier observation of their monocations.     

Is the protein surrounding the active site critical for hydrogen peroxide reduction by selenoprotein glutathione peroxidase? An ONIOM study

In this ONIOM(QM:MM) study, we evaluate the role of the protein surroundings in the mechanism of H2O2 reduction catalyzed by the glutathione peroxidase enzyme, using the whole monomer (3113 atoms in 196 amino acid residues) as a model. A new optimization scheme that allows the full optimization of transition states for large systems has been utilized. It was found that in the presence of the surrounding protein the optimized active site structure bears a closer resemblance to the one in the X-ray structure than that without the surrounding protein. H2O2 reduction occurs through a two-step mechanism. In the first step, the selenolate anion (E-Se(-)) formation occurs with a barrier of 16.4 kcal/mol and is endothermic by 12.0 kcal/mol. The Gln83 residue plays the key role of the proton abstractor, which is in line with the experimental suggestion. In the second step, the O-O bond is cleaved, and selenenic acid (R-Se-OH) and a water molecule are formed. The calculated barrier for this process is 6.0 kcal/mol, and it is exothermic by 80.9 kcal/mol. The overall barrier of 18.0 kcal/mol for H2O2 reduction is in reasonable agreement with the experimentally measured barrier of 14.9 kcal/mol. The protein surroundings has been calculated to exert a net effect of only 0.70 kcal/mol (in comparison to the "active site only" model including solvent effects) on the overall barrier, which is most likely due to the active site being located at the enzyme surface.     

Study of implanted nickel hydride via resistivity and hyperfine interaction experiments

Conclusion We have shown that, in contrast to what is observed when hydrogen is diffused into Ni (formation of a two-phased alloy up to a saturation concentration x0.70), a unique supersaturated -phase is formed by low-temperature H implantation up to x=1.0. The magnetism of this phase decreases linearly and reaches zero at x=(0.83±0.10), as predicted by recent band structure calculations and as also found in the case of a diffused alloy.By combining the HFI experiments with resistivity experiments, we followed the evolution of a Ni H_1.0 implanted alloy with temperature up to H desorption. A sharp drop in the resistivity curve around 150 K, correlated with the appearance of a fraction of¹¹¹In in pure magnetic sites, showed that the supersaturated -phase is no longer stable at this temperature. Part of the hydrogen (15%) may precipitate or desorb out of the sample.Finally, we note that the HFI probe technique used here is only appropriate in systems where the probe impurity does not interact with the implanted ions.     

Interaction between recoil-implanted111In impurities and radiation damage in silver

We report a study of the impurity-radiation damage interaction for In in silver. Time-differential perturbed angular correlation experiments were carried out on¹¹¹Cd, the radioactive daughter of¹¹¹In produced and recoiled in Ag following the¹⁰⁹Ag(, 2n)¹¹¹In nuclear reaction. All irradiations were performed at 80 K. The dose-rate and annealing temperature dependences of the defect-induced quadrupole interaction on¹¹¹Cd were investigated. Clear differences are observed between the effects of correlated damage (the impurity interacting with the damage created by its own recoil) and uncorrelated damage (where the impurity is submitted to the influence of other damage cascades). In the former case (low dose-rate experiments), about 6% of the¹¹¹In impurities are subject to a well-defined quadrupole interaction which is ascribed to a near-neighbor vacancy. This proportion is temperature-independent up to 500 K, but the amplitude of the quadrupole frequency is reduced by a factor of three between 300 and 400 K. Assuming vacancy release by the impurity, this would lead to a binding energy of 0.34(15) eV. The remainder of the¹¹¹In nuclei are subject to a distribution of quadrupole interactions presumably due to a statistical repartition of lattice defects at some distance from the impurities. The width of the distribution changes upon annealing: a clear transition takes place at 200 K, which is the divacancy mobility threshold in Ag deduced from resistivity experiments. When cascade overlap occurs (high dose-rate experiments), no impurity-defect trapping is evidenced but a distribution of quadrupole interactions due to surrounding damage is still found. From these experiments, an order of magnitude of the damage cascade size is deduced. Our results are discussed in terms of available information on the impurity-damage interaction in various hosts; a compilation of all results on the impurity-vacancy interaction known to us is also presented in the paper.This work is part of a Ph.D. Thesis submitted by L. Thomé to Université Paris-Sud.     

Electrochemical stability of self-assembled monolayers of biphenyl based thiols studied by cyclic voltammetry and second harmonic generation

The reductive desorption of self-assembled monolayers (SAMs) of ω-(4′-methyl-biphenyl-4-yl)-alkanethiols (CH₃-C₆H₄-C₆H₄-(CH₂)_n-SH, BPn) on Au(1 1 1) on mica was studied in 0.5 M KOH solution as a function of the length of the aliphatic spacer chain (n = 1-6 and 12) and for two different preparations temperatures (295 K and 343 K). Second harmonic generation (SHG) was applied in situ parallel to cyclic voltammetry (CV). Odd-even differences in the structure of the BPn monolayers are clearly reflected in the electrochemical stability, as well as by the charge and shape of the desorption peak. For n = 1-5 a single desorption peak is detected whereas multiple peaks occur for BP6 similar to hexadecane thiol which was also studied for comparison. An increased preparation temperature affects the shape and width of the desorption peak but not the position. BP1 exhibits a temperature dependence different from the other homologues. The relationship between coverage monitored by SHG and desorption charge determined from the CVs is found to be linear and surprisingly independent from the details of the SAMs. The combined SHG and CV experiments suggest that capacitive and faradaic current are always closely coupled even for BP6 and hexadecane thiol which exhibit multiple desorption peaks.     

Sulfuration and reversion reactions of brominated poly(isobutylene‐co‐isoprene)

The sulfuration and reversion products of brominated poly(isobutylene‐co‐isoprene) (BIIR) were characterized through the use of a model compound, brominated 2,2,4,8,8‐pentamethyl‐4‐nonene (BPMN). The reaction of BPMN with S₈ produced bisallylic polysulfides of various ranks, yielding sulfur bromide intermediates that likely contributed to the rapid oxidation of allylic sulfides into thiophenes. Reductive cure reversion to pentamethylnonene was also observed in the latter stages of vulcanization. The reaction of 2,2′‐dithiobisbenzothiazole with BIIR and BPMN produced a stable adduct that reduced the concentration of allylic bromide available for vulcanization. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1915–1926, 2003     

Dominance Criteria for the Capacitated Warehouse Location Problem

The paper examines the capacitated warehouse location problem where fixed costs, generally relating to the installation of warehouses and variable costs, consisting mainly of transportation costs, are minimized. The minimization of each kind of cost drives the solution towards opposite directions with respect to the number of warehouses to be opened/closed. Therefore, dominance criteria between fixed and variable costs are examined. This leads to exact tests as well as greedy heuristics, the latter known in the literature as ADD/DROP techniques.