Patrice Moreau

Transition-metal ligands bound onto the micelle-templated silica surface

Abstract This paper reviews recent works on the design of immobilized transition-metal ligands on solid supports. After an overview of some results concerning ligand anchorage on polymeric supports and encapsulation of transition-metal complexes inside layered or zeolitic minerals, the grafting of ligands onto the silicic wall surface of micelle-templated silicas (MTS) is reported. MTS silicas featuring a regular mesoporous system of pore-monodispersed size and exhibiting larger pores than zeolites, provide a new opportunity to allow anchorage of organic moieties through the silanation procedure. Mn(III) Salpr and t Salpr complexes bound onto the MTS surface are active in epoxidation reaction using PhIO as oxygen donor. Anchorage of (1 R ,2 S )-ephedrine has been also investigated with the aim to obtain benefit from the MTS structure effect. These new supported chiral catalysts are active in alkylation of benzaldehyde with diethylzinc although less enantioselective than the corresponding homogeneous catalyst. The effect of dispersion of active sites and of surface passivation has been investigated and discussed in terms of the nature of the support surface.

Liquid phase hydrogenation of cinnamaldehyde over supported ruthenium catalysts: Influence of particle size, bimetallics and nature of support

Abstract Hydrogenation of cinnamaldehyde was studied over supported Ru based catalysts. The influence of particle size, bimetallic formulation and nature of support on activity and selectivity to the desired product, cinnamyl alcohol, was investigated. Ru particle size had a pronounced effect on both TOF and selectivity. Larger Ru particles showed higher selectivity to cinnamyl alcohol, as well as higher TOF. Addition of Sn, Fe, Ge, Sb, Zn and Ag as a second metal improved both activity and selectivity. RuSn/Al2O3 showed sustained improvement in selectivity, even at higher conversions, probably due to the presence of RuSnδ+ sites and/or due to the electron transfer from Sn to Ru. ZrO2 was found to be the most effective support in terms of selectivity. Possible reasons for this behaviour of ZrO2 are delineated.

The Lysine Decarboxylase CadA Protects Escherichia coli Starved of Phosphate against Fermentation Acids

Conflicting results have been reported for the rate and extent of cell death during a prolonged stationary phase. It is shown here that the viability of wild-type cells (MG1655) could decrease >or=10(8)-fold between days 1 and 14 and between days 1 and 6 of incubation under aerobic and anaerobic phosphate (P(i)) starvation conditions, respectively, whereas the cell viability decreased moderately under ammonium and glucose starvation conditions. Several lines of evidence indicated that the loss of viability of P(i)-starved cells resulted primarily from the catabolism of glucose into organic acids through pyruvate oxidase (PoxB) and pyruvate-formate lyase (PflB) under aerobic and anaerobic conditions, respectively. Weak organic acids that are excreted into the medium can reenter the cell and dissociate into protons and anions, thereby triggering cell death. However, P(i)-starved cells were efficiently protected by the activity of the inducible GadABC glutamate-dependent acid resistance system. Glutamate decarboxylation consumes one proton, which contributes to the internal pH homeostasis, and removes one intracellular negative charge, which might compensate for the accumulated weak acid anions. Unexpectedly, the tolerance of P(i)-starved cells to fermentation acids was markedly increased as a result of the activity of the inducible CadBA lysine-dependent acid resistance system that consumes one proton and produces the diamine cadaverine. CadA plays a key role in the defense of Salmonella at pH 3 but was thought to be ineffective in Escherichia coli since the protection of E. coli challenged at pH 2.5 by lysine is much weaker than the protection by glutamate. CadA activity was favored in P(i)-starved cells probably because weak organic acids slowly reenter cells fermenting glucose. Since the environmental conditions that trigger the death of P(i)-starved cells are strikingly similar to the conditions that are thought to prevail in the human colon (i.e., a combination of low levels of P(i) and oxygen and high levels of carbohydrates, inducing the microbiota to excrete high levels of organic acids), it is tempting to speculate that E. coli can survive in the gut because of the activity of the GadABC and CadBA glutamate- and lysine-dependent acid resistance systems.

Hydrogenation of α,β-unsaturated carbonyls: Acrolein hydrogenation on Group VIII metal catalysts

Abstract Hydrogenation of acrolein was studied in the vapor phase on various supported Group VIII metals. The distribution of products changes rapidly with time on stream. When supported on alumina, silica, titania, zirconia or graphite, Co, Ni and Ru based catalysts show good initial selectivity to allyl alcohol, while Pt — with a higher activity — is less selective, as is Ir. The other products formed in major quantities were propanal and acetone, while the activities for n-propanol and light compounds are short lived. It is proposed that acetone is formed through a 1,4-diadsorbed intermediate on some specific electron-deficient sites which could be situated at the interface between the metal particles and the support. Acetone is not formed when graphite or zirconia is used as the support. From the study of the influence of the size of Ru particles supported on alumina on the hydrogenation of acrolein, it turns out that the highly dispersed Ru/Al2O3 catalyst is the least selective to allyl alcohol. In other respects, the zirconia-supported ruthenium catalyst exhibits the highest selectivity to allyl alcohol, possibly owing to some alloying occurring between ruthenium and zirconium.

Liquid phase selective alkylation of naphthalene with t-butanol over large pore zeolites

Abstract Liquid phase alkylation of naphthalene with t -butanol has been studied using HY and H-beta zeolites with varying silicon to aluminum ratios. Over both series of zeolites,2-( t -butyl)naphthalene (2-TBN) was observed as only monoalkylated product. Up to 84%2,6-di( t -butyl)naphthalene (2,6-DTBN) selectivity with β,β′-selectivity (2,6-+2,7-) 98–99% and2,6-/2,7- ratios from 5.6 to 5.9 were obtained on silica rich HY zeolites HY(20) and HY(6) under mild reaction conditions (160°C, 2 h reaction). H-beta zeolites show selective effect only for the formation of 2-TBN. For the different zeolites, the activity order isHY(20)≈HY(6)>H-beta(26)≈H-beta(13)>HY(2.5), while the selectivity to 2,6-DTBN stands in the orderHY(20)≈HY(6)>HY(2.5)>H-beta(26)≈H-beta(13). Kinetic parameters (temperature, t -butanol/naphthalene ratio) influencing the reaction were investigated using HY zeolites. Higher temperatures than 160°C cause a decrease in 2,6-DTBN selectivity due to secondary reactions; large t -butanol/naphthalene ratio results in lower activity, possibly caused by counter diffusion effect. The results show that alkylation of naphthalene with t -butanol should be much practical attractive not only because of the high activity, 2,6-DTBN selectivity and relatively mild reaction conditions but also due to an easy separation of the desired product (2,6-DTBN) from the reaction mixtures by crystallization.

Diversion of the Metabolic Flux from Pyruvate Dehydrogenase to Pyruvate Oxidase Decreases Oxidative Stress during Glucose Metabolism in Nongrowing Escherichia coli Cells Incubated under Aerobic, Phosphate Starvation Conditions†

Ongoing aerobic metabolism in nongrowing cells may generate oxidative stress. It is shown here that the levels of thiobarbituric acid-reactive substances (TBARSs), which measure fragmentation products of oxidized molecules, increased strongly at the onset of starvation for phosphate (P(i)). This increase in TBARS levels required the activity of the histone-like nucleoid-structuring (H-NS) protein. TBARS levels weakly increased further in DeltaahpCF mutants deficient in alkyl hydroperoxide reductase (AHP) activity during prolonged metabolism of glucose to acetate. Inactivation of pyruvate oxidase (PoxB) activity decreased the production of acetate by half and significantly increased the production of TBARS. Overall, these data suggest that during incubation under aerobic, P(i) starvation conditions, metabolic flux is diverted from the pyruvate dehydrogenase (PDH) complex (NAD dependent) to PoxB (NAD independent). This shift may decrease the production of NADH and in turn the adventitious production of H(2)O(2) by NADH dehydrogenase in the respiratory chain. The residual low levels of H(2)O(2) produced during prolonged incubation can be scavenged efficiently by AHP. However, high levels of H(2)O(2) may be reached transiently at the onset of stationary phase, primarily because H-NS may delay the metabolic shift from PDH to PoxB.

Acetylation of dimethoxybenzenes with acetic anhydride in the presence of acidic zeolites

Abstract The acetylation of 1,2-, 1,3- and 1,4-dimethoxybenzenes with acetic anhydride has been investigated in the liquid phase (chlorobenzene as solvent) over the H-forms of various zeolites. H-Y and H-Beta have been shown to be efficient catalysts in such a reaction, and led to the selective formation of the corresponding dimethoxyacetophenones. 1,2-Dimethoxybenzene (veratrole) has been chosen as a model substrate for the kinetic study of the reaction over H-Y (Si/Al=15) as catalyst. The reaction proceeds through a modified Eley–Rideal type mechanism, wherein the chemisorbed acetic anhydride generates the electrophilic acylium ion, which then reacts with veratrole in the liquid phase. The competitive adsorption of both reactants and products has been evidenced. It has been especially shown that the deactivation of the catalyst was due to a partial adsorption on the active sites of the catalyst of either the reaction product (3′,4′-dimethoxyacetophenone in the case of 1,2-dimethoxybenzene as the substrate) or/and of acetic acid formed in the reaction.

Control of the LexA regulon by pH: evidence for a reversible inactivation of the LexA repressor during the growth cycle of Escherichia coli

The LexA repressor controls the expression of several genes, including lexA, recA, and sfiA, which are induced when exponentially growing bacteria are exposed to DNA‐damaging agents, Induction of this so‐called SOS response takes place while LexA is cleaved in a reaction that requires the RecA protein and damaged DNA. We have shown that large fluctuations in the cellular concentration of the LexA repressor and in the rate of transcription of the sfiA gene also occur spontaneously during bacterial growth in complex medium such as LB. The possibility that changes in external or internal pH may explain these fluctuations has been explored. A consistent pattern was established whereby conditions leading to either increased or decreased pH were associated with altered expression of the LexA and SfiA genes. These data can be explained by a model in which the LexA repressor exists in either of two forms in equilibrium: a form favoured at homeostatic internal pH, which has a low affinity for the operators of LexA‐controlled genes; and a form accumulated in response to a transient decrease in internal pH, which has a high affinity for operators.

Non‐growing Escherichia coli cells starved for glucose or phosphate use different mechanisms to survive oxidative stress‡

Recent data suggest that superoxide dismutases are important in preventing lethal oxidative damage of proteins in Escherichia coli cells incubated under aerobic, carbon starvation conditions. Here, we show that the alkylhydroperoxide reductase AhpCF (AHP) is specifically required to protect cells incubated under aerobic, phosphate (Pi) starvation conditions. Additional loss of the HP‐I (KatG) hydroperoxidase activity dramatically accelerated the death rate of AHP‐deficient cells. Investigation of the composition of spent culture media indicates that ΔahpCF katG cells leak nutrients, which suggests that membrane lipids are the principal target of peroxides produced in Pi‐starved cells. In fact, the introduction of various mutations inactivating repair activities revealed no obvious role for protein or DNA lesions in the viability of ahp cells. Because the death of ahp cells was directly related to ongoing aerobic glucose metabolism, we wondered how glycolysis, which requires free Pi, could proceed. 31P nuclear magnetic resonance spectra showed that Pi‐starved cells consumed Pi but were apparently able to liberate Pi from phosphorylated products, notably through the synthesis of UDP‐glucose. Whereas expression of the ahpCF and katG genes is enhanced in an OxyR‐dependent manner in response to H2O2 challenge, we found that the inactivation of oxyR and both oxyR and rpoS genes had little effect on the viability of Pi‐starved cells. In stark contrast, the inactivation of both oxyR and rpoS genes dramatically decreased the viability of glucose‐starved cells.

Role of Escherichia coli RpoS, LexA and H-NS global regulators in metabolism and survival under aerobic, phosphate-starvation conditions.

It has been suggested that Escherichia coli can resist aerobic, glucose-starvation conditions by switching rapidly from an aerobic to a fermentative metabolism, thereby preventing the production by the respiratory chain of reactive oxygen species (ROS) that can damage cellular constituents. In contrast, it has been reported that E. coli cannot resist aerobic, phosphate (Pi)-starvation conditions, probably because of the maintenance of an aerobic metabolism and the continuous production of ROS. This paper presents evidence that E. coli cells starved for Pi under aerobic conditions indeed maintain an active aerobic metabolism for about 3 d, which allows the complete degradation of exogenous nutrients such as arginine (metabolized probably to putrescine via the SpeA-initiated pathway) and glucose (metabolized notably to acetate), but cell viability is not significantly affected because of the protection afforded against ROS through the expression of the RpoS and LexA regulons. The involvement of the LexA-controlled RuvAB and RecA proteins with the RecG and RecBCD proteins in metabolism and cell viability implies that DNA double-strand breaks (DSB), and thus hydroxyl radicals that normally generate this type of damage, are produced in Pi-starved cells. It is shown that induction of the LexA regulon, which helps protect Pi-starved cells, is totally prevented by introduction of a recB mutation, which indicates that DSB are actually the main DNA lesion generated in Pi-starved cells. The requirement of RpoS for survival of cells starved for Pi may thus be explained by the role played by various RpoS-controlled gene products such as KatE, KatG and Dps in the protection of DNA against ROS. In the same light, the degradation of arginine and threonine may be accounted for by the synthesis of polyamines (putrescine and spermidine) that protect nucleic acids from ROS. Besides LexA and RpoS, a third global regulator, the nucleoid-associated protein H-NS, is also shown to play a key role in Pi-starved cells. Through a modulation of the metabolism during Pi starvation, H-NS may perform two complementary tasks: it helps maintain a rapid metabolism of glucose and arginine, probably by favouring the activity of aerobic enzymes such as the NAD-dependent pyruvate dehydrogenase complex, and it may enhance the cellular defences against ROS which are then produced by increasing RpoS activity via the synthesis of acetate and presumably homoserine lactone.