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The cold-adapted cellulase CelG has been purified from the culture supernatant of the Antarctic bacterium Pseudoalteromonas haloplanktis and the gene coding for this enzyme has been cloned, sequenced and expressed in Escherichia coli . This cellulase is composed of three structurally and functionally distinct regions: an N-terminal catalytic domain belonging to glycosidase family 5 and a C-terminal cellulose-binding domain belonging to carbohydrate-binding module family 5. The linker of 107 residues connecting both domains is one of the longest found in cellulases, and optimizes substrate accessibility to the catalytic domain by drastically increasing the surface of cellulose available to a bound enzyme molecule. The psychrophilic enzyme is closely related to the cellulase Cel5 from Erwinia chrysanthemi . Both k cat and k cat / K m values at 4 °C for the psychrophilic cellulase are similar to the values for Cel5 at 30–35 °C, suggesting temperature adaptation of the kinetic parameters. The thermodynamic parameters of activation of CelG suggest a heat-labile, relatively disordered active site with low substrate affinity, in agreement with the experimental data. The structure of CelG has been constructed by homology modelling with a molecule of cellotetraose docked into the active site. No structural alteration related to cold-activity can be found in the catalytic cleft, whereas several structural factors in the overall structure can explain the weak thermal stability, suggesting that the loss of stability provides the required active-site mobility at low temperatures.

By using modifiers in platinum catalysts it is possible to modulate the hydrogenation selectivity in α , β -unsaturated aldehydes carbonyl bond with regard to olefinic bond hydrogenation. The challenge is to find an emergent catalytic material able to produce 100% unsaturated alcohol. In this field, 1% Pt/ZnO catalysts were tested in hydrogenation of crotonaldehyde, in gas phase, at atmospheric pressure. The catalysts were prepared by impregnation of ZnO from two different precursors, H 2 PtCl 6 and Pt(NH 3 ) 4 (NO 3 ) 2 , followed by calcination at 673 K and tested after reduction at different temperatures, 473–773 K. The ex-nitrate and ex-chloride catalysts showed quite different catalytic behaviors. The ex-chloride was much more active and selective; in certain conditions 90% crotyl alcohol selectivity was reached. Different techniques (TPR, XRD, TEM, XPS, FTIR) were used to characterize both types of catalysts and to correlate the structure and the reactivity. The Lewis acidity of ZnO after the reduction treatment and the formation of PtZn alloy were found to be the key factors leading to high crotyl alcohol yield. It was proposed that modification of the electronic properties of platinum both by alloying to zinc and by Lewis acidity of the support reinforced by chlorine changes the adsorption mode of crotonaldehyde by favoring the binding of the terminal oxygen of the molecule to the electronically modified platinum atom.

Surface properties of unsupported MoS 2 have been studied by means of IR spectroscopy of a wide range of adsorbed molecules. After contact with air, surface of MoS 2 is partially oxidised and contains adsorbed water, MoO groups, sulphate and thiosulphate-like anions. These species are removable by treatment in vacuum or in H 2 S–H 2 mixture. On MoS 2 , only molecular adsorption was established for SO 2 , CO 2 , COS and CH 3 SH showing weak basicity of surface sulphur ions. Unsupported MoS 2 presents also very weak acidic properties, as revealed by pyridine, 2,6-dimethylpyridine and ethylene adsorption. However, ammonia could be protonated or coordinately bound to MoS 2 surface. CO and NO interact with the coordinately unsaturated Mo sites of the sulphide, providing evidence for an important back donation effect. Comparison with surface properties of supported MoS 2 points to the lower acidic and higher metallic character of Mo sites of the unsupported sulphide.

Le traitement d'une zeolithe NH 4 EMT par une solution de (NH 4 ) 2 SiF 6 fournit un solide comportant une phase SiO 2 amorphe intimement melangee aux parties intactes de la zeolithe. Sur ce support acide EMT-Si, on a greffe un sulfure de NiMo afin de preparer un catalyseur bifonctionnel qui a ete teste dans les reactions d'hydrogenation du benzene et d'hydrocraquage du n-heptane. Ce catalyseur NiMo/EMT-Si s'avere plus actif en hydrogenation que son analogue NiMo/HY, en raison d'une meilleure dispersion du sulfure de NiMo. Sur le solide EMT modifie, le sulfure se repartit a peu pres egalement entre les mesopores internes et les fissures crees dans les grains de zeolithe lors de l'insertion du sulfure de NiMo. Au contraire sur le support Y, une partie du sulfure est externe aux grains de zeolithe et inactive en catalyse. Le catalyseur NiMo/EMT-Si est aussi trouve plus actif que le NiMo/HY en hydrocraquage du n-heptane, et un peu plus selectif en isomeres.

The lipase from the Antarctic psychrophilic bacterium Psychrobacter immobilis B10 shows a very limited thermal stability when compared to the lipase from a mesophilic strain of Pseudomonas aeruginosa . The thermal dependence of its activity is shifted by at least 30 °C towards low temperatures and its activation energy is reduced by a factor of 2. The three-dimensional model of the P. immobilis lipase reveals several features typical of cold-adapted enzymes: a very low proportion of arginine residues as compared to lysines, a low content in proline residues, a small hydrophobic core, a very small number of salt bridges and of aromatic-aromatic interactions. All these properties should confer on the enzyme a more flexible structure, in accord with its low activation energy and its low thermal stability.

Sous helium, le benzaldehyde se transforme selectivement en alcool benzylique par une reaction de type Cannizzaro. Les activites observees en debut de reaction dependent uniquement du nombre d'hydroxyles basiques forts initiaux et conduisent au classement suivant : a(MgO) » a(ZrO 2 ) > a(ZnO) O En presence de H 2 , la transformation de l'aldehyde benzylique en alcool benzylique est une reaction catalytique, elle peut etre decrite par deux schemas reactionnels en competition. Le premier necessite une dissociation reversible du dihydrogene et semble lie a la reductibilite de l'oxyde, le second prevoit la liberation de certains sites benzoates et la regeneration d'hydroxyles basiques forts. Le classement suivant les activites decroissantes presente precedemment est completement inverse: a(ZnO) » a(ZrO 2 ) > a(MgO) = 0.

Two zeolites were coked using cyclohexene at 620 K; one had a low Si/Alf ratio (HY), the other was ultrastabilized (HYS). It was found that oxygen regeneration occurred at 770 K or above, whereas ozone was effective at 450 K or less,e.g. under conditions such that the HY sample was still thermally stable.

Serine beta-lactamases contribute widely to the beta-lactam resistance phenomena. Unfortunately, the intimate details of their catalytic mechanism remain elusive and subject to some controversy even though many "natural" and "artificial" mutants of these different enzymes have been isolated. This paper is essentially focused on class C beta-lactamases, which contain a Tyr (Tyr150) as the first residue of the second conserved element, in contrast to their class A counterparts, in which a Ser is found in the corresponding position. We have modified this Tyr residue by site-directed mutagenesis. On the basis of the three-dimensional structure of the Enterobacter cloacae P99 enzyme, it seemed that residues Glu272 and His314 might also be important. They were similarly substituted. The modified enzymes were isolated and their catalytic properties determined. Our results indicated that His314 was not required for catalysis and that Glu272 did not play an important role in acylation but was involved to a small extent in the deacylation process. Conversely, Tyr150 was confirmed to be central for catalysis, at least with the best substrates. On the basis of a comparison of data obtained for several class C enzyme mutants and in agreement with recent structural data, we propose that the phenolate anion of Tyr150, in conjunction with the alkyl ammonium of Lys315, acts as the general base responsible for the activation of the active-site Ser64 during the acylation step and for the subsequent activation of a water molecule in the deacylation process. The evolution of the important superfamily of penicillin-recognizing enzymes is further discussed in the light of this proposed mechanism.

FT i.r. spectroscopy has been applied in studying the reaction between methanol and hydrogen sulfide on NaX and NaY zeolites. Hydrogen-bonded methanol and methoxy and carbonate species were found on both of the zeolites after methanol adsorption. Pyrrole and pyridine preadsorption experiments have shown that methanol is chemisorbed on basic and/or cationic sites on the zeolites used. H 2 S is adsorbed on the same sites. In the reaction between methanol and hydrogen sulfide on sodium zeolites methoxy species or HS − ions are involved. Carbonate species can also react with H 2 S producing hydrogenthiocarbonates or thiocarbonates. However, this reaction does not play any role in the interaction between CH 3 OH and H 2 S under the conditions used.

Adsorption of CH3SH and CH3OH on activated NaX and NaY zeolites has been studied by IR spectroscopy. Only NaX is able to provoke the dissociative adsorption of CH3SH at room temperature and that of CH3OH at 623 K. Results are discussed taking into account sodium cations in the SIII position.

Carbon monoxide hydrogenation over ceria-promoted Rh or Rh/SiO{sub 2} catalysts was investigated by means of Fourier transform infrared spectroscopy. In the chemisorption of CO at room temperature, a new absorption band was observed near 1,725 or 1,696 cm{sup {minus}1} on a reduced 5% Rh-5% CeO{sub 2}/SiO{sub 2} or 5% Rh/CeO{sub 2} catalyst, respectively. This new absorption band was attributed to a C- and O-bonded species, located at the metal/support interface. The appearance of this species was related to the shift to lower temperatures of the peak ascribed to CO dissociation in the temperature-programmed desorption spectra after CO adsorption. A flow high-pressure IR cell allowed to study the surface species in situ during CO + H{sub 2} reaction and to follow the reactivity of these species toward pure H{sub 2}. The results suggest that the drop in methane formation may be related to a lower reactivity of surface carbon in the presence of the promoter.

The reactivity towards O3 of the different species formed from phenol adsorption on silica and ceria has been studied using IR spectroscopy by increasing the temperature from 77 K. It clearly appeared that on silica H-bonded phenol species were more reactive than phenoxy species, whereas on ceria monodentate phenoxy species were more active than bidentate species. Intermediate products of phenol oxidation were characterized by bands between 1800 and 1600 cm−1, that at 1750 cm−1 being probably due to the formation of a carboxylic acid.

The catalytic properties of six "natural" mutants of the TEM-1 beta-lactamase have been studied in detail, with special emphasis on their activity versus third-generation cephalosporins. On the basis of the recently determined high-resolution structure of the wild-type enzyme, and of the substrates' structures optimized by the AMI quantum chemistry method, we have attempted to explain the influences of the mutations on the substrate profiles of the enzymes. Some of the kinetic results have thus received a satisfactory, semi-quantitative interpretation, especially in the case of single mutations. Analysis of the double mutants proved more hazardous. Extending the comparison to some other class A beta-lactamases showed that similar properties could result from different sequences, supplying an interesting example of convergent evolution within a generally diverging family.

Beta-lactamases are responsible for resistance to penicillins and related beta-lactam compounds. Despite numerous studies, the identity of the general base involved in the acylation step is still unclear. It has been proposed, on the basis of a previous pKa calculation and analysis of structural data, that the unprotonated Lys73 in the active site could act as the general base. Using a continuum electrostatic model with an improved treatment of the multiple titration site problem, we calculated the pKa values of all titratable residues in the substrate-free TEM-1 and Bacillus licheniformis class A beta-lactamases. The pKa of Lys73 in both enzymes was computed to be above 10, in good agreement with recent experimental data on the TEM-1 beta-lactamase, but inconsistent with the proposal that Lys73 acts as the general base. Even when the closest titratable residue, Glu166, is mutated to a neutral residue, the predicted downward shift of the pKa of Lys73 shows that it is unlikely to act as a proton abstractor in either enzyme. These results support a mechanism in which the proton of the active Ser70 is transferred to the carboxylate group of Glu166.