Unexpected tricovalent binding mode of boronic acids within the active site of a penicillin-binding protein

Boronic acids bearing appropriate side chains are good inhibitors of serine amidohydrolases. The boron usually adopts a tetrahedral conformation, bound to the nucleophilic serine of the active site and mimicking the transition state of the enzymatic reaction. We have solved the structures of complexes of a penicillin-binding protein, the DD-peptidase from Actinomadura sp. R39, with four amidomethylboronic acids (2,6-dimethoxybenzamidomethylboronic acid, phenylacetamidomethylboronic acid, 2-chlorobenzamidomethylboronic acid, and 2-nitrobenzamidomethylboronic acid) and the pinacol ester derived from phenylacetamidomethylboronic acid. We found that, in each case, the boron forms a tricovalent adduct with Oγ of Ser49, Ser298, and the terminal amine group of Lys410, three key residues involved in the catalytic mechanism of penicillin-binding proteins. This represents the first tricovalent enzyme-inhibitor adducts observed by crystallography. In two of the five R39-boronate structures, the boronic acid is found as a tricovalent adduct in two monomers of the asymmetric unit and as a monocovalent adduct with the active serine in the two remaining monomers of the asymmetric unit. Formation of the tricovalent complex from a classical monocovalent complex may involve rotation around the Ser49 Cα-Cβ bond to place the boron in a position to interact with Ser298 and Lys410, and a twisting of the side-chain amide such that its carbonyl oxygen is able to hydrogen bond to the oxyanion hole NH of Thr413. Biphasic kinetics were observed in three of the five cases, and details of the reaction between R39 and 2,6-dimethoxybenzamidomethylboronic acid were studied. Observation of biphasic kinetics was not, however, thought to be correlated to formation of tricovalent complexes, assuming that the latter do form in solution. On the basis of the crystallographic and kinetic results, a reaction scheme for this unexpected inhibition by boronic acids is proposed.     

Compacité faible dans l'espaceL E 1 et dans l'espace des multifonctions intégralement bornées,et minimisation (*).

Summary Some compactness results in the space of Bochner integrable functions in Banach spaces and in the space of integrably bounded multifunction with non empty convex weakly compact values are presented. Applications to minimization problems are given.

---

---

Pour terminer, nous remercions M.Valadier qui a lu attentivement ce papier.     

Thermodynamics of liquid mixtures of argon+ carbon monoxide

The heat of mixing of liquid argon (component 1) and liquid carbon monoxide has been measured by direct calorimetry for several mixtures of composition ranging from x₁ ? 0.2 to x₁ ? 0.8 at 85.79 K. At this temperature H^E is positive and an almost parabolic function of mole fraction, with a maximum value of H^E = 95.9 J mol^?1 at x₁ = 0.569.The experimental results are compared with those derived from the Gibbons-Rigby equation of state, and from conformal solution theory.     

Crystal structure of BRL 42715, C6-(N1-methyl-1,2,3-triazolylmethylene)penem, in complex with Enterobacter cloacae 908R beta-lactamase: evidence for a stereoselective mechanism from docking studies

BRL 42715, C6-(N1-methyl-1,2,3-triazolylmethylene)penem, is an active-site-directed inactivator of bacterial beta-lactamases. The crystal structure of Enterobacter cloacae 908R class C beta-lactamase in complex with BRL 42715, docking, and energy minimization studies explain stereoselectivity of the binding of C6-(heterocyclic methylene)penems against class C beta-lactamase.     

Preparation and certification of a reference material for the determination of nutrients in seawater

There is an urgent need for natural water reference materials certified for nutrients. In 1996, NRC collected seawater for a proposed CRM at a depth of 200 m in the North Atlantic; this was immediately filtered through 0.05-m cartridge filters into 50-L carboys. The water was later homogenized in the NRC laboratories in Ottawa and stabilized via gamma irradiation. Over six years of stability testing no significant deterioration was detected. In addition to the usual customary standard colorimetric procedures, alternative analytical methods were developed to enable the certification process. The production of a CRM called MOOS-1 will be discussed. Certified values, with uncertainty components addressing the homogeneity, stability, and characterization of the material, were calculated to be: orthophosphate=1.56±0.07 µmol L^–1, silicate=26.0±1.0 µmol L^–1, nitrite=3.06±0.15 µmol L^–1, and nitrite and nitrate=23.7±0.9 µmol L^–1.     

A fluorine-substituted hexakisdecyloxy- hexa-peri-hexabenzocoronene

[structure: see text] The first fluorine-substituted hexabenzocoronene has been synthesized and is much more readily reduced than its nonfluorinated analogues. Thin films of this material show columnar order at room temperature, and the charge-carrier mobility measured by the pulse-radiolysis time-resolved microwave conductivity technique is comparable to that of other hexabenzocoronene materials.     

Nanomaterials and catalysis for green chemistry

1. Download : Download high-res image (245KB)
2. Download : Download full-size image

     

Le microdosage de l'azote amidé etα aminé par la méthode aux acides chlorhydrique et nitrique concentrés

Résumé L'étude de la réaction du mélange acides chlorhydrique et nitrique concentrés sur les acides-monoaminés monocarboxylés a montré que celle-ci s'effectue d'après le schéma: R-CHNH₂-COOH + HCl + HNO₃ = R-CHCl-COOH + N₂O + 2 H₂O.L'étude quantitative de cette réaction a, en particulier, montré que le volume de gaz recueilli correspond rigoureusement à l'azote aminé de la molécule. Cette constatation nous a permis, lorsque la molécule n'est pas susceptible de donner des réactions secondaires avec l'acide nitrique, de développer à l'échelle micro une méthode simple et précise de dosage de l'azote aminé des acides-aminés et des acides-aminosulfonique ainsi que de l'azote amidé des amides et des sulfonamides.

---

Summary The study of the reaction of a mixture of concentrated hydrochloric acid and nitric acid on the mono-carboxylated monamines has shown that it proceeds: R—CHNH₂—COOH + HCl + HNO₃ = R—CHCl—COOH + N₂O + 2 H₂O.The quantitative study of this reaction has proved, in particular, that the volume of gas collected corresponds rigorously to the amine nitrogen of the molecule. This finding has made it possible for the authors, provided the molecule is not susceptible to yielding secondary reactions with nitric acid, to develop on the microscale, a simple and precise method of determining the amine nitrogen of amino acids and aminosulfonic acids, as well as the amide nitrogen of amides and sulfonamides.

---

Zusammenfassung Das Studium der Umsetzung eines Gemisches von konzentrierter Salzsäure und Salpetersäure mit-Monoaminomonocarbonsäuren ergab die Erkenntnis folgenden Reaktionsablaufes: R—CHNH₂—COOH + HCl + HNO₃ = R—CHCl—COOH + N₂O + 2 H₂O. Die quantitative Untersuchung dieser Reaktion ergab im besonderen, daß das erhaltene Gasvolumen dem vorhandenen Aminostickstoff genau entspricht. Sofern die Substanz keiner Sekundärreaktion mit Salpetersäure unterliegt, ermöglichte diese Feststellung die Ausarbeitung einer einfachen und genauen Mikrobestimmung des Aminostickstoffes in-Aminosäuren und-Aminosulfonsäuren und ebenfalls des Amidostickstoffes in Amiden und Sulfonamiden.

---

---

Actuellement chargé de cours à l'Institut Agronomique de l'Etat à Gembloux.

---

Avec 1 figure.