Guido Sello

Silver as a powerful electrocatalyst for organic halide reduction: The critical role of molecular structure

Abstract The remarkable electrocatalytic properties of silver for organic halide reductions, related to its strong specific interactions with halide ions, and therefore modulated by the surface state and by the nature of the supporting electrolyte, have been shown by us recently. The key role played by the molecular structure is now described together with its effect on the reaction pathway, in terms of not only the intrinsic R⋯X reactivity modification but also of elements connected to the heterogeneous nature of the process, including the accessibility of the leaving group and the possible presence of adsorption auxiliary groups stabilizing the postulated R⋯X⋯Me intermediate. The present discussion is supported by: (a) cyclovoltammetric investigations on a wide set of aliphatic and aromatic halides performed on silver, mercury and glassy carbon; and (b) a systematic program of preparative electroreductions carried out on variously configurated haloadamantanes in different operating conditions. The haloadamantane case, yielding a mixture of adamantane and dimers in a ratio heavily affected by the operating conditions, is very appropriate for an elucidation of the factors favouring monoelectronic dimerisation versus bielectronic halogen replacement by hydrogen.

Electrocatalytic potentialities of silver as a cathode for organic halide reductions

The peculiar halide affinity for silver results in an extraordinary electrocatalytic activity for the reduction of halides (either glycosyl halides or, more generally, aryl and alkyl halides). The most striking features are: (a) a reduction potential shift in the positive direction of about 1000 mV with respect to glassy carbon and 500 mV with respect to mercury; (b) a cage effect, evidenced in previous synthetic work concerning bromosugars, promoted by the halide acting as a bridge between the electrode surface and the reacting substrate, which mainly results in dimerization and/or addition products. The above electrocatalytic effect is here investigated by means of a systematic reactivity study on Ag, Hg and glassy carbon cathodes, with a variety of substrates. The effect of the supporting electrolyte is also analysed in detail, providing a first inspection on specific halide/silver interactions in acetonitrile media.

Organization and Regulation of meta Cleavage Pathway Genes for Toluene and o-Xylene Derivative Degradation in Pseudomonas stutzeri OX1

ABSTRACT Pseudomonas stutzeri OX1 meta pathway genes for toluene and o-xylene catabolism were analyzed, and loci encoding phenol hydroxylase, catechol 2,3-dioxygenase, 2-hydroxymuconate semialdehyde dehydrogenase, and 2-hydroxymuconate semialdehyde hydrolase were mapped. Phenol hydroxylase converted a broad range of substrates, as it was also able to transform the nongrowth substrates 2,4-dimethylphenol and 2,5-dimethylphenol into 3,5-dimethylcatechol and 3,6-dimethylcatechol, respectively, which, however, were not cleaved by catechol 2,3-dioxygenase. The identified gene cluster displayed a gene order similar to that of thePseudomonas sp. strain CF600 dmp operon for phenol catabolism and was found to be coregulated by thetou operon activator TouR. A hypothesis about the evolution of the toluene and o-xylene catabolic pathway inP. stutzeri OX1 is discussed.

Styrene lower catabolic pathway in Pseudomonas fluorescens ST: identification and characterization of genes for phenylacetic acid degradation

Pseudomonas fluorescens ST is a styrene degrading microorganism that, by the sequential oxidation of the vinyl side chain, converts styrene to phenylacetic acid. The cluster of styrene upper pathway catabolic genes (sty genes) has been previously localized on a chromosomal region. This report describes the isolation, sequencing and analysis of a new chromosomal fragment deriving from the ST strain genomic bank that contains the styrene lower degradative pathway genes (paa genes), involved in the metabolism of phenylacetic acid. Analysis of the paa gene cluster led to the description of 14 putative genes: a gene encoding a phenylacetyl-CoA ligase (paaF), the enzyme required for the activation of phenylacetic acid; five ORFs encoding the subunits of a ring hydroxylation multienzymatic system (paaGHIJK); the gene paaW encoding a membrane protein of unknown function; five genes for a β-oxidation-like system (paaABCDE), involved in the steps following the aromatic ring cleavage; a gene encoding a putative permease (paaL) and a gene (paaN) probably involved in the aromatic ring cleavage. The function of some of the isolated genes has been proved by means of biotransformation experiments.

Characterization of Rhodococcus opacus R7, a strain able to degrade naphthalene and o-xylene isolated from a polycyclic aromatic hydrocarbon-contaminated soil

Rhodococcus opacus R7 was isolated from a soil contaminated with polycyclic aromatic hydrocarbons for its ability to grow on naphthalene. The strain was also able to degrade o-xylene, the isomer of xylenes most recalcitrant to microbial degradation. The catabolic pathways for naphthalene and o-xylene were investigated by identification of metabolites in R. opacus R7 cultures performed with the two hydrocarbons and by evaluation of some enzymes involved in the metabolism of these compounds. 1,2-Dihydro-1,2-dihydroxynaphthalene, salicylic and gentisic acids were identified as metabolites in cultures exposed to naphthalene. This suggests that the degradation occurs through the dioxygenation of the aromatic ring with the formation of 1,2-dihydro-1,2-dihydroxynaphthalene, dehydrogenated to the corresponding 1,2-dihydroxy derivative which is further oxidized to salicylic acid, a key intermediate of naphthalene metabolism; this compound is converted to gentisic acid cleaved by a gentisate 1,2-dioxygenase. From R. opacus R7 cultures supplied with o-xylene, 2,3-dimethylphenol and 3,4-dimethylcatechol were observed. The pathway of o-xylene involves the monooxygenation of the benzene nucleus leading to dimethylphenol which is further metabolised to 3,4-dimethylcatechol, followed by a meta cleavage reaction, catalyzed by the catechol 2,3-dioxygenase. R. opacus R7 is the first strain thus far described both in Gram-negative and Gram-positive bacteria which has the ability to degrade both a polycyclic aromatic hydrocarbon such as naphthalene and a monocyclic aromatic hydrocarbon such as o-xylene.

Bioconversion of substituted naphthalenes to the corresponding 1,2-dihydro-1,2-dihydroxy derivatives. Determination of the regio- and stereochemistry of the oxidation reactions

A mutant (TTC1) derived from Pseudomonas fluorescens N3 has been obtained for use in the bioconversion of several naphthalene derivatives to the corresponding optically active cis-dihydrodiols on a milligrams-to-grams scale. All main compounds have been characterized, their relative and absolute configuration assigned, and their enantiomeric purity determined. The regio- and stereoselectivity of the transformation has been established. The procedure therefore represents a valid method for the convenient preparation of a pool of valuable chiral syntons and auxiliaries.

Bioconversion of substituted styrenes to the corresponding enantiomerically pure epoxides by a recombinant Escherichia coli strain

Abstract Enantiomerically pure epoxides are produced by bioconversion of the corresponding styrenes using a recombinant Escherichia coli strain containing the styrene monooxygenase gene cloned from Pseudomonas fluorescens ST. Different procedures were used to optimise yields and to permit product isolation. Conversion rates depend on the position and nature of the styrene substituent.

Glycosyl Halides as Building Blocks for the Electrosynthesis of Glycosides

The importance and variety of glycosides in nature together with the lack of a conclusive synthetic methodology prompted us to study the electrochemical reductive behavior of glycosyl halides to generate suitable intermediates for the glycosidation of both aliphatic and aromatic acceptors. A special case considered here is the almost quantitative synthesis of C-C glycosyl dimers.

Production of substituted naphthalene dihydrodiols by engineered Escherichia coli containing the cloned naphthalene 1,2-dioxygenase gene from Pseudomonas fluorescens N3

Naphthalene dioxygenase, a key enzyme in the dihydroxylation of naphthalene, is encoded by the plasmid pN3, responsible for naphthalene metabolism in Pseudomonas fluorescens N3. The naphthalene dioxygenase, including all the sequences for its expression and the regulatory region, has been localized on the 4.3-kb HindIII-ClaI fragment and on the 3.5-kb HindIII fragment of the plasmid pN3, by Southern analysis using as probes nahA and nahR genes, the homologous genes of the plasmid NAH7 from Pseudomonas putida G7. We cloned in Escherichia coli JM109 the dioxygenase gene and its regulatory region and developed an efficient bacterial system inducible by salicylic acid, able to produce dihydrodiols. E. coli containing recombinant plasmids carrying the dioxygenase gene were analysed for their potential as a biocatalytic tool to produce dihydrodiols from different naphthalenes with the substituent on the aromatic ring at the alpha or beta position. The dihydrodiols, identified by HPLC (high-performance liquid chromatography) and 1H-NMR (nuclear magnetic resonance) were produced with yields ranging from 50 to 94%. The degree of bioconversion efficiency depends on the nature and the position of the substituent and indicates the broad substrate specificity of this dioxygenase and its potential for the production of a wide variety of fine chemicals.

Cathode and medium effects on the electroreductive glucosidation of phenols

The electroreductive pathway to phenol glucosidation, recently introduced by our research group, is analysed here in detail for both mechanism elucidation and choice of operating conditions. Preparative electrosyntheses were carried out on model substrates, varying either the cathode material, the supporting electrolyte, and/or the potential/current electrolysis conditions, to study their effects on the glucosidation yields and stereochemistry. Special care was devoted to the analysis of the reaction mixtures, leading to the identification and characterisation of several new products.