Yohan Chan

Molecular dissection of bacterial acrylate catabolism – unexpected links with dimethylsulfoniopropionate catabolism and dimethyl sulfide production

A bacterium in the genus Halomonas that grew on dimethylsulfoniopropionate (DMSP) or acrylate as sole carbon sources and that liberated the climate-changing gas dimethyl sulfide in media containing DMSP was obtained from the phylloplane of the macroalga Ulva. We identified a cluster that contains genes specifically involved in DMSP catabolism (dddD, dddT) or in degrading acrylate (acuN, acuK) or that are required to break down both substrates (dddC, dddA). Using NMR and HPLC analyses to trace 13C- or 14C-labelled acrylate and DMSP in strains of Escherichia coli with various combinations of cloned ddd and/or acu genes, we deduced that DMSP is imported by the BCCT-type transporter DddT, then converted by DddD to 3-OH-propionate (3HP), liberating dimethyl sulfide in the process. As DddD is a predicted acyl CoA transferase, there may be an earlier, unidentified catabolite of DMSP. Acrylate is also converted to 3HP, via a CoA transferase (AcuN) and a hydratase (AcuK). The 3HP is predicted to be catabolized by an alcohol dehydrogenase, DddA, to malonate semialdehyde, thence by an aldehyde dehydrogenase, DddC, to acyl CoA plus CO2. The regulation of the ddd and acu genes is unusual, as a catabolite, 3HP, was a co-inducer of their transcription. This first description of genes involved in acrylate catabolism in any organism shows that the relationship between the catabolic pathways of acrylate and DMSP differs from that which had been suggested in other bacteria.

Dimethylsulfoniopropionate biosynthesis in marine bacteria and identification of the key gene in this process

Dimethylsulfoniopropionate (DMSP) is one of the Earths most abundant organosulfur molecules, a signalling molecule1, a key nutrient for marine microorganisms2,3 and the major precursor for gaseous dimethyl sulfide (DMS). DMS, another infochemical in signalling pathways4, is important in global sulfur cycling2 and affects the Earths albedo, and potentially climate, via sulfate aerosol and cloud condensation nuclei production5,6. It was thought that only eukaryotes produce significant amounts of DMSP7–9, but here we demonstrate that many marine heterotrophic bacteria also produce DMSP, probably using the same methionine (Met) transamination pathway as macroalgae and phytoplankton10. We identify the first DMSP synthesis gene in any organism, dsyB, which encodes the key methyltransferase enzyme of this pathway and is a reliable reporter for bacterial DMSP synthesis in marine Alphaproteobacteria. DMSP production and dsyB transcription are upregulated by increased salinity, nitrogen limitation and lower temperatures in our model DMSP-producing bacterium Labrenzia aggregata LZB033. With significant numbers of dsyB homologues in marine metagenomes, we propose that bacteria probably make a significant contribution to oceanic DMSP production. Furthermore, because DMSP production is not solely associated with obligate phototrophs, the process need not be confined to the photic zones of marine environments and, as such, may have been underestimated.

Asymmetric Epoxidation Using Iminium Salt Organocatalysts Featuring Dynamically Controlled Atropoisomerism

Introduction of a pseudoaxial substituent at a stereogenic center adjacent to the nitrogen atom in binaphthyl- and biphenyl-derived azepinium salt organocatalysts affords improved enantioselectivities and yields in the epoxidation of unfunctionalized alkenes. In the biphenyl-derived catalysts, the atropoisomerism at the biphenyl axis is controlled by the interaction of this substituent with the chiral substituent at nitrogen.

Enantioselective Total Synthesis of (+)-Scuteflorin A Using Organocatalytic Asymmetric Epoxidation

We report the first enantioselective total synthesis of (+)-scuteflorin A in 14% overall yield, employing a chiral iminium salt to effect an organocatalytic asymmetric epoxidation of xanthyletin in >99% ee as the key step.

Kinetic Resolution in Asymmetric Epoxidation using Iminium Salt Catalysis

The first reported examples of kinetic resolution in epoxidation reactions using iminium salt catalysis are described, providing up to 99% ee in the epoxidation of racemic cis-chromenes.

A gold-gold oil microtrench electrode for liquid-liquid anion transfer voltammetry

Two flat gold electrodes are placed vis‐à‐vis with an epoxy spacer layer that is etched out to give a ca. 100 μm‐deep electrochemically active trench. A water‐insoluble oil phase, here the redox system N,N‐diethyl‐N′N′‐didodecyl‐phenylenediamine (DDPD) in 4‐(3‐phenylpropyl)‐pyridine (PPP), is immobilized into the trench to allow anion transfer upon oxidation of DDPD (oil) to DDPD+ (oil). In “mono‐potentiostatic mode” quantitative transfer/expulsion of anions into the trench oil phase occurs. However, in “bi‐potentiostatic mode” feedback currents dominated by rapid plate‐to‐plate diffusion normal to the electrode surfaces are observed. Comparison of “normal” diffusion and “lateral” diffusion shows that the rate of diffusion–migration charge transport across the oil film is anion hydrophobicity dependent.

Liquid-liquid ion transport junctions based on paired gold electrodes in generator-collector mode.

Simultaneous electrochemically driven double anion transfer across liquid–liquid interfaces is demonstrated at a gold–gold junction electrode. In the presence of two closely spaced electrodes (generator and collector), anion uptake into the organic phase (oxidation) and anion expulsion into the aqueous phase (reduction) can be combined to result in a generator–collector anion transport system across the liquid–liquid interface. In this report we are employing a paired gold junction grown by electro‐deposition to ca. 5 μm gap size with the N,N‐diethyl‐N′,N′‐didodecyl‐phenylene‐diamine water immiscible redox liquid immobilized into the gap to demonstrate simultaneous perchlorate anion uptake and expulsion. The effects of redox liquid volume and scan rate on the magnitude of currents and two mechanistic pathways for ion transport are discussed in the context of micro‐electrophoretic processes.

Atropo- and Diastereoselective Construction of Tetracyclic Biphenylazepinium Salts Derived from Aminoalcohols: Use as Catalysts in Enantioselective Asymmetric Epoxidation

A range of new biphenylazepinium salt organocatalysts effective for asymmetric epoxidation has been developed incorporating an additional substituted oxazolidine ring, and providing improved enantiocontrol in alkene epoxidation over the parent structure. Starting from enantiomerically pure aminoalcohols, tetracyclic iminium salts were obtained as single diastereoisomers through an atroposelective oxazolidine formation.

New Non-Toxic Semi-Synthetic Derivatives from Natural Diterpenes Displaying Anti-Tuberculosis Activity

We report herein the synthesis of six diterpene derivatives, three of which are new, generated through known organic chemistry reactions that allowed structural modification of the existing natural products kaurenoic acid (1) and copalic acid (2). The new compounds were fully characterized using high resolution mass spectrometry, infrared spectroscopy, 1H- and 13C-NMR experiments. We also report the evaluation of the anti-tuberculosis potential for all compounds, which showed some promising results for Micobacterium tuberculosis inhibition. Moreover, the toxicity for each of the most active compounds was also assessed.

Formal total synthesis of (+)-C9-deoxyomuralide from L-leucine using a double sacrificial chirality transfer approach

Formal stereocontrolled syntheses of (±)- and (+)-C9-deoxyomuralide is reported, constituting one of the shortest routes to the full carbon skeleton reported to date.