Benjamin Siegler

Role of mannitol metabolism in the pathogenicity of the necrotrophic fungus Alternaria brassicicola

In this study, the physiological functions of fungal mannitol metabolism in the pathogenicity and protection against environmental stresses were investigated in the necrotrophic fungus Alternaria brassicicola. Mannitol metabolism was examined during infection of Brassica oleracea leaves by sequential HPLC quantification of the major soluble carbohydrates and expression analysis of genes encoding two proteins of mannitol metabolism, i.e., a mannitol dehydrogenase (AbMdh), and a mannitol-1-phosphate dehydrogenase (AbMpd). Knockout mutants deficient for AbMdh or AbMpd and a double mutant lacking both enzyme activities were constructed. Their capacity to cope with various oxidative and drought stresses and their pathogenic behavior were evaluated. Metabolic and gene expression profiling indicated an increase in mannitol production during plant infection. Depending on the mutants, distinct pathogenic processes, such as leaf and silique colonization, sporulation, survival on seeds, were impaired by comparison to the wild-type. This pathogenic alteration could be partly explained by the differential susceptibilities of mutants to oxidative and drought stresses. These results highlight the importance of mannitol metabolism with respect to the ability of A. brassicicola to efficiently accomplish key steps of its pathogenic life cycle.

Synthesis and conformation of a novel fluorescein–Zn-porphyrin dyad and intramolecular energy transfer

This study describes the synthesis and characterization of a new zinc porphyrin–fluorescein dyad, the two chromophoric units being covalently linked by a 1,2,3-triazole bridge. The latter was formed by Cu-catalyzed Huisgen 1,3-dipolar cycloaddition. The conformational analysis of this dyad (1) was performed by NOESY experiments, suggesting interactions between moieties; density functional theory (DFT) calculations confirmed clear evidence of a folded conformer, which is stabilized by electrostatic and CH–π interactions. Photophysical measurements demonstrated solvent-dependent energy transfer, with an efficiency of about 40%.

Bromination of the benzothioxanthene Bloc: toward new π-conjugated systems for organic electronic applications

A selective and efficient method to afford a monobrominated benzothioxanthene (Br-BTXI) derivative is reported. Br-BTXI was extensively employed in common palladium catalyzed coupling reactions. Finally, as a proof of concept, a BTXI based molecular donor was synthesized and evaluated in bulk heterojunction solar cells.

The secreted polyketide boydone A is responsible for the anti-Staphylococcus aureus activity of Scedosporium boydii.

&NA; Usually living as a soil saprophyte, the filamentous fungus Scedosporium boydii may also cause various infections in human. Particularly, it is one of the major causative agents of fungal colonization of the airways in patients with cystic fibrosis (CF). To compete with other microorganisms in the environment, fungi have evolved sophisticated strategies, including the production of secondary metabolites with antimicrobial activity that may also help them to establish successfully within the respiratory tract of receptive hosts. Here, the culture filtrate from a human pathogenic strain of S. boydii was investigated searching for an antibacterial activity, mainly against the major CF bacterial pathogens. A high antibacterial activity against Staphylococcus aureus, including methicillin‐resistant strains of this species, was observed. Bio‐guided fractionation and analysis of the active fractions by nuclear magnetic resonance or by high‐performance liquid chromatography and high‐resolution electrospray ionization‐mass spectrometry allowed us to identify boydone A as responsible for this antibacterial activity. Together, these results suggest that this six‐membered cyclic polyketide could be one of the virulence factors of the fungus. Genes involved in the synthesis of this secreted metabolite are currently being identified in order to confirm the role of this polyketide in pathogenesis.

NMR diffusometry data sampling optimization for mixture analysis

HighlightsOptimization of acquisition parameters for NMR diffusometry mixture analysis.Optimal signal decay sampling prediction tool adapted to mixture composition.Validation on unimer/micelle or liposomal drug mixtures. ABSTRACT NMR diffusometry is a powerful but challenging method to analyze complex mixture. Each component diffuses differently, from the faster small species to the slower large species, corresponding to different signal attenuation. However, the method is highly sensitive to the quality of the acquired data and the performance of the processing used to resolve multiexponential signals influences. Adapting the signal decay sampling to the mixture composition is one way to improve the precision of the measure. In this work, we propose a prediction tool, based on the calculation of the Cramér‐Rao lower bound to minimize the variance of diffusion coefficient estimation in order to determine the optimal number of diffusion gradient steps, the best diffusion gradient sampling (among linear, exponential, quadratic and sigmoidal ones) and the optimal maximum diffusion factor. The tool was validated experimentally on a unimer/micelle solution of sodium dodecyl sulfate and on Caelyx®, a commercial liposomal preparation containing a mixture of pegylated‐liposomes and sucrose.

13C-NMR dereplication of Garcinia extracts: Predicted chemical shifts as reliable databases

Usually isolated from Garcinia (Clusiaceae) or Hypericum (Hypericaceae) species, some Polycyclic Polyprenylated AcylPhloroglucinols (PPAPs) have been recently reported as potential research tools for immunotherapy. Aiming at exploring the chemodiversity of PPAPs amongst Garcinia genus, a dereplication process suitable for such natural compounds has been developed. Although less sensitive than mass spectrometry, NMR spectroscopy is perfectly reproducible and allows stereoisomers distinction, justifying the development of 13C-NMR strategies. Dereplication requires the use of databases (DBs). To define if predicted DBs were accurate enough as dereplication tools, experimental and predicted δC of natural products usually isolated from Clusiaceae were compared. The ACD/Labs commercial software allowed to predict 73% of δC in a 1.25 ppm range around the experimental values. Consequently, with these parameters, the major PPAPs from a Garcinia bancana extract were successfully identified using a predicted DB.