Siderophore production by pathogenic mucorales and uptake of deferoxamine B.

Clinical reports have established that mucormycosis, mainly caused by Rhizopus spp., frequently occurs in patients treated with deferoxamine B (DFO, Desferal(®)) which is misappropriated by these fungi. Siderophore production by twenty mucoralean isolates was therefore investigated using a commercial iron-depleted culture medium. Siderophore production was detected for most of the isolates. Our experiments confirmed that feroxamine B (iron chelate of DFO) promoted in vitro growth of Rhizopus arrhizus. Electrophoretic analysis of somatic extracts revealed iron-regulated proteins of 60 and 32 kDa which were lacking in iron-depleted culture conditions. Using a fluorescent derivative of deferoxamine B, we showed by fluorescence microscopy the entry of the siderophore within the fungal cells, thus suggesting a shuttle mechanism encompassing the uptake of the entire siderophore-ion complex into the cell. This useful tool renders possible a better understanding of iron metabolism in Mucorales which could lead to the development of new diagnostic method or new antifungal therapy using siderophores as imaging contrast agents or active drug vectors.

A facile route to steady redox-modulated nitroxide spin-labeled surfaces based on diazonium chemistry.

A TEMPO derivative was covalently grafted onto carbon and gold surfaces via the diazonium chemistry. The acid-dependent redox properties of the nitroxyl group were exploited to elaborate electro-switchable magnetic surfaces. ESR characterization demonstrated the reversible and permanent magnetic character of the material.

Electrochemical transduction on self-assembled monolayers: are covalent links essential?

Electrochemical transduction without covalent links between redox and complexant units in a complexing self-assembled monolayer has been established. The results demonstrate that transduction depends on the crown ether/ferrocene ratio and appears to be tunable.

Selective detection of alkaloids in MALDI-TOF: the introduction of a novel matrix molecule.

The current manuscript presents 3-[5'-(methylthio)-2,2'-bithiophen-5-ylthio]propanenitrile (MT3P), as a novel matrix molecule, which facilitates the selective ionization of alkaloids in matrix-assisted laser desorption/ionization mass spectrometry. Exhibiting strong ionizing properties at low levels of laser energy, MT3P was evaluated on 55 compounds belonging to various chemical families. The observed molecular ion yields induced by MT3P were compared with those obtained by commercially available matrices such as 1,8-dihydroxy-9,10-dihydroanthracen-9-one, α-cyano-4-hydroxycinnamic acid, 2,2':5',2"-terthiophene and 2,5-dihydroxybenzoic acid. In conclusion, MT3P displayed excellent ionization properties for 23 out of 25 investigated alkaloids, while showing little to no interaction with compounds from different chemical origin. Further, in comparison to other tested matrices, MT3P generally facilitated better ionization of alkaloids. Eventually, levels of laser energy were adjusted to obtain spectra with significantly reduced matrix noise.

Free and immobilized matrix molecules: impairing ionization by quenching secondary ion formation in laser desorption MS.

Within the last 25 years, matrix-assisted laser desorption ionization (MALDI) has become a powerful analytical tool in mass spectrometry (MS). While the method has been successfully applied to characterize large organic molecules such as proteins, sugars and polymers, its utilization for small molecules (≤ 600 Da) is significantly impaired by the coformation of matrix ions. Reducing or eliminating matrix-related signals has been subject of many studies. Some of which propose the enhancement of so-called matrix suppression effects, while others suggest the replacement of matrix molecules by materials such as microporous silicon. Alternatively, the immobilization of matrix molecules by utilizing them as self-assembled monolayers (SAMs) has been discussed. In continuation of this research, the current manuscript focuses on the elucidation of ion formation processes occurring on the surface of light absorbing SAMs. Ion yields obtained by free and immobilized matrix molecules as well as those generated by matrix-free gold film-assisted laser desorption ionization (GF-LDI) were compared. Experiments showed that the formation of strong analyte signals essentially required the presence of free matrix molecules, while the immobilization of the latter severely impaired ionization. The observed effect inversely correlated with the surface coverage of SAMs determined by cyclic voltammetry (CV). Based on these findings, the MS signal generated on light absorbing SAMs could be used supplementary to CV for determining the surface coverage of light absorbing SAMs.

Phase segregation on electroactive self-assembled monolayers: a numerical approach for describing lateral interactions between redox centers.

A numerical method is proposed in order to differentiate a random distribution from a phase segregation of redox centers on (mixed) SAMs. This approach is compared to Laviron's interactions model and voltammetric data of nitroxylalkanethiolate SAMs.

Evaluation of a new matrix-free laser desorption/ionization method through statistic studies: comparison of the DIAMS (desorption/ionization on self-assembled monolayer surface) method with the MALDI and TGFA-LDI techniques.

This work demonstrates that the desorption/ionization on self-assembled monolayer surface (DIAMS) mass spectrometry, a recent matrix-free laser desorption/ionization (LDI) method based on an organic target plate, is as statistically repeatable and reproducible as matrix assisted laser desorption ionization (MALDI) and thin gold film-assisted laser desorption/ionization (TGFA-LDI) mass spectrometries. On lipophilic DIAMS of target plates with a mixture of glycerides, repeatability/reproducibility has been estimated at 15 and 30% and the relative detection limit has been evaluated at 0.3 and 3 pmol, with and without NaI respectively. Salicylic acid and its d(6)-isomer analysis confirm the applicability of the DIAMS method in the detection of compounds of low molecular weight.

Trafficking of siderophore transporters in Saccharomyces cerevisiae and intracellular fate of ferrioxamine B conjugates.

We have studied the intracellular trafficking of Sit1 [ferrioxamine B (FOB) transporter] and Enb1 (enterobactin transporter) in Saccharomyces cerevisiae using green fluorescent protein (GFP) fusion proteins. Enb1 was constitutively targeted to the plasma membrane. Sit1 was essentially targeted to the vacuolar degradation pathway when synthesized in the absence of substrate. Massive plasma membrane sorting of Sit1 was induced by various siderophore substrates of Sit1, and by coprogen, which is not a substrate of Sit1. Thus, different siderophore transporters use different regulated trafficking processes. We also studied the fate of Sit1-mediated internalized siderophores. Ferrioxamine B was recovered in isolated vacuolar fractions, where it could be detected spectrophotometrically. Ferrioxamine B coupled to an inhibitor of mitochondrial protoporphyrinogen oxidase (acifluorfen) could not reach its target unless the cells were disrupted, confirming the tight compartmentalization of siderophores within cells. Ferrioxamine B coupled to a fluorescent moiety, FOB-nitrobenz-2-oxa-1,3-diazole, used as a Sit1-dependent iron source, accumulated in the vacuolar lumen even in mutants displaying a steady-state accumulation of Sit1 at the plasma membrane or in endosomal compartments. Thus, the fates of siderophore transporters and siderophores diverge early in the trafficking process.

Structural control of the electronic properties of photodynamic azobenzene-derivatized pi-conjugated oligothiophenes.

Quaterthiophenes containing a median 3,3'-dimethoxybithiophene (4MTZ) or bis(3,4-ethylenedioxythiophene) (4ETZ) block and an azobenzene group attached at two internal beta-positions of the end thiophene rings have been synthesized. The analysis of the crystal structure of the two compounds by X-ray diffraction shows that, for 4MTZ, the quaterthiophene chain adopts a syn-anti-syn conformation similar to the parent system based on unsubstituted quaterthiophene. In contrast, 4ETZ presents an all-anti conformation stabilized by noncovalent intramolecular interactions between oxygen atoms and thiophenic sulfur atoms. The effect of the photoisomerization of the azobenzene group on the electronic properties of the attached pi-conjugated quaterthiophene chain has been analyzed by UV-vis spectroscopy and cyclic voltammetry. The results obtained for 4MTZ suggest very limited geometrical changes due to the restricted rotation around the central interannular single bond caused by steric interactions between the methoxy groups. In contrast, upon trans to cis photoisomerization of the attached azobenzene group of 4ETZ, the quaterthiophene chain undergoes a reversible conformational switch to a final state with a lower HOMO level and a larger HOMO-LUMO gap than the initial state.

Synthesis and electrochemical and photophysical studies of tetrathiafulvalene-annulated phthalocyanines.

The synthesis of tetrakis(tetrathiafulvalene)-annulated metal-free and metallophthalocyanines 5-8 via the tetramerization of the phthalonitrile derivative 4 is reported. All of them have been fully characterized by electronic absorption spectroscopy, thin-layer cyclic voltammetry, mass spectrometry, and elemental analysis. Their solution electrochemical data show two reversible four-electron oxidation waves, indicating that these fused systems are strong pi-electron donors, which give rise to tetra- or octaradical cation species. For the metal-free phthalocyanine 5, additionally a reversible one-electron wave was found in the negative direction arising from the reduction of the macrocycle. Moreover, the tetrathiafulvalene unit acts as an efficient reductive electron-transfer quencher for the phthalocyanine emission, but upon its oxidation, an intense luminescence is switched on.