Acide citrique | EPFL Graph Search

L'acide citrique est un acide tricarboxylique α-hydroxylé présent en abondance dans le citron, d'où son nom. Il s'agit d'un acide faible qui joue un rôle important en biochimie comme métabolite du cycle de Krebs, une voie métabolique majeure chez tous les organismes aérobies. Les citrates émis par les racines de certaines plantes jouent aussi un rôle important en écologie et agroécologie, car ils rendent le phosphore plus facilement bioassimilable par les plantes. Plus d'un million de tonnes d'acide citrique de synthèse sont produites industriellement chaque année. Il est largement utilisé comme exhausteur de goût, comme régulateur alimentaire de pH et comme chélateur. Propriétés gauche|vignette|Diagramme de spéciation d'une solution d'acide citrique à . L'acide citrique a été isolé pour la première fois en 1784 par le chimiste suédois Carl Wilhelm Scheele à partir de jus de citron. Il peut exister sous forme anhydre ou monohydratée. La forme anhydre cristallise dans l'e

Bioactive Compounds of Juice and Peels of Yuzu Fruits Cultivated in Switzerland

Aurélie Sprunger

Yuzu is a citrus fruit cultivated mainly in northeast Asia. Due to the huge gastronomic interest emerging in recent years, some attempts to grow yuzu in Europe are being made. Juice and peel of yuzu cultivated in Switzerland have been characterised in this study. Peel constituted the major part of yuzu fruit followed by flesh, juice and seeds. The fruit degree of maturity was investigated by measuring pH, total titratable acidity and total soluble solids of the juice. The analyses were pursued by determining the content of vitamin C, malic and citric acids amounting to 0.560, 6.18 and 44.7 g/L, respectively. Four flavanones: naringin, hesperidin, naringenin, and hesperetin were identified and quantified in both juice and peel. The most abundant flavanone was hesperidin with 47 mu g/mL and 640 mu g/g dry matter (DM) in juice and peel, respectively. For the first time yuzu grown in Switzerland have been analysed and the obtained values have been compared with literature for other citrus fruits. Yuzu juice showed higher contents of citric and malic acid. Yuzu cultivated in Switzerland contained nearly twice as much vitamin C as yuzu juices from different regions of Japan. The content of vitamin C of yuzu was as high or higher than in most other citrus fruits. Large differences in the content of flavanones in yuzu juice from different regions of Japan were reported, in general the values noted were lower than those in the present study.

2018

Novel Chemosensors for Biologically Important Analytes

Ziya Köstereli

In this work, fluorescent-based chemosensors for the detection of important analytes in aqueous solution are described. The sensors are based on different detection concepts, including analyte-induced aggregation/de-aggregation, pattern-based sensing and micelle-based sensing. In chapter 2, we present an analyte-induced aggregation-type chemosensor for the sensing of spermine. A charge-frustrated amphiphile composed of a highly fluorescent pyrene-1,3,6-trisulfonate head group and an eicosane side chain was used as a fluorescence chemosensor. Analyte-induced aggregation of the dye upon addition of spermine results in pronounced fluorescence quenching. The sensor enables the detection of spermine down to the nanomolar concentration range with good selectivity over closely related biogenic amines such as spermidine. In chapter 3, we describe a conceptually new âone-cuvetteâ sensing system for the pattern-based analysis of seven aminoglycosides antibiotics. The antibiotics include amikacin, apramycin, gentamicin, kanamycin A and B, neomycin, and paromomycin. A mixture of two amphiphiles with fluorescent head groups was used as a sensing ensemble. In buffered aqueous solution, the amphiphiles form a dynamic mixture of micellar aggregates. In the presence of aminoglycosides, the relative amount and the composition of the micelles is modified. Accurate differentiation in the low micromolar concentration range is achieved by a principal component analysis of the spectral data. Also, the sensing system allows the differentiation of pure aminoglycosides from their mixtures. In chapter 4, a fluorescent chemosensor based on analyte-induced aggregation/de-aggregation is described for the sensing of Al3+ and citric acid. In buffered aqueous solution, the amphiphilic dye can form aggregates and the aggregation of the dye is associated with a strong quenching of its fluorescence. The Al3+-induced aggregation is used to sense Al3+ in the low micromolar concentration range with high selectivity. Furthermore, we demonstrate that the non-fluorescent dye-Al3+ complex can be used as a sensing ensemble for the detection of citric acid. The assay is able to quantify the citric acid content of commercial beverages such as energy drinks. A simple micelle-based assay for the fluorescence sensing of vitamin K1 is described. In the following chapter 5, the assay enables the detection of vitamin K1 in the low micromolar concentration range. As a sensing ensemble, a mixture of the surfactant triton X-100 and 1 aminopyrene in buffered aqueous solution is employed. Vitamin K1 co-localizes with the fluorescent pyrene dye in the micelle, resulting in fluorescence attenuation by dynamic quenching. The assay displays good selectivity and can be used to determine the concentration of vitamin K1 in a commercial preparation. In chapter 6, a fluorescent-based sensor array for the optical analysis of purine derivatives is described. The array is composed of four polysufonated fluorescent dyes. The complexation of the analytes results in partial quenching of the fluorescence. The Sensor array enables the identification of ten purine derivatives including caffeine, theophylline, theobromine, purine, hypoxanthine, paraxanthine, 8-chlorotheophylline, 6-mercaptopurine, cladribine and penciclovir at low millimolar concentration. Furthermore, it is possible to use the array for obtaining information about the quantity and the purity of samples.

EPFL2015

Interactions of silver and polystyrene nanoparticles with algae

Xiaomei Li

Nanoparticles have various physicochemical properties attributed by their small dimensions. The global production of nanoparticles lead to a raised concern for their environmental impacts. Algae are of highly ecological importance functioning as primary producers for almost all aquatic life. In the present thesis, interactions of nanoparticles with different fresh water algal strains were examined. First, the toxicity and uptake of citrate-coated silver nanoparticles (AgNP) and AgNO3 were examined in the alga Euglena gracilis, which has no cell wall but a pellicle. Secondly, to examine the role of algal cell wall in determining the particles interactions with algae, four strains were selected, including Euglena gracilis, Haematococcus pluvialis, and Chlamydomonas reinhardtii wild type and a cell wall free mutant. Their interactions with polystyrene nanoparticles (PSNP) of two sizes, 50 nm (PSNP50) and 500 nm (PSNP500), were investigated. Third, interactions of three differently coated AgNP with alkaline phosphatase (AP), an extracellular enzyme used for phosphorus acquisition, were assessed. The selected coatings were citrate (CIT), polyvinylpyrrolidone (PVP) and gelatin (GEL). Exposure to AgNP and AgNO3 for 1-2 hours led to decrease in photosynthetic yield, in a concentration-dependent manner, and changes in cell morphology in E. gracilis. Based on total silver added, AgNP were less toxic than AgNO3. Concentrations causing a 50% reduction in photosynthetic yield were 1.9 µM and 85 for AgNP and AgNO3, respectively. Damaging effects of AgNP were completely prevented by cysteine, suggesting that the toxicity of AgNP was mediated by Ag+ ions. Uptake studies showed that the maximal cell-associated silver was higher in AgNP compared to AgNO3, and the higher silver level was shown to correspond to particles adsorbed to the pellicle. By examining four algal stains, it was found that no strain internalized PSNP, emphasizing the role of algal cell walls as barrier for nanoparticle uptake. Interactions of PSNP with algae were found to be unique for each strain, and depend on particle size. PSNP50 were associated with E. gracilis cells displaying a non-homogenous distribution on pellicle. In H. pluvialis, PSNP50 distributed homogenously around the cells. The wild type and cell wall free mutant of C. reinhardtii cells exposed to PSNP50 were found to clump together packed in the extracellular polymeric substances (EPS). The particles were associated with the EPS. The larger PSNP500 were observed to interact only with the two C. reinhardtii strains. Taken together, these results indicate that the algal cell walls hinder the crossing of nanoparticles. Assessing the sorption of AP to AgNPCIT, AgNPPVP, and AgNPGEL showed that the physiochemical properties of both the particle coatings and the enzyme were determinant for the binding. The enzyme adsorbed to AgNPCIT and AgNPPVP, leading to a 10% and 70% coverage of the particle surface area, respectively. No adsorption was found in the case of AgNPGEL. The three types of AgNP decreased AP activity, however, the inhibitory effects only occurred when the AgNP were added after addition of the substrate to the enzyme, not vice versa. AgNO3 did not affect the AP activity. Thus, the results of this study indicate particle-specific effects due to interactions with the AP-substrate intermediate.

EPFL2015

Novel Chemosensors for Biologically Important Analytes

Ziya Köstereli

EPFL2015

Interactions of silver and polystyrene nanoparticles with algae

Xiaomei Li

EPFL2015