F.F. de la Rosa

Determination of Trivalent and Hexavalent Chromium in Waste Water by Flow Injection Chemiluminescence Analysis

Abstract Flow injection analysis (FIA) has been applied to the determination of both Cr(III) and Cr(VI) in waste water. The method is based on the measurement of Cr(III)-catalyzed light emission from luminol oxidation by hydrogen peroxide and the apparatus consists of a FIA system with a flow cell suitable for chemiluminescence detection. Cr(III) is determined directly by the chemiluminescence, meanwhile Cr(VI) is reduced previously to Cr(III) by H2O2 in acidic medium and then the total amount of chromium is determined. The concentration of Cr(VI) is obtained by the difference between the Cr(III) and Cr(VI) determinations. We have analyzed synthetic mixtures of both species, Cr(VI) and Cr(III), using this method and its application to waste water has been shown to be very efficient. The method is simple, inexpensive, sensitive (subnanomolar concentrations), selective and rapid. Tens of samples per hour can be performed with tolerance to potential interferants.

The microalga Chlamydomonas reinhardtii CW-15 as a solar cell for hydrogen peroxide photoproduction: Comparison between free and immobilized cells and thylakoids for energy conversion efficiency

Abstract Immobilized cells and thylakoid vesicles of the microalga Chlamydomonas reinhardtii CW-15 have been developed as a solar cell because of their capabilities of producing hydrogen peroxide. This compound is an efficient and clean fuel used for rocket propulsion, motors and for heating. Hydrogen peroxide is produced by the photosystem in a catalyst cycle in which a redox mediator (methyl viologen) is reduced by electrons obtained from water by the photosynthetic apparatus of the microalga and it is re-oxidized by the oxygen dissolved in the solution. The photoproduction has been investigated using a discontinuous system with whole cells, or thylakoid vesicles, free or immobilized on alginate. The stimulation by azide as an inhibitor of catalase has also been analyzed. Under determined optimum conditions, the photoproduction by Ca-alginate entrapped cells, with a rate of 33 μmol H2O2/mg Chl.h, was maintained for several hours with an energy conversion efficiency of 0.25%.

A flow injection chemiluminescence method using Cr(III) as a catalyst for determining hydrogen peroxide. Application to H2O2 determination in cultures of microalgae

Flow injection analysis has been applied to the determination of hydrogen peroxide produced by some different species of microalgae. The method is based on the luminol-H(2)O(2) chemiluminescence reaction using Cr(III) as a catalyst. Optimum experimental conditions for the method have been studied and trace amounts of hydrogen peroxide determined with detection limits of 4 10(-8) mol/L. The method using Cr(III) was compared with that using horseradish peroxidase as the catalyst.

Hydrogen peroxide photoproduction by immobilized cells of the blue-green alga Anabaena variabilis: A way to solar energy conversion

A photosystem for hydrogen peroxide photoproduction formed by immobilized cells of the blue-green alga, Anabaena variabilis and the redox mediator methyl viologen is described. Hydrogen peroxide is produced in a redox catalyst cycle in which methyl viologen is reduced by electrons from water obtained by the photosynthetic apparatus of the algae using solar energy, and reoxidized by the introduction of oxygen into the solution. Hydrogen peroxide is produced during methyl viologen re-oxidation in two steps by means of the formation of superoxide. Experimental conditions for maximum photoproduction (catalyst charge, chlorophyll, and agar final concentration for cell immobilization) have been investigated using a continuous photosystem with immobilized A. variabilis as photocatalyst. Under the determined optimum conditions, the photosystem with immobilized A. variabilis is photocatalyst. Under the determined optimum conditions, the photosystem produces hydrogen peroxide at a rate of 100 {mu}moles/mg Chl{center dot}h, maintaining the production for several hours, and with an energy conversion efficiency of about 2%. Taking into account the use of hydrogen peroxide as fuel, this photosystem can be a useful tool in the storage of solar energy.

Light-driven hydrogen peroxide production as a way to solar energy conversion

Abstract The existence of a number of biochemical pathways through which molecular oxygen is reduced to hydrogen peroxide, either directly in a two-electron transfer process or indirectly via superoxide, is well known. The Mehler reaction, which is the photoreduction of oxygen by isolated chloroplasts in light with the concomitant formation of hydrogen peroxide, is a worthwhile example of such biochemical pathways. Taking advantage of these facts, the possibility of designing a photobiochemical system for solar energy conversion aimed to generate a valuable product such as hydrogen peroxide — which is both a powerful source of energy (it releases more than 100 kJ per mol upon decomposition into water and oxygen) and a powerful source of oxygen (it is a strong bleaching agent widely used in industry) — from oxygen reduced by illuminated chloroplasts has been considered. Likewise, the construction of several artificial photosystems for the production of hydrogen peroxide was also studied. Such model systems, which try to simulate the natural process with a higher efficiency and simplicity, are based on the use of a visible light-absorbing pigment (flavins, for example) as a photosensitizer.

Energy transduction by bioelectrochemical systems

Summary Electronic energy seems to be the obligatory link between the different forms of energy (light, redox, acid-base, phosphate-bond) transduced by the bioelectrochemical systems. These energy-transducing systems can operate, according to their nature and depending on their energization state, either at two alternate midpoint redox potentials ( U 0 ′ and U 0 ′* ), or at two p K a ′ s (p K a and p K a * ), or at two phosphate transfer potentials ( PTP and PTP * ). The key point in energy coupling between any two of these biochemical systems lies apparently in the fact that both of them share a common intermediate, which cyclically participates in the overall process by alternating between its electronically energized state and its unenergized basal state. Electronic energization of the coupling intermediate may proceed in one or two steps and can oscillate between approximately 0.2 and 1 eV molecule −1 , i.e., between 20 and 100 kJ mole −1 .

pH-Dependent interconversion between the two redox forms of chloroplast cytochrome b-559

Summary Spinach chloroplast cytochrome b-559 is characterized (cf. Ref. 1) by exhibiting both a pH-independent hydroquinone-reducible high-potential form (midpoint potential U0′, pH 7=340 mV) and a pH-dependent (pKa=7.6) dithionite-reducible low-potential form (midpoint potential U0′, pH 7=135 mV). Untreated fresh chloroplasts present about two-thirds of their cytochrome b-559 in its high-potential form, which is rapidly reduced by hydroquinone. However, fresh chloroplasts treated in the light with a high concentration of the protonophoric uncoupler carbonylcyanide-p-trifluoromethoxy-phenylhydrazone (FCCP) contain only the low-potential form not reducible by hydroquinone, a change that is brought about by the irreversible conversion of the high-potential form into the low-potential one. In the presence of a lower concentration of the uncoupler, the high-potential form likewise becomes transformed into the low-potential form, but remarkably this can then be subsequently reduced by hydroquinone, although only at a comparatively slow rate, thus indicating reversibility of the conversion process between both redox couples. These results, which are discussed in detail in a broad context, reinforce our view (cf. Ref. 1) that cytochrome b-559 operates not only as a redox but also as an acid-base energy-transducing system, of both the energized oxidized- and basic-form type at two altermate potentials and pKas. Accordingly, a minimal model for the mutual coupling between redox energy and acid-base energy FCCP would exert its uncoupling action by removing the proton from the energized oxidized form of the cytochrome high-potential couple. Conversely, protonation of the basal oxidized form of the low-potential couple-either directly at low pH or indirectly at high pH through redox reactions—would cause its energization.

Restoration of high-potential cytochrome b-564 by integration of baker's yeast complex III into liposomes

Cytochrome b-564 in isolated complex III from bakers yeast mitochondria exhibits the midpoint redox potential proper to the low-potential couple (+60 mV, pH 7.2). Incorporation of the complex into liposomes promotes total conversion to the high-potential couple (+170 mV, pH 7.2). The reconstituted system shows electrogenic proton translocation, which is inhibited by the uncoupler CCCP. Deenergizing treatments result, moreover, in reversal of the redox potential change. These results support our previous proposal that cytochrome b-564 acts as a transducer of redox energy into acid-base energy in the complex III region of the respiratory chain.

Spinach nitrate reductase. Kinetic studies of NADH-diaphorase

Abstract NADH-cytochrome c reductase (diaphorase), a functional moiety of the spinach NADH-nitrate reductase complex, uses one molecule of NADH to reduce two of ferricytochrome c . Initial velocity experiments in the absence of products produce patterns of parallel lines when plotting 1/ v vs. 1/(substrate) at different fixed concentrations of the other substrate. The results agree with a Ping Pong mechanism in which the first product (NAD + ) is released, with formation of a reduced free form of the enzyme, prior to binding of the first molecule of the second substrate (ferricytochrome c ). Product inhibition experiments indicate the production of an abortive complex between NAD + and the oxidized enzyme form that binds NADH. The data are consistent with a Hexa Uni Ping Pong mechanism in which an abortive complex is formed between NAD + and the enzyme form binding NADH.

LASER FLASH PHOTOLYSIS STUDIES OF ELECTRON TRANSFER IN COMPLEX III FROM YEAST MITOCHONDRIA

Complex III (ubiquinol cytochrome c reductase, EC 1.10.2.2) contains, in addition of ubiquinone and iron-sulfur protein, two cytochromes b (b H, bL) and a cytochrome type c (C 1). This complex shows proton translocation with two protons appearing on the outside of the membrane for each electron transfer from ubiquinol to cytochrome c