Redox-controlled fluorescence

Scheme 15.1 Principle of a switchable, redox-controlled fluorescence in D-A systems (D = TTF).

Scheme 15.8 The photo- and redox-controlled fluorescence switch based on bis-thiaxanthy-lidenes 49 and 50.

The efficient on/off switching of fluorescence from substituted zinc porphyrin-ferrocene dyads 16a and 16b is achieved through redox control of the excited-state electron transfer quenching.26 This redox fluorescence switch is based on the switching of the excited-state electron transfer from the ferrocene to the zinc porphyrin through the use of the ferrocene/ferrocenium (Fc/Fc +) redox couple. 

Because the redox state of the complex ZQB controls fluorescence yield, Z must be inactivated in order to probe QB interactions. A pretreatment of chloroplasts with high concentrations of hydroxylamine will completely destroy the oxidizing side of PS II (Z) and produce some reduction of B (13). The experimental protocol was the following first, a chloroplast suspension was treated in the dark with 10 mM hydroxylamine and was measured immediately. After a 5 min dark incubation period, the chloroplasts received a strong actinic illumination that gave rise to the QB form. After complete relaxation (15 min), DCMU-type inhibitors were added producing Q B and difference between the two values was evaluated in relation to the inhibitor concentrations and a half-effect determined. 

We then tried to discriminate between a quenching effect under redox control of the PQ pool and specific quenching under ATP control per se. To this end, we attempted to revert the FCCP-induced and AA+SHAM-induced low fluorescence state by a white light illumination in presence of DCMU, a situation expected to reoxidise the PQ pool. As shown in figure 3 A, for AA+SHAM-treated cells, the Fmax level rose next to that in State I with a half time of 6 min, similar to that of a state II to state I transition 330 sec (6). Similar results were obtained for FCCP-treated cells (fig.3) when the FCCP concentration uncouple the mitochondria, but not the chloroplast. In contrast, no... 

The molecular orientation and interactions of redox chromophores are very important in controlling photoresponses at the molecular level. Absorption and fluorescence spectra will give important information on them. We have studied, photoresponses, specific interactions, in-plane and out-of-plane orientation of various chomophores in LB films composed of amphiphiles shown in Figure 1 [3-12]. 

We have shown that redox chromophores organized in LB films with resped to their orientation, alignment, or electronic interactions make very useful and specific photoresponses such as amplified fluorescence quenching, photocurrents controlled at the molecular level, photoinduced anisotropic eledrochromism, and photochemically modulated second harmonic generation. These results may contribute to facilitate the design and construction of novel photonic devices in the near future. 

The LB deposition is one of the best methods to prepare highly organized molecular systems, in which various molecular parameters such as distance, orientation, extent of chromophore interaction, or redox potential can be controlled in each monolayer. We have been studying photophysical and photochemical properties of LB films in order to construct molecular electronic and photonic devices. The molecular orientation and interactions of redox chromophores are very important in controlling photoresponses at the molecular level. Absorption and fluorescence spectra give important information on them. We have studied photoresponses, specific interactions, and in-plane and out-of-plane orientation of various chromophores in LB films [3-11], In addition to the change of absorp-... 

Sterically hindered alkenes derived from naphtho[2,l-A thiopyrans continue to attract attention as molecular motors tethering the motor to a quartz surface enables controlled rotary motion in monolayer assemblies to be accomplished <2007AGE1278>. The reversible interconversion between a non-fluorescent and two fluorescent states of crowded bisthioxanthylidenes can be achieved through irradiation and temperature and redox changes <2006JA12412>. 

Fig. 2a-c. Growth of Bacillus stearothermophilus PV72 in continuous culture on a synthetic medium containing glucose (8 gH) as the sole carbon and energy source. The dissolved oxygen concentration was controlled at 50 % and the dilution rate was 0.3 h-1. As derived from the measured process variables, variant formation started at about 15-16 h after inoculation. Shown are the measures for a Respiration activity b External and internal reduction state (redox potential and culture fluorescence) c Cell density (Reprinted from J. Biotechnol. 54, K.C. Schuster et al., p. 19,1997, with permission from Elsevier Science)... 

The ability of tetraazamacrocycles (106) to stabilize nickel ions in the +3 oxidation state has been employed to demonstrate redox switching of fluorescence. The wavelength of the emitted light is controlled through the choice of pendant group... 

The effect of the metal oxidation state on the emission intensity has been also investigated by performing controlled potential exhaustive electrolysis experiments. When an MeCN, poorly emissive solution of the Cu complex is reduced cathodi-cally to the corresponding colorless Cu species, a strong fluorescence enhancement is observed. As the redox process is fully reversible, fluorescence can be switched on/off at will, by setting the potential of the working electrode at the proper potentials for Cu -to-Cu (on) and Cu -to-Cu (off ) changes to take place. 

An equally compelling piece of evidence that protein phosphorylation is controlled by the redox state ofplastoquinone pool was provided by Horton, Allen, Black and Bennett " using potentiometric titration of both fluorescence and LHC II phosphorylation, as shown in Fig. 7 (B). Details on the use of redox titration ofPS-II fluorescence-yield changes to monitor the redox-state changes of the plastoquinone pool in PS II will be discussed in Chapter 17. The data points for fluorescence as well as for phosphorylation, as measured by the level of radioactive P-labeled LHC II, fit the Nernst equation [see footnote on p. 92] with an (at pH 7.8) of 0 mV and an n-value of 2, reflecting a two-electron change for the electron carrier involved, presumably PQ. Thus crucial links are established between the redox state of plastoquinone, the activation of kinase for LHC II phosphorylation and changes in chlorophyll fluorescence. 

The more analytical tools that are available and the better the understanding of critical biochemical pathways, the more rapidly fermentation processes can be developed. Besides those previously mentioned, a munber of different parameters have been monitored on-line in fermentation development [7], including exhaust gas analysis and gas fluxes [46], cell density [47], redox potential [48], IR [49], culture fluorescence [50], biological activities [45 ], and viscosity. It is important to iterate that small-scale fermentation studies should aim to develop relatively simple control systems that are easily scaled. As an example, although HPLC systems are routinely set-up on line to measure and control laboratory scale fermentations, the robustness of such a system and its utility in a manufacturing facility remains debatable. 

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