Photosynthesis, artificial fluorescence

Vegetative microspores of Cryptogams are unicellular objects with hard cover, blue-fluorescence in ultra-violet light and elaters [serve as anchor to a substrate (soil)]. The cells are diploid and have autotrophic nutrition due to the presence of chloroplasts, where photosynthesis occurs. They germinate well in artificial nutrient medium or in water and ultra-violet light induces significant autofluorescence. 

Photoinduced electron transfer (PET) is often responsible for fluorescence quenching. This process is involved in many organic photochemical reactions. It plays a major role in photosynthesis and in artificial systems for the conversion of solar energy based on photoinduced charge separation. Fluorescence quenching experiments provide a useful insight into the electron transfer processes occurring in these systems. 

Non-Forster fluorescence quenching of trans-etiochlorin by magnesium oc-taethylporphine in phosphatidylcholine vesicles gives evidence for a statistical pair energy trap. Energy transfer also occurs in the excited singlet manifold of chlorophyll. " The photophysics of bis(chlorophyll)-cyclophanes, models of photosynthetic reaction centres, have been explored for use in artificial photosynthesis.Picosecond time-resolved energy transfer in phycobilosomes have also been studied with a tunable laser. The effect of pH on photoreaction cycles of bacteriorhodopsin, " the fluorescence polarization spectra of cells, chromatophores, and chromatophore fractions of Rhodospirillum rubrum, and a brief review of the mechanism and application of artifical photosynthesis are all relevant to the subject of this Chapter. 

HPTS is a pH-sensitive fluorophore (pk, 7.3) [6]. The opposite pH sensitivity of the two excitation maxima permits the ratiometric (i.e. unambiguous) detection of pH changes in double-channel fluorescence measurements. The activity of synthetic ion channels is determined in the HPTS assay by following the collapse of an applied pH gradient. In response to an external base pulse, a synthetic ion channel can accelerate intravesicular pH increase by facilitating either proton efflux or OH influx (Fig. 11.5c). These transmembrane charge translocations require compensation by either cation influx for proton efflux or anion efflux for OH influx, i.e. cation or anion antiport (Fig. 11.5a). Unidirectional ion parr movement is osmotically disfavored (i.e. OH /M or X /H symport). HPTS efflux is possible with pores only (compare Fig. 11.5b/c). Modified HPTS assays to detect endovesiculation (Fig. 11.1c) [16], artificial photosynthesis [17] and catalysis by pores [18] exist. 

There continues to be an enormous amount of activity in the area of PET, much of it directed towards the development of systems capable of delivering artificial photosynthesis. Many of these systems involve porphyrin units as electron-donors and thus it is appropriate to consider them in this section of the review. A number of new fullerene-porphyrin dyads have been reported. A pyrazolinofullerene (155) has been constructed which facilitates efficient PET when strong donors such as iV,Ar-diethylaniline or ferrocene are linked to the pyrazoline ring. A photosynthetic multi-step ET model (156) based on a triad consisting of a meso,meso- inked porphyrin dimer connected to ferrocene and Ceo as electron-donor and electron-acceptor, respectively, has been synthesized and its ET dynamics (Scheme 38) have been investigated using time-resolved transient absorption spectroscopy and fluorescence lifetime measurements. ... 

More recently, vesicle bilayer nanocapsules constructed using perylene diimide molecules have been prepared in which they encapsulated donor molecules for studying artificial photosynthesis." The fascinating functional feature of these loaded vesicles is that they provide ultrasensitive pH information about their aqueous environment by displaying pH-dependent fluorescence color changes covering... 

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