Light-dependent cyclic electron transfer

Also procedures for the isolation of inside-out membranes, by French Press treatment of intact bacteria, have been described. In phototrophic organisms, these membranes are derived from the invaginations of the plasma membrane and are called chromatophores. These preparations have been extensively used for studies on light-dependent cyclic electron transfer and photophosphorylation. In non-phototrophic bacteria the resulting structures are often called inverted membranes or membrane particles, in analogy with sub-mitochondrial particles. Amongst others, these preparations have been isolated from Azotobacter vinelandii and E. coli. These inverted membranes can be used for the study of oxidative phosphorylation and the determination of H /e stoicheiometries since the enzymatic machinery for these processes is located on the external surface of these membranes. Also excretion of ions (like ) from intact cells can be studied conveniently in these preparations because these ions are accumulated in inverted membranes. 

A cartoon of a fluorescent switch , turned on or off (quenched) depending on the absence or presence of a metal ion. The ionophore (the cyclic polyether) is the metal-binding component, the fluorophore (the fused-ring aromatic unit) is the component activated by light. Complexation stops electron transfer that otherwise quenches fluorescence. 

Such an interaction between the electron transfer system and solute transport carriers is not specific for Rps. sphaeroides. In a strain of Rps. sphaeroides in which the E. coli transport protein for lactose (the M-protein) was incorporated by genetic transformation a similar interaction between the rate of cyclic electron transfer and lactose transport was observed (Elferink et al., 1983). Kinetic analysis of the changes in the initial rate of lactose uptake indicates that the regulation is due to a light-dependent change in the. number of active carrier molecules in the membrane. 

Reaction Centers (RC s) from phototrophic bacteria catalyze light-driven transmembrane electron transfer as a first step in the (cyclic) electron transfer chain of such bacteria (for a review see Okamura et al., 1983 and Dutton et al., 1982). Many of the structural and functional features of RC s have already been elucidated the remaining questions mainly focus on (i) the effects of transmembrane gradients (of redox potential and electrochemical potential of protons) on the reactions catalyzed by the RC s and (ii) the interactions between RC s and physiological and artificial electron donors and acceptors. Many of the unsolved aspects can be optimally investigated under conditions, in which the RC s have been reconstituted into artificial membranes either in planar (Schonfeld et al., 1979) or vesicular form (Crofts et al., 1977 Pachence et al., 1979). Here I report on the structure of reconstituted RC vesicles and light-dependent unidirectional proton translocation catalyzed by these vesicles. 

Electrochemical measurements of the Cu(II/I) potentials with the nS4 ligands (n = 12-16) indicate that the Cu(II) and Cu(I) species each exist in two different conformational states [170]. Conformational rearrangement may either precede or succeed electron transfer. Rorabacher and coworkers interpreted their results in light of a square mechanistic scheme that neatly reconciles the sweep rate dependence of the cyclic voltammograms with the requisite change in coordination geometry at Cu. Kinetic studies on the electron transfer [149, 170, 176-177] support this scheme application of the Marcus cross relationship to reduction of Cu(II) and oxidation of Cu(I) yields widely discrepant values, presumably because of the different conformational states involved. 

These seemingly easily distinguished mechanisms are considered together as an alternative to the diradical mechanism for reasons which will become apparent. Electron transfer is particularly important in the more electron deficient, cyclic per-ester dioxetanones. Dioxetans in general do not respond to activators -fluorescent compounds of low ionisation potential (see p. 40) almost certainly because they are poorer oxidants than dioxetanones. There is however a hint that simple dioxetans, if sufficiently strained may accept electrons from an activator [36]. The light emitted from the cyclic compound (S) shows a linear dependence on DP A concentration, and the ionisation potential of several fluorescent compounds. However, true electron exchange luminescence cannot be... 

The kinetically best characterized system for which a bimolecular reductive elimination has been proposed is a neutral hydrido porphyrin derivative of Ru(III) [4]. Cyclic voltammetry and double potential step chronoamperometry afford data that are more consistent with a second order than with a first order decay for the 17-electron RuH(OEP)(L) (OEP = octaethylporphyrin L = THE, 1 -tert-bytyl-S-phenylimidazole) complexes in THF as solvent. The second order dependence of the rate constant and the independence on the parent 18-electron anion concentration exclude the proton transfer mechanism. The possibility of a disproportionation mechanism (which would afford the same second order dependence, see section 6.5.2), however, has not been considered, nor were studies in solvents other than THF carried out. In the light of the gathered information, the mechanism shown in Scheme 17 was proposed. 

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