Gramicidin in chromatophores of Rhodobacter sphaeroides

Chromatophores of Rhodobacter sphaeroides were excited with light flashes to generate a transmembrane electrical potential difference. The electric relaxation was measured by electrochromic absorption changes as a function of added gramicidin. At low gramicidin/bacteriochlorophyll (BChl) molar ratios the decay of the electrochromic absorption changes showed a biphasic behaviour, with a fast phase relaxing at some s, and a slow phase relaxing at more than 100 ms. This was attributable to a mixture of vesicles containing gramicidin dimers with others containing none. The concentration dependence of this effect was linear. This implied full dimerization of gramicidin. The data were interpreted to yield an average bacteriochlorophyll content per chromatophore of 770(±150) and the conductance of a single gramicidin dimer in the chromatophore membrane of 15(±4) pS (in about 115 mM KCl).Abbreviations BChl Bacteriochlorphyll - tricine N-Tris[hydroxymethyllmethylglycine Offprint requests to: W. Junge     

Activation of the non-electrochromic component in the 518 nm absorbance change by photosystem I

The flash-induced absorbance change measured at 518 nm (P515) in intact chloroplasts consists of at least 4 kinetically different components. Here the non-electrochromic component, either called phase d or reaction 3, is studied in some detail. The effect of DCMU, DQH₂ and DBMIB on the amplitude of reaction 3 and the turnover of cytochrome f and P700 have been monitored, suggesting an involvement of photosystem 1 in the activation of the non-electrochromic absorbance change. This is confirmed by the parallel oscillation pattern found in P700 rereduction and the amplitude of reaction 3.     

Electrochromic properties of mixed valence binuclear ruthenium complexes adsorbed on nanocrystalline SnO2 films

The electrochromic properties of two new mixed valence ruthenium complexes: K[(NC₅H₄CH₂PO₃H₂)Ru^III(NH₃)₄(NC)Ru^II(CN)₅] and K[(NC₅H₄PO₃H₂)Ru^III(NH₃)₄(NC)Ru^II(CN)₅], where phosphonic acid groups have been introduced at the pyridine ligand, have been studied in homogeneous solution and adsorbed on transparent nanocrystalline SnO₂ electrodes. These species exhibit a superior stability with respect to the previously studied, K[(NC₅H₄CO₂H)Ru^III(NH₃)₄NCRu^II(CN)₅] complex, showing negligible optical density changes after cycling 20 000 times the electrodes between −0.5 and 0.5 V versus SCE.     

Respiratory control over photosynthetic electron transport in chloroplasts of higher-plant cells: evidence for chlororespiration

Flash-induced primary charge separation, detected as electrochromic absorbance change, the operation of the cytochrome b/f complex and the redox state of the plastoquinone pool were measured in leaves, protoplasts and open-cell preparations of tobacco (Nicotiana tabacum L.), and in isolated intact chloroplasts of peas (Pisum sativum L.). Addition of 0.5–5 mM KCN to these samples resulted in a large increase in the slow electrochromic rise originating from the electrogenic activity of the cytochrome b/f complex. The enhancement was also demonstrated by monitoring the absorbance transients of cytochrome f and b ₆ between 540 and 572 nm. In isolated, intact chloroplasts with an inhibited photosystem (PS) II, low concentrations of dithionite or ascorbate rendered turnover of only 60% of the PSI reaction centers, KCN being required to reactivate the remainder. Silent PSI reaction centers which could be reactivated by KCN were shown to occur in protoplasts both in the absence and presence of a PSII inhibitor. Contrasting spectroscopic data obtained for chloroplasts before and after isolation indicated the existence of a continuous supply of reducing equivalents from the cytosol.Our data indicate that: (i) A respiratory electron-transport pathway involving a cyanide-sensitive component is located in chloroplasts and competes with photosynthetic electron transport for reducing equivalents from the plastoquinone pool. This chlororespiratory pathway appears to be similar to that found in photosynthetic prokaryotes and green algae. (ii) There is an influx of reducing equivalents from the cytosol to the plastoquinone pool. These may be indicative of a complex respiratory control of photosynthetic electron transport in higher-plant cells.Abbreviations and symbols A₅₁₅ flash-induced electrochromic absorbance change at 515 nm - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - PS photosystem - SHAM salicylhydroxamic acid     

Theory of the Stark Effect in protein systems containing an electron donor–acceptor couple

Mixed-valency can occur in a variety of biological systems, such as the Cu(I)–Cu(II) pair in hemocyanin, Fe(II)–Fe(III) in many iron–oxo and iron–sulfur proteins, and Mn(II)–Mn(III) or Mn(III)–Mn(IV) in the photosynthetic water oxidase. The characterization of the ground states of such systems often has been controversial. Stark Effect spectroscopy is proving to be a valuable tool for the elucidation of systems of this type. The purpose of the present work is to develop theory for the spectral lineshape for the case where the electron donor and acceptor are coupled directly in a strong electric field. A mixed-valence dimer with an applied electric field aligned along the internuclear axis is studied using a two-site small-polaron model. Potential energy surfaces are calculated in the adiabatic (Born–Oppenheimer) approximation. It is shown that two nuclear coordinates (one totally symmetric and one antisymmetric) are coupled to the electronic motion, whereas only the antisymmetric coordinate is coupled in the absence of an electric field. For a strongly localized system, such as a protein system where electron donor and acceptor sites are separated by large distances, the potential surfaces become highly asymmetrical, but coupling to the totally symmetric mode is not significant. For a localized case corresponding to a valence-trapped two-metal cluster, the displacement along the totally symmetric coordinate is directly proportional to the applied field strength. Along the antisymmetric coordinate, the lowest potential surface is an asymmetric double well. For a delocalized (valence-averaged) two-metal cluster, there is significant displacement along the antisymmetric coordinate, an effect which also vanishes in the absence of an applied field. Contributions to the linewidth are estimated. Localized systems show larger field-induced shift in frequency maximum, whereas delocalized systems show greater field-induced line broadening.