The field indicating electrochromic 515 nm absorption change has been measured under different excitation conditions in DCMU poisoned chloroplasts in the presence of benzylviologen as electron acceptor. It has been found: 1. The amplitude of the 515 nm absorption change is nearly completely suppressed under repetitive single turnover flash excitation conditions which kinetically block the back reaction around system II (P. Bennoun, Biochim. Biophys. Acta 216, 357 [1970]). 2. The amplitude of the 515 nm absorption change measured under repetitive single turnover flash excitation conditions which allow the completion of the back reaction during the dark time between the flashes (measuring light beam switched off) amounts in the presence of 2 mum DCMU nearly 50% of the electrochromic 515 nm amplitude obtained in the absence of DCMU. In DCMU poisoned chloroplasts this amplitude is significantly decreased by hydroxylaminhydrochloride, but nearly doubled in the presence of CDIP+ascorbate. 3. The dependence of the 515 nm amplitude on the time td between the flashes kinetically resembles the back reaction around system ?II. The time course of the back reaction can be fairly described either by a second order reaction or by a two phase exponential kinetics. 4. 1,3-dinitrobenzene (DNE) or alpha-bromo-alpha-benzylmalodinitril (BBMD) reduce the 515 nm amplitude in DCMU poisoned chloroplasts, but seem to influecne only slightly the kinetics of the back reaction. 5. The dependence of the 515 nm amplitude on the flash light intensity (the amplitude normalized to 1 at 100% flash light intensity) is not changed by DNB. Based on these experimental data it has been concluded that in DCMU poisoned chloroplasts the amplitude of the 515 nm absorption change reflects the functional state of photosystem II centers (designated as photoelectric dipole generators II) under suitable excitation conditions. Furthermore, it is inferred that in DCMU poisoned chlorplasts the photoelectric dipole generators II either cooperate (probably as twin-pairs) or exist in two functionally different forms. With respect to BBMD and DNB it is assumed that these agents transform the phtooelectric dipole generators II into powerful nonphotochemical quenchers, which significantly reduce the variable fluorescence in DCMU-poisoned chloroplasts.

Benzyl Compounds↗

PubMedDate not supplied

Studies on the nature of the primary reactions of photosystem II in photosynthesis. II. The modification of the functional integrity of the photochemical active centers of system II by a alpha-bromo-benzyl-malodinitril and accompyaning effects on chloroplasts.

The effect of alpha-bromo-alpha-benzylmalodinitril (BBMD) on the oxygen evolution and on the absorption changes at 515 nm and 704 nm has been investigated in spinach chloroplasts. It has been found: 1. Under repetitive flash excitation conditions, where the back reaction around system II is practically excluded for kinetical reasons, BBMD does not resotre the 515 nm absorption change in DCMU poisoned chloroplasts. 2. Under single flash excitation conditions, where the back reaction around system II becomes prominent in the presence of DCMU, BBMD moderately inhibits this back reaction. The deleterious effect is pronounced by preillumination with short flashes during the BBMD incubation period of the chloroplasts in the absence of DCMU. 3. Incubation of the chloroplasts with BBMD leads to an activity loss of oxygen evolution which increases with the time td between the repetitive short excitation flashes and with the dark incubation time. Preillumination during the incubation period with tbbmd significantly enhances the effect. 4. In the absence of artificial electron acceptors BBMD suppresses in DCMU poisoned chloroplasts the 704 nm absorption change reflecting an internal cyclic electron flow around system I. On the other hand the linear electron transport at system I mediated by DCIP plus ascorbate as electron donor couple and benzylviologen as electron acceptor is not distrubed by BBMD. 5. BBMD incubation of chloroplasts accelerates the decay rate of the field indicating 515 nm absorption change. Based on these experimental findings the conclusion has been drawn, that -- in contrast to the assumption of Brandon and Elgersma (Biochim. Biophys. Acta 292, 753--762 [1973]) -- BBMD does not accept electrons from the primary electron acceptor X 320 of system II in DCMU poisoned chloroplasts. BBMD rather acts as a system I electron acceptor. Furthermore, BBMD exerts deleterious effects on the center of photosystem II, accompanied by a weak ADRY-effect on the water-splitting enzyme system Y. As a tentative explanation of the BBMD-action on system II it is assumed that BBMD transforms the photochemical centers of system II into dissipative energy sinks.

Benzyl Compounds↗

PubMedDate not supplied

Indirect evidence for a very fast recovery kinetics of chlorophyll-aII in spinach chloroplasts.

The 690 nm absorption change reflecting the turnover of the system-II-reaction center chlorophyll, Chl-aII (often referred to as P 680), has been investigated under different experimental conditions in spinach chloroplasts. A comparison was made with oxygen evolution and with absorption changes of Chl-aI measured at 703 nm, both indicating the number of electrons produced by system II. It was found: 1. The dependency on actinic flash intensity of the initial amplitudes of the measured 690 nm absorption change, deltaalpha0(Chl-aII) in Tris-washed chloroplasts is similar to that for the total amplutude of the 703 nm absorption change, deltaalpha0(Chl-aI) in normal chloroplasts, and can be described by an exponential function. On the other hand, deltaalphao(chl-aII) in normal chloroplasts exhibits a more complex biphasic dependency and much higher flash intensities are required for saturation. 3. Unver repetitive flash group excitation and in the presence of an ADRY(= acceleration of the deactivation reactions of the water-splitting enzyme system Y)-reagent the initial amplitude of the 690 nm absorption change oscillates in the same characteristic pattern as the oxygen evolution. 4. The initial amplitude of the 690 nm absorption change, deltaalpha0(Chl-aII), IN Tris-washed chloroplasts becomes significantly smaller (more than 50%) by the addition of system-II-electron donors (benzidine, p-phenylendiamine, tetraphenylboron), whereas the total amplitude of the 703 nm absorption change, detalalpha0)Chl-aI) increases 3-4-fold. In order to explain these results, the existance of a very fast reduction kinetics of Chl-aII+ is postulated, which is not detectable by our measuring equipment. The half time of this reaction is less than or equal to mus. Reaction centers with the very gast "undetected" Chl-aII+-reduction are photochemically transformed into slower one by double hit processes with a comparatively low quantum yield. Furthermore, it is inferred, that the dark recovery kinetics of Chl-aII is dependent on be charge accumulation state of the watersplitting enzyme system Y. This phenomenon is shown to explain also the oscillation pattern of delayed fluorescence. On the basis of the present results two alternative reaction schemes for the functional organization of the electron transport on the donor side of system II are discussed.

Chlorophyll↗

PubMedDate not supplied

Effect of an antiserum to a thylakoid membrane polypeptide on the primary photoreaction of photosystem II.

As was described previously, an antiserum to polypeptide 11000 inhibited photosynthetic electron transport on the oxygen evolving side of photosystem II. The effect of the antiserum on chloroplasts from two tobacco mutants also clearly showed that the inhibition site is on the photosystem II-side of the electron transport chain. One of the two tobacco mutants lacks the oxygen evolving capacity but exhibits some electron transport with tetramethyl benzidine, an artificial donor to PS II. In this mutant electron transport was barely inhibited. The effect of the antiserum on the primary photoevents showed that the initial amplitude of the absorption change of chlorophyll aII at 690 nm and that of the primary electron acceptor X320 at 334 nm both diminished in the presence of the antiserum. Both signals were restored upon addition of diphenylcarbazide another artificial donor to photosystem II. Comparison of the degree of inhibition on the amplitudes of the fast and slow components of the 690 nm absorption change with the manometrically measured inhibition of electron transport shows that besides a full inactivation of a part of the reaction centers of photosystem II another part apparently mediates a fast cyclic electron flow around photosystem II as reported by Renger and Wolff earlier for tris-treated chloroplasts. Moreover, the antiserum affects the low temperature fluorescence in a way which is opposite to Murata's effect of the Mg2+-ion induced inhibition of energy spill-over from photosystem II to phytosystem I. The antiserum under the condition in which the Hill reaction is inhibited lowered the 686 nm emission and enhanced the 732 nm emission which indicates an enhanced energy spill-over to photosystem I.

Chloroplasts↗

PubMedDate not supplied

On the correlation between the amplitude of the electrochromic absorption changes and the number of bulk pigments.

A correlation between the amplitude of the electrochromic absorption changes measured at 480 nm and 520 nm and the size of the photosynthetic unit is found. Three tobacco mutants which differ with respect to their photosynthetic unit size exhibit decreased amplitudes of the electrochromic absorption changes which parallel the size of the photosynthetic units. Under these conditions the ATP-yield per flash is unaffected by the photosynthetic unit size. From this it is deduced that the correlation between the amplitude of the electrochromic absorption change and the photosynthetic unit size is due to the fact that the number of pigments undergoing an electrochromic bandshift is proportional to the total pigment content. As the influence on the electrochromic effect at 520 nm is too pronounced as to be simply explainable by the variation of the carotenoid content in the different mutants it is inferred that either structural effects leading to a modification of the carotenoid orientation in the mutants or an indirect influence of chlorophyll on the electrochromic bandshift which depend on the number of chlorophyll molecules are responsible.

Adenosine Triphosphate↗

PubMedDate not supplied

[Effects of relaxation processes on the temperature dependence of oxidation rate of photooxidized bacteriochlorophyll on the primary quinone in reaction centers of Rhodobacter sphaeroides].

The temperature dependence of the time of dark recombination of charges between photooxidized bacteriochlorophyll and reduced primary quinone acceptor (tau e) in Rhodobacter sphaeroides photosynthetic reaction centers was studied in the temperature range 140-320 K. It was found that the function tau e = tau e(T) is nonmonotonous: in the temperature range from 140 to 290 K, tau e is increased from 40 to 100 ms; however, under further heating to 320 K, tau e decreased to 80 ms. The replacement of H2O by D2O in these preparations caused an acceleration of the recombination process in the range of physiological temperatures, but the nonmonotonous character of the function tau e(T) remained. The theoretical interpretation of the results was made in the framework of the theory of electron-phonon interactions with allowance for the relaxation processes.

Algorithms↗

PubMedDate not supplied

[Possible effect of structural phase transition in the reactive center of Rhodobacter sphaeroides on the rate of dark adaptation of photooxidized bacteriochlorophyll from primary quinone].

The dependence of the rate of dark recombination between the photooxidized primary donor--dimer bacteriochlorophyll molecule (P) and reduced primary quinone acceptor (QA), P+QA(-)-->PQA was studied in photosynthetic reaction centers (RC) from Rhodobacter sphaeroides in the temperature range of 100-320 K. Control RC preparations, RC species with the removed H-subunit as well as RC samples with the hydrogen bonds network modified by isotopic D2O-H2O substitution were investigated. An anomalous temperature dependence of the recombination time (tau rec) of dark reaction P+QA(-)-->PQA was found for all RC samples. It was found that upon heating from 120 to 290 K tau rec increased 2.5 fold. However, upon further heating to 320 K, tau rec decreased again. The temperature dependences of the P+QA(-)-->PQA recombination time were compared with those of the thermodepolarization current of RC preparations in the same temperature range. The temperature curve of the thermodepolarization current was also nonmonotonous. The theoretical interpretation of the temperature dependence of tau rec as well as of the thermodepolarization current was made in the framework of the theory of structural phase transitions within the hydrogen bond network in the water-protein surrounding of the redox centers participating in the electron transfer reactions.

Bacteriochlorophylls↗

PubMedDate not supplied

[The application of PS II model for the analysis of fluorescence yield transients induced by actinic single turnover flash in the time range from 100 ns to 10 s].

Changes in flash-induced fluorescence yield in preparations of thermophilic Chlorella pyrenoidosa Chick cells (native and in the presence of DCMU) were investigated in the time range from 100 ns to 10 s using a new measuring system. The results were analyzed by mathematical modeling of processes in photosystem II. It was shown that the detailed description of recombination (including nonradiative) processes in photosystem II is important to simulate the fluorescence yield transients induced by an actinic single turnover flash. The model photosystem II parameters were modified to describe the light-induced effects in the presence of DCMU. By comparing the theoretical fluorescence curves with experimental ones, we obtained the values of relative fluorescence yield and the FM/F0 ratio, which is typical for experimental data. As a result of simulation, the values of the model parameters (rate constants of electron transfers at the donor and acceptor sites of photosystem II, pH in stroma, initial redox state of the plastoquinone pool, the rate of plastoquinol oxidation, and the rate constants of nonradiative recombination processes) were determined.

Chlorella↗