The electron acceptor side

Fortunately, the electron-acceptor side of PSII can be exploited to allow turnover control of the S states in highly concentrated samples. A number of herbicides are known that bind tightly to the QB site and block electron transfer past the primary quinone electron acceptor (QA) (13). Some examples are shown in Figure 3. Equations 4 and 5 show the reactions of PSII in the presence of 3-(3,4-dichlorophenyl)-l,l-di-methylurea (DCMU, Figure 3). 

As far as we know, the cyclic electron transfer pathways in Chloroflexus are similar to those found in purple bacteria [13,35]. As shown in Fig. 3, there appear to be cyclic pathways involving b- and c-type cytochromes, Fe-S centers, and qui-nones in both kinds of bacteria grown phototrophically. However, Chloroflexus contains only MQ [32], while any given purple bacterium will always contain ubiquinone (UQ) with or without some MQ. On the electron acceptor side of the RC in Chloroflexus there are apparently two MQ molecules in series (MQ and MQb), which are assumed to feed electrons into an MQ pool [30,36]. Electrons presumably enter a Cyt b c complex from the MQ pool and leave the complex via Cyt c = 0.21 V) and Cyt c-554 j = 0.28 V) [35]. The membrane-bound Cyt c-554 ( m,8 +0.26 V, = 43 000) [37] is the direct electron donor to P-865 in the RC [22,30,36]. It contains two hemes with redox potentials of +0.14 and +0.26 V respectively [36] and is absent from aerobically grown cells [38]. 

In an effort to generate images of the morphology of thylakoid membranes inside plant chloroplasts, isolated chloroplasts resuspended in a buffer solution containing a herbicide, 3-(3,4-dichlorophenyl)-l,l-dimethylurea (DCMU), have often been used [10, 11]. The DCMU inhibits photosynthesis by blocking electron transport at the electron acceptor side of PSII and enhances chlorophyll fluorescence from PSII. 

The figure shows that subsequent electron transfers lead, on the electron-acceptor side, to reduction of Qa and then, in the absence ofDCMU, to the reduction of Qb while on the electron-donor side, an S-state transition occurs ... 

Bicarbonate (or CD2) has been shown to stimulate the electron flow from the primary plastoquinone to Qg, the secondary plastoquinone of Photosystem II (PS II) and, then, to the intersystem plastoquinone pool in formate-treated thylakoids (1, 2). We call this the " bicarbonate effect . The site of action of these anions appears to be located on the electron acceptor side in PS II no evidence exists for its action on the oxygen evolving complex (3). 

Abstract. Treatment with formate (1), nitrite or azide of Svnechocvstis 6803 thylakoids caused a slowing down of the oxidation of 0 , as calculated form Chi a fluorescence decay after saturating flakes. Addition of 2.5 to 5 itiM HQ03 fully reversed this inhibition in formate- and nitrite-treated sairples however, in 100 mM azide-treated saitples only 50% inhibition was reversed at 2 ms after the actinic flash. The anion treatment (bicarbonate depletion) affects the electron acceptor side of PSII between 0 and the PQ pool. Hill reaction in bicarbonate-depleted Svnechocvstis cells was stimulated more than 4 fold by 5 itiM bicarbonate. The pH range for the optimum stimulatory effect was around 6.7. 

Anion Effects on the Electron Acceptor Side of Photosystem II in a Transformable... 

FT photooxygenation of 1,1-diarylethylenes in the presence of electron acceptors such as cyanoaromatics, Lewis acids, and dyes occurs efficiently to yield 1,2-dioxanes as the main product in addition to diarylketones as side products. The yield of the 1,2-dioxane derivatives proved to be dependent on aryl substitution, solvent polarity, the electron acceptor, and the excitation wavelength (Scheme 39). 

The electron acceptors on the reducing side of photosystem II resemble those of purple bacterial reaction centers. The acceptor that removes an electron from P680 is a molecule of pheophytin a. The second and third acceptors are plastoquinones (see fig. 15.10). As in bacterial reaction centers, electrons move one at a time from the first quinone to the second. When the second quinone becomes doubly reduced, it picks up protons from the stromal side of the thylakoid membrane and dissociates from the reaction center. 

The intramolecular mechanism, illustrated on the left-hand side of Figure 6.8, is based on four separate operations [52]. (a) Destabilization of the stable translational isomer light excitation of the photoactive unit P (step 1) is followed by the transfer of an electron from the excited state to the Al station, which is encircled by the macrocycle (step 2) with the consequent deactivation of this station such a photoinduced electron-transfer process has to compete with the intrinsic decay of P (step 3). (b) Ring displacement the ring moves from the reduced station Ah to A2 (step 4), a step that has to compete with the back electron-transfer process from Ah (still encircled by the macrocycle) to the oxidized photoactive unit P+ (step 5). This is the most difficult requirement to meet in the intramolecular mechanism, (c) Electronic reset a back electron-transfer process from the free reduced station Ah to P+ (step 6) restores the electron-acceptor power to the Al station, (d) Nuclear reset as a consequence of the electronic reset, back movement of the ring from A2 to Al takes place (step 7). 

The qualitative features of the spectra are in conformity with the observations made on the ether. HF or ether. HC1 systems. Millen and Zabicky24 examined the complexes of methanol with trimethylamine, dimethylamine, methylamine, ammonia and aziridine. In all cases methanol is the proton donor and the nitrogen lone pair is the electron acceptor. In the methanol trimethylamine-complex spectrum the central band is at 3355 cm-1. One subband is resolved at each side, at about 3495 and 3200 cm-1. Using a large excess of amine the absorbance of the bands is proportional to the product of the pressures of the two components so the spectrum must be attributed to the 1 1 complex. From the spacing a value of 145 cm-1 can be infered for v . 

The accumulation of minority carriers and the corresponding shift of energy bands can be avoided if a suitable redox system is added to the electrolyte. For instance in the case of p-GaAs, the energy bands are shifted downward back to its original dark value upon addition of [Fe(CN)e] " as an electron acceptor. It is believed here that the electrons captured by surface states, are transferred from these states to the electron acceptor in the solution, as illustrated on the right side of Fig. 10 [70]. 

In environmental engineering, it is customary to call the substance oxidized as the electron donor and the substance reduced as the electron acceptor. The electron donor is normally considered as food. In the context of nitrogen removal, the foods are the nitrites, nitrates, and ammonia. Equation (15.10) is an example of an electron donor reaction. Zn is the donor of the electrons portrayed on the right-hand side of the half-cell reaction. On the other hand, the reverse of the equation is an example of an electron acceptor reaction. Zn would be the electron acceptor. McCarty (1975) derived values for free energy changes of half-reactions for various electron donors and acceptors utilized in a bacterial systems. The ones specific for the nitrogen species removal are shown in Table 15.2. 

The evolution of O2 from water has been shown to occur every 4th flash, when flashes of saturating intensity, short enough to allow only one turnover of the PS II reaction centres, are fired, separated by a dark period long enough to permit the reoxidation of the electron acceptors on the reducing side of PS II [7]. This observation has been the basis of the S states model. Each flash promotes the transition from the state S to S +, in the sequence [8,9] ... 

When the components of the PS II reaction centre are drawn on a redox scale and compared in this way to those of the purple bacterial reaction centre, a remarkable similarity can be seen between the electron acceptors in each system (Fig. 4). The chemical natures of these components are extremely similar, being made up of a complex of two quinones, an iron atom and a pheophytin (a bacteriopheo-phytin in bacteria). The donor side of PS II in the redox scheme is, however, not comparable to that in bacteria. P-680 may appear to be structurally similar to P-870 in bacteria in that it is made up of chlorophyll (bacteriochlorophyll in bacteria) and that is acts as the primary electron donor however, the P-680/P-680+ redox couple is approximately 600-800 mV more oxidizing than the equivalent bacterial redox couple P-870/P-870, = +450 mV). In addition, PS II has an array of high-potential components which make up the 02-evolving enzyme and which are clearly unique to that system. 

Fig. 3 shows the arrangement of electron transfer cofactors in PS I and PS II as given by their X-ray crystal structures (2, 3). The two structures are similar, and each has a so-called special pair of chlorophylls located on the stromal side of the complex and shown in green in Fig. 2. Extending across the membrane from the respective special pairs are two branches of cofactors that act as the electron acceptors.

Small phosphatidylcholine vesicles (d = 25 nm) containing magnesium octa-ethylporphyrin (MgOEP) in the presence of the water-soluble electron acceptors ferricyanide or dimethylviologen produce cation radicals upon flash photolysis. When the electron acceptor is present on both sides of the vesicle bilayer, approximately triple the amount of porphyrin cations are produced than by the reaction with the electron acceptor on the outside. Furthermore decay of triplet absorption is scarcely effected when M is primarily on the outside of the vesicles, but occurs much more rapidly where MV is present on both sides. 

A subsequent measurement of the structure of crystals formed from the 252-residue, lumen-side domain at 1.96 A by Martinez et revealed the presence of an L-shaped array of five water molecules embedded near the heme, as shown in Fig. 6 (D), left, and in an enlarged view for the water chain and its heme environment in Fig. 6 (D), right. The water chain extends in two directions from ligand His-25. The longer branch extends 11 A in the direction of Lys-66, which is known to be in the basic patch at the top of the domain and has been implicated in the interaction with the electron acceptor, plastocyanin. The authors suggested that the water chain may function as an exit port on the lumen side for protons translocated by the Cyt b(f complex. 

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