Quinone Molecule

The functional reaction center contains two quinone molecules. One of these, Qb (Figure 12.15), is loosely bound and can be lost during purification. The reason for the difference in the strength of binding between Qa and Qb is unknown, but as we will see later, it probably reflects a functional asymmetry in the molecule as a whole. Qa is positioned between the Fe atom and one of the pheophytin molecules (Figure 12.15). The polar-head group is outside the membrane, bound to a loop region, whereas the hydrophobic tail is... 

In the bacterial reaction center the photons are absorbed by the special pair of chlorophyll molecules on the periplasmic side of the membrane (see Figure 12.14). Spectroscopic measurements have shown that when a photon is absorbed by the special pair of chlorophylls, an electron is moved from the special pair to one of the pheophytin molecules. The close association and the parallel orientation of the chlorophyll ring systems in the special pair facilitates the excitation of an electron so that it is easily released. This process is very fast it occurs within 2 picoseconds. From the pheophytin the electron moves to a molecule of quinone, Qa, in a slower process that takes about 200 picoseconds. The electron then passes through the protein, to the second quinone molecule, Qb. This is a comparatively slow process, taking about 100 microseconds. 

FIGURE 22.17 The R. viridis reaction center is coupled to the cytochrome h/Cl complex through the quinone pool (Q). Quinone molecules are photore-duced at the reaction center Qb site (2 e [2 hv] per Q reduced) and then diffuse to the cytochrome h/ci complex, where they are reoxidized. Note that e flow from cytochrome h/ci back to the reaction center occurs via the periplasmic protein cytochrome co- Note also that 3 to 4 are translocated into the periplasmic space for each Q molecule oxidized at cytochrome h/ci. The resultant proton-motive force drives ATP synthesis by the bacterial FiFo ATP synthase. (Adapted from Deisenhofer, and Michel, H., 1989. The photosynthetic reaction center from the purple bac-terinm Rhod.opseud.omoaas viridis. Science 245 1463.)... 

Quinhydrone, a solid-state associate of quinone and hydroquinone, decomposes in solution to its components. The quinhy drone electrode is an example of more complex organic redox electrodes whose potential is affected by the pH of the solution. If the quinone molecule is denoted as Ox and the hydroquinone molecule as H2Red, then the actual half-cell reaction... 

PS I has a reaction center designated as p700 and has a high ratio of chlorophyll a to chlorophyll b. It consists of 13 polypeptide chains, 1 quinone molecule, and 4 Fe-4S clusters. 

Our current work is concentrated on the series of porphyrin-quinone molecules (V). These are very similar to I in that amide linkages have replaced the ester linkages. We chose to study these molecules not only because they provided a definite change... 

Once the special pair has absorbed a photon of solar energy, the excited electron is rapidly removed from the vicinity of the reaction centre to prevent any back reactions. The path it takes is as follows within 3 ps (3 X 10 12 s) it has passed to the bacteriopheophytin (a chlorophyll molecule that has two protons instead of Mg2+ at its centre), without apparently becoming closely associated with the nearby accessory bacteriochlorophyll molecule. Some 200 ps later it is transferred to the quinone. Within the next 100 ps the special pair has been reduced (by electrons coming from an electron transport chain that terminates with the cytochrome situated just above it), eliminating the positive charge, while the excited electron migrates to a second quinone molecule. 

Distance-Dependent Rates of Photoinduced Charge Separation and Dark Charge Recombination in Fixed-Distance Porphyrin-Quinone Molecules... 

Trifonov and Panayotov (41) attempted to carry out anionic polymerizations of vinyl monomers with semiquinones generated at a cathode. Since semiquinones inhibit free-radical polymerization, anionic polymerization alone should take place in the system. When electrolysis of quinones was conducted in a solution of LiCl or N(CaH6)4I in DMF with mercury cathode, the catholyte turned to red or purple red in accordance with the semiquinones. The presence of free-radical produced on the quinone molecule was proved from the ESR spectrum. When each of the monomers, styrene, acrylonitrile and methyl methacrylate were added to the colored solutions, polymers were obtained. 

The observation of a photosynthetic reaction center in green sulfur bacteria dates back to 1963.39 Green sulfur bacteria RCs are of the type I or the Fe-S-type (photosystem I). Here the electron acceptor is not the quinine instead, chlorophyll molecules (BChl 663, 81 -OII-Chi a, or Chi a) serve as primary electron acceptors, and three Fe4S4 centers (ferredoxins) serve as secondary acceptors. A quinone molecule may or may not serve as an intermediate carrier between the primary electron acceptor (Chi) and the secondary acceptor (Fe-S centers).40 The process sequence leading to the energy conversion in RCI is shown in Figure 21. 

Enhanced photovoltage and photocurrent signals were observed by the authors of Refs. [183,184] with linked porphyrin-quinone molecules in planar bilayer lipid membranes (BLM) as compared with preparations containing the non-Iinked components. They interposed BLM between two aqueous compartments containing a secondary electron donor on one side and a secondary acceptor on the other side. The efficiency of PET increased when the P-L-Q molecules were oriented in the membrane. 

The fluorescence decay has a major component of 1 ns which is likely to correspond to unquenched porphyrin, whereas a much shorter component of -100 ps can also be detected which is attributed to quenched porphyrin by quinone molecules which sit within a distance corresponding to the radius of an active-sphere of quenching (Perrin, 1924). 

Q-pool outside. The minimum distance between myxothiazol and an-timycin A molecules is only 18 A this indicates that the two sites can exchange a quinol/quinone molecule very quickly by simply flipping the head group. 

A typical CIDEP initial polarization system is summarized in Fig. 11. For the quenching by a hydrogen donor (SH) of triplet quinone molecules, a plot of V-- - versus [SH] 1 will yield a straight line (see eq. 38), the ordinate intercept of which will give V"1 and from the slope an estimate of Tjt (or Tf) kq can be made. Because Vo can be obtained from the intercept and eliminated from the slope to obtain kq, it can be seen that only relative values of V are needed (i.e., the absolute are not required). (Absolute can be directly measured in a time-resolved CIDEP experiment, if needed.) Obviously, if the value of 3Tl is known, kq (absolute) can be calculated. Unfortunately, Tj values are usually not known. However, by studying a series of hydrogen donors, the relative quenching efficiencies can be estimated from the ratios of the slopes for plots of eq. 38. 

Despite lack of sequence homology, the function of the quinone reduction site (Qi site) is similar to that of the secondary quinone-binding site (Qb site) of bacterial reaction centers. Both sites have a conserved histidine residue as quinone ligand and both quinone molecules are reduced to hydroquinone in two consecutive one-electron transfer steps. The midpoint potential for the first step is pH-independent at near neutrality, whereas that for the second reduction varies by 120mV per pH unit (Robertson et al., 1984). This suggests a reaction pathway Q —> Q" QH2, with both protons added concomitantly with the second electron. A stable semi-quinone anion intermediate can be detected by EPR spectroscopy of samples frozen during turnover (Yu et al., 1980 de Vries et al., 1980) or with the redox potential adjusted near the midpoint of ubiquinone (Robertson et al., 1984 Ohnishi and Trumpower, 1980). The semiquinone signal is not observed in the presence of antimycin, which is consistent with the proposal that antimycin inhibits the reaction at the site (Mitchell, 1976 Mitchell, 1975). 

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