The Quinone Acceptors

3 The Quinone Acceptor. - In the bRC two quinones act in sequence in the electron-transfer process. They are coupled to a high spin Fe2+ (S — 2). QA accepts only one electron whereas QB can be doubly reduced and protonated. QbH2 leaves the RC and releases its electron and protons to neighboring membrane complexes. It is replaced by an oxidized quinone from the pool in the membrane. The strikingly different physical properties of QA and QB in the bRC can only be explained by a different protein surrounding. 

The extensive EPR work performed on the radical-ions of QA and QB in the last decades has been thoroughly described by Mobius103 and Weber45. This will therefore only briefly be summarized here before recent articles are discussed. 

Removal of the iron or replacement with a diamagnetic divalent ion like Zn2+ allows the observation of narrow and intense signals of both and Qy. Feher 

The -tensor anisotropy of the quinone radical-anions is much larger than that of P+ or BChl a +. It can thus often be fully resolved at W-band or even at Q-band frequencies when deuterated samples are employed. An overview of the measured 0-tensor components has already been given by Weber45, and an interpretation of 0-tensors and their usefulness can be found in ref. 131. Significant progress in the calculation of g values of quinone radical-anions has been made, see references 134-139. 

It is clear that the g values are very sensitive to the presence or absence of hydrogen bonding to the quinone oxygens and to the hydrophobicity of the solvent surrounding or the protein pocket. Also the structure of the quinone itself plays a role.140 However, since the 0-tensor reflects only the global properties of the wavefunction it is still difficult to draw far reaching conclusions from this spectroscopic parameter. This situation will hopefully change when more reliable 0-tensor calculations, e.g. for radicals embedded in proteins, become available, e.g. on the QM/MM level. 

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The Quinone Acceptors. - The two plastoquinones (PQ-9) QA and QB act as sequential electron acceptors in PS II, QA being a one- and QB a two-electron acceptor. Both quinones are coupled to a high-spin Fe2+ (S = 2) that is coordinated by four histidines (two from D1 and two from D2 protein). The 5th and 6th coordination position are occupied by bicarbonate. A number of small molecules can replace bicarbonate (OH-, CN-, NO etc.) by which the spin/charge state of the complex and the ET rate can be influenced. 

In 12, the donor and acceptor moieties are chiral, and, as noted by the authors, the molecule therefore exists as a pair of diastereomers which are separable by chromatography. However, rotation about the linkages between the porphyrin macrocycle and the attached aryl rings must be very slow on the time scale of electron transfer. Thus, non-interconverting diastereomers should be present, with slightly different separations and orientations between the aniline donor and the quinone acceptor. This distribution would be expected to influence the decay kinetics of D+-P-QT if charge recombination is via direct electron transfer. If a... 

The photochemistry of the reaction center takes place one electron at a time. However, one of the products of the electron transfer process is a reduced ubiquinone, which has taken up two electrons as well as two protons. To form this species, the reaction center must turn over twice, with electrons entering the complex by donation of cytochrome ci with the oxidized special pair. The electrons accumulate in the quinone acceptors and protons are taken up from the surrounding medium. Finally, a fidly reduced ubiquinol is formed, which is released from the complex into the hydrocarbon portion of the membrane. The quinol is subsequently reoxidized at the cytochrome bc complex (described below). 

There is some confusion as to which electron acceptor should be called primary. Historically, in purple bacteria the quinone acceptor, Q, was so named. Later it was found that a BPh molecule accepts an electron before Q, and possibly even earlier acceptors, or charge transfer states, exist. Since the latter matter is still under debate (see Chapter 3), one might prudently keep the label primary for the quinone acceptor with the understanding that it is the first stable (on a time scale of ms) acceptor. 

The absorption of a photon by P produces the lowest excited singlet state of P ( P ) that irreversibly donates an electron to I and leads to formation of a singlet radical pair ( [P T X] ) in about 2.8 ps. In the normal biological reaction, the [P r X] radical pair transfers an electron to the quinone acceptor (X) to produce [P K ] in about 200 ps. The lower free energy of [P K ] makes this an irreversible process. Because the above-mentioned rates are much faster the rate of the S-T conversion of the radical pairs, no appreciable MFE can be observed in the normal reaction. Instead, if X is chemically reduced (XT ) to prevent it from accepting an electron, the reaction of such a system occurs as follows ... 

Gust, Moore, Moore and coworkers covalent cartenoid-porphyrin-quinone molecular triads 55-60 contain a cyclized hydrogen bond within the quinone acceptor framework [143], The naphthaquinone moiety of 55 is fused to a norbomene system whose bridgehead position bears a carboxylic acid, which can hydrogen bond to an adjacent quinone. Photoinduced electron transfer from the porphyrin to the quinone leads to a marked p/fg increase of the latter, resulting in a fast proton transfer ( pt 10 s ) to form the semiquinone. Back electron transfer from the semiquinone is attenuated as a consequence of the proton-stabilized charge-separated species. This leads to a two-fold increase in the quantum yield of the charge-separated state of 55, as compared to those of the reference triads 56 and 57 (see Volume III, Part 2, Chapter 2). 

Cyclodextrin-appended porphyrins are one of the suitable types of compound for ET models, since cyclodextrin (CD) has a hydrophobic cavity able to capture a small nonpolar molecule such as a quinone in aqueous solution. The fir.st example of a porphyrin- -CD photochemical system 137 was reported by Bolton, Weedon and coworkers in 1984. They monitored the photoinduced ET reaction in a frozen mixture of porphyrin 137 and p-benzoquinone by ESR spectroscopy. Irradiation of the solution showed the characteristic single ESR signal due to the generation of porphyrin cation radical and quinone anion radical species. The signal intensities in the ESR spectra, using a variety of quinones, indicated that the efficiency of the ET depended upon the reduction potential of the quinone acceptors. 

Isolated RC s from both Rb, sphaeroides R26 and Rps. viridis can be depleted of the natural quinones and then reconstituted with artificial quinones of different thermodynamic and stereochemical properties. Assaying these RC s with optical flash and ESR spectroscopy allows to gain insight in the thermodynamics of the primary reactions in photosynthesis, and in the structural requirements of the binding site of the quinone acceptors. 

To understand the nature of the pH dependence of Lab on pH, we consider a simple phenomenological model in which we suppose only electrostatic interactions between reactants and in which only two amino acid residues, close to Qb, are involved in the proton uptake by RCs after the first flash. The two amino acids, designated here as D (i.e.. Asp) and E (i.e., Glu), generate four different protonation states DHEH, D-EH, DH E- and D E. Electrostatic interactions between amino acid residues and quinone acceptors give rise to different pKs and rate constants for the various ionization states of D and E and the charge states of Ae quinones. Instead of one state of the quinone acceptors, for example QaQb> it is necessary to consider four QaQb(DHEH), QaQb(D"EH), (JaQbCDHE ) and QaQb(D E ) One-electron transfer between the quinones involves 12 possible states, with 10 independent equilibrium constants and pKs [17] ... 

Penetration of the polar portions of the RC protein by salts and their screening effect encourages the reinvestigation of all characteristics of the quinone acceptors as a function of salt concentration. 

In Rps. viridis no site-directed mutants are available. However, a number of herbicide-resistant mutants have been characterized (Sinning et al., 1989 Ewald et al., 1990). Herbicides of the triazine type compete with Qb for the binding pocket and their binding is controlled by nearby residues which also are potential candidates for proton donors to reduced Qb- Recently, we have characterized the kinetics and pH-dependence of the electron transfer in the quinone acceptor complex in Rps. viridis wild type (Wt). To get further insight into the mechanisms of quinone reduction we studied these reactions in herbicide-resistant mutants. In this contribution we present a first glance on these experimental results. 

We studied the kinetics of electron transfer in the quinone acceptor complex in Wt (Leibl and Breton, 1991) and in the following three herbicide-resistant mutants of Rps,... 

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